:>V;>y«".5.i.:i';<v.:,*  .  $3 


THE  PRINCIPLES   OF 


ANIMAL    NUTRITION. 


WITH  SPECIAL  REFERENCE  TO   THE 
NUTRITION  OF  FARM  ANIMALS. 


BY 

HENRY   PRENTISS   ARMSBY,    Ph.D., 

Director  of  The  Pennsylvania  State  College  Agricultural  Experiment  Station 
Expert  in  Animal  Nutrition,  United  States  Department  of  Agriculture. 


FIRST  EDITION. 
FIRST    THOUSAND. 


NEW  YORK : 

JOHN  WILEY   &   SONS. 

London:  CHAPMAN   &  HALL,   Limited. 

1903. 


Copyright,  1903, 

BY 

HENRY  P.  ARMSBY. 


ROBERT   DRUMMOND,    PRINTER,    NEW   YORK. 


PREFACE. 


The  past  two  decades  have  not  only  witnessed  great  activity  in 
the  study  of  the  various  problems  of  animal  nutrition,  but  they  are 
especially  distinguished  by  the  new  point  of  view  from  which  these 
problems  have  come  to  be  regarded.  Speaking  broadly,  it  may  be 
said  that  to  an  increasing  knowledge  of  the  chemistry  of  nutrition 
has  been  added  a  clear  and  fairly  definite  general  conception  of  the 
vital  activities  as  transformations  of  energy  and  of  the  food  as 
essentially  the  vehicle  for  supplying  that  energy  to  the  organism. 
This  conception  of  the  function  of  nutrition  has  been  a  fruitful 
one,  and  in  particular  has  tended  to  introduce  greater  simplicity  and 
unity  into  thought  and  discussion.  Much  exceedingly  valuable 
work  has  been  done  under  its  guidance,  while  it  points  the  way 
toward  even  more  important  results  in  the  future.  The  following 
pages  are  not  a  treatise  upon  stock-feeding,  but  are  an  attempt  to 
present  in  systematic  form  to  students  of  that  subject  a  summary  of 
our  present  knowledge  of  some  of  the  fundamental  principles  of  ani- 
mal nutrition,  particularly  from  the  standpoint  of  energy  relations, 
with  special  reference  to  their  bearings  upon  the  nutrition  of  farm 
animals.  Should  the  attempt  at  systematization  appear  in  some 
instances  premature  or  ill-advised,  the  writer  can  only  plead  that 
even  a  temporary  or  tentative  system,  if  clearly  recognized  as  such, 
may  be  preferable  to  unorganized  knowledge.  The  scaffolding 
has  its  uses,  even  though  it  form  no  part  of  the  completed  building. 

The  attentive  reader,  should  there  be  such,  will  not  fail  to  note 
that  the  work  is  limited  to  those  aspects  of  the  subject  included 
under  the  more  technical  term  of  "The  Statistics  of  Nutrition," 
and  that  even  in  this  restricted  field  some  important  branches  of 
the  subject  have  been  omitted  on  account  of  what  has  seemed  to 


e^ 


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— >  <^  & 


iv  PREFACE. 

the  writer  a  lack  of  sufficient  accurate  scientific  data  for  their  profit- 
able discussion.  Moreover,  many  principles  which  are  already 
familiar  have  been  considered  rather  cursorily  in  order  to  ahow  a 
more  full  treatment  of  less  well-known  aspects  of  the  subject,  even 
at  the  expense  of  literary  proportion. 

The  substance  of  this  volume  was  given  in  the  form  of  lectures 
before  the  Graduate  Summer  School  of  Agriculture  at  the  Ohio 
State  University  in  1902,  and  has  been  prepared  for  publication  at 
the  request  of  instructors  and  students  of  that  school.  In  thus 
presenting  it  to  a  somewhat  larger  public  the  author  ventures  to 
hope  that  it  may  tend  in  some  degree  to  promote  the  rational  study 
of  stock-feeding  and  to  aid  and  stimulate  systematic  investigation 
into  both  its  principles  and  practice. 

State  College,  Pa.,  November,  1902. 


CONTENTS. 


Introduction * 

The  Statistics  of  Nutrition 3 

PART   I. 
THE   INCOME   AND   EXPENDITURE    OF    MATTER. 

CHAPTER    I. 

The  Food 5 

CHAPTER   II. 

Metabolism 14 

§  1.  Carbohydrate  Metabolism 17 

§  2.  Fat  Metabolism 29 

§  3.  Proteid  Metabolism 38 

Anabolism 38 

Katabolism 41 

The  Non-proteids 52 

CHAPTER    III. 

Methods  of  Investigation 59 

CHAPTER  IV. 

The  Fasting  Metabolism 80 

§  1.  The  Proteid  Metabolism 81 

§  2.  The  Total  Metabolism 83 

CHAPTER  V. 

The  Relations  of  Metabolism  to  Food-supply 93 

§  1.  The  Proteid  Supply 94 

Effects  on  Proteid  Metabolism 94 

Effects  on  Total  Metabolism 104 

Formation  of  Fat  from  Proteida 107 

v 


CONTENTS. 

Fim 

52.  Th«  Non-nitrogenous  Nutrients 114 

Effects  on  Proteid  Metabolism 114 

The  Minimum  of  Proteids 133 

Effects  on  Total  Metabolism 144 

Mutual  Replacement  of  Nutrients 148 

Utilization  of  Excess — Sources  of  Fat 162 


CHAPTER  VI. 

The  Influence  of  Muscular  Exertion  upon  Metabolism 185 

§  1.  General  Features  of  Muscular  Activity 185 

Muscular  Contraction 185 

Secondary  Effects  of  Muscular  Exertion 191 

§  2.  Effects  upon  Metabolism 193 

Upon  the  Proteid  Metabolism 194 

Upon  the  Carbon  Metabolism 209 


PART   II. 

THE  INCOME  AND   EXPENDITURE   OF  ENERGY. 

CHAPTER  VII 
Force  and  Energt 226 

CHAPTER  VIII. 
Methods  of  Investigation 234 

CHAPTER  IX. 
The  Conservation  of  Energy  in  the  Animal  Body 258 

CHAPTER  X. 

The  Food  as  a  Source  of  Energy — Metabolizable  Energy 269 

§  1.  Experiments  on  Carnivora 272 

§  2.  Experiments  on  Man 277 

§  3.  Experiments  on  Herbivora 281 

Metabolizable  Energy  of  Organic  Matter 284 

Total  Organic  Matter 285 

Digestible  Organic  Matter 297 

Energy  of  Digestible  Nutrients 302 

Gross  Energy 302 

Metabolizable  Energy 310 


CONTENTS. 


CHAPTER  XI. 

PAGE 

Internal,  Work 336 

§  1.  The  Expenditure  of  Energy  by  the  Body 336 

§  2.  The  Fasting  Metabolism 340 

Nature  of  Demands  for  Energy 340 

Heat  Production 344 

Influence  of  Thermal  Environment 347 

Influence  of  Size  of  Animal 359 

§  3.  The  Expenditure  of  Energy  in  Digestion  and  Assimilation 372 

CHAPTER  XII. 

Net  Available  Energy — Maintenance 394 

§  1.  Replacement  Values 396 

§  2.  Modified  Conception  of  Replacement  Values 405 

§  3.  Net  Availability 412 

Determinations  of  Net  Availability 413 

Discussion  of  Results 430 

Influence  of  Amount  of  Food 430 

Character  of  Food 431 

The  Maintenance  Ration 432 


CHAPTER  XIII. 

The  Utilization  op  Energy 444 

§  1.  Utilization  for  Tissue  Building 448 

Experimental  Results 448 

Discussion  of  Results 465 

Influence  of  Amount  of  Food 466 

Influence  of  Thermal  Environment 471 

Influence  of  Character  of  Food 472 

The  Expenditure  of  Energy  in  Digestion,  Assimilation 

and  Tissue  Building 491 

§  2    Utilization  for  Muscular  Work 494 

Utilization  of  Net  Available  Energy 497 

The  Efficiency  of  the  Animal  as  a  Motor 498 

Conditions  determining  Efficiency 511 

Utilization  of  Metabolizable  Energy 525 

Wolff's  Investigations 528 


THE  PRINCIPLES  OF  ANIMAL  NUTRITION. 


INTRODUCTION. 

The  body  of  an  animal,  regarded  from  a  chemical  point  of  view, 
consists  of  an  aggregate  of  a  great  variety  of  substances,  of  which 
water,  protein,  and  the  fats,  with  smaller  amounts  of  certain 
carbohydrates,  largely  predominate.  By  far  the  greater  portion  of 
the  substance  of  the  body,  aside  from  its  water,  consists  of  so- 
called  "  organic  "  compounds;  i.e.,  compounds  of  carbon  with  hydro- 
gen, oxygen,  nitrogen,  and,  to  a  smaller  extent,  with  sulphur  and 
phosphorus.  These  compounds  are  in  many  cases  very  complex, 
and  all  of  them  have  this  in  common,  that  they  contain  a  con- 
siderable store  of  potential  energy. 

It  is  through  these  complex  "  organic  "  compounds  that  the  phe- 
nomena of  life  are  manifested.  All  forms  of  life  with  which  we  are 
acquainted  are  intimately  associated  with  the  conversion  of  com- 
plex into  simpler  compounds  by  a  series  of  changes  which,  regarded 
as  a  whole,  partake  of  the  nature  of  oxidations.  During  this  break- 
ing down  and  oxidation  more  or  less  of  the  potential  energy  of  these 
compounds  is  liberated,  and  it  is  this  liberation  of  energy  which  is 
the  essential  end  and  object  of  the  whole  process  and  which,  if  not 
synonymous  with  life  itself,  is  the  objective  manifestation  of  life. 
This  is  equally  true  of  the  plant  and  the  animal,  although  masked 
in  the  case  of  green  plants  by  the  synthetic  activity  of  the  chloro- 
phyl  in  the  presence  of  light.  The  process  is  most  manifest  in  the 
animal,  however,  both  on  account  of  the  inability  of  the  latter  to 
utilize  the  radiant  energy  of  the  sun  and  on  account  of  the  greater 
intensity  of  the  process  itself. 

Setting  aside  for  the  moment  any  storing  up  of  material,  and 


PBOrtRTY  LKRARY 
tf.  C  State  College 


2  PRINCIPLES   OF  ANIMAL   NUTRITION. 

therefore  of  potential  energy,  in  the  body  for  the  future  use  of  the 
animal  itself  or  of  its  offspring  as  being,  from  a  physiological  point  of 
view,  temporary  and  incidental,  the  sole  useful  product  of  the  animal 
is  energy.  All  the  physical  effect  which  we  can  produce,  either 
through  our  own  bodies  or  those  of  our  domestic  animals,  is  simply 
to  move  something,  and  moving  something  is  equivalent  to  the 
exertion  of  energy.  This  motion  may  be  the  motion  of  visible 
masses  of  matter  in  the  performance  of  useful  work  or  the  invisible 
molecular  motion  of  heat,  which  is  economically  a  waste  product, 
but  in  either  case  the  animal  is  a  source  of  energy  which  is  imparted 
to  its  surroundings.  From  this  point  of  view,  then,  we  may  look 
upon  the  animal  as  a  mechanism  for  transforming  the  stored-up 
energy  of  the  sun's  rays,  contained  in  its  tissues,  into  the  active  or 
"  kinetic  "  forms  of  heat  and  motion.  The  various  cells  and  tissues 
of  the  living  animal  body,  in  the  performance  of  their  several  func- 
tions, break  down  and  oxidize  the  proteids,  fats,  carbohydrates, 
and  other  materials  of  which  they  are  composed  or  which  are  con- 
tained in  them,  seizing,  as  it  were,  upon  the  energy  thus  liberated 
and  converting  it,  here  into  heat,  there  into  motion,  again  into  the 
energy  of  chemical  change,  as  the  needs  of  the  organism  demand. 

The  very  definition  of  physical  life,  then,  implies  that  the  living 
animal  is  constantly  consuming  its  own  substance,  rejecting  the 
simpler  compounds  which  result  and  giving  off  energy  in  the  various 
forms  characteristic  of  living  beings.  Obviously,  this  process,  if 
unchecked,  would  soon  lead  to  the  destruction  of  the  organism  and 
the  dissipation  of  its  store  of  potential  energy.  To  prevent  this 
catastrophe  is  the  object  of  the  great  function  of  nutrition. 

This  function,  in  its  broader  outlines,  is  familiar  to  us  all  through 
daily  experience  and  observation.  The  living  animal  requires  to 
be  frequently  supplied  with  certain  substances  which  collectively 
constitute  its  food.  This  food  contains  a  great  variety  of  chemical 
ingredients,  but  much  the  larger  part  of  it  consists  of  "  organic  " 
compounds  belonging  to  the  three  great  groups  already  noted  as 
making  up  the  larger  share  of  the  organic  matter  of  the  body,  viz., 
the  proteids,  the  fats,  and  especially  the  carbohydrates,  and  while 
the  individual  members  of  these  groups  differ  in  the  two  cases,  the 
ingredients  of  the  food,  like  those  of  the  body,  contain  a  large  store 
of    potential    energy.      These    and    other    "organic"  substances, 


INTRODUCTION.  3 

together  with  more  or  less  mineral  matter,  are  separated  by  the 
organism,  in  the  processes  of  digestion  and  resorption,  from  the  un- 
essential or  unavailable  matters  of  the  food.  The  latter  are  rejected 
from  the  body,  while  the  former  are  used  by  it  to  take  the  place 
of  the  material  broken  down  and  excreted  by  its  vital  activities, 
and  thus  serve  to  maintain  its  capital  of  matter  and  of  potential 
energy. 

In  other  words,  the  food  may  be  regarded  as  the  vehicle  by 
means  of  which  a  little  portion  of  the  "infinite  and  eternal  energy 
from  which  all  things  proceed  "  is  put  for  the  time  being  at  the 
service  of  the  individual  ;  as  being  not  so  much  a  supply  of  matter 
to  make  good  the  waste  of  tissue  as  a  supply  of  energy  for  the  mani- 
festations of  life. 

The  animal  body,  then,  from  our  present  standpoint,  consists  of 
a  certain  amount  of  matter  which  has  been  temporarily  segregated 
from  the  rest  of  the  universe  and  which  represents  a  certain  store 
or  capital  of  potential  energy.  This  aggregate  of  matter  and 
energy  is  in  a  constant  state  of  change  or  flux.  On  the  one  hand, 
its  vital  activities  are  continually  drawing  upon  its  capital.  By 
the  very  act  of  living  it  expends  matter  and  energy.  On  the  other 
hand,  by  means  of  the  function  of  nutrition,  it  is  continually  receiv- 
ing supplies  of  matter  and  energy  from  its  environment  and  adding 
them  to  its  capital.  Plainly,  then,  the  growth;  the  maintenance,  or 
the  decay  of  the  body  depends  upon  the  relation  which  it  is  able  to 
maintain  between  the  income  and  the  expenditure  of  matter  and 
energy.  If  the  two  are  equal,  the  animal  is  simply  maintained 
without  increase  or  decrease;  if  the  income  is  greater  than  the 
expenditure,  the  body  adds  to  its  capital  of  matter  and  energy,  if 
the  income  is  less  than  the  expenditure,  the  necessary  result  is  a 
diminution  in  the  accumulated  capital  which;  if  continued,  must 
ultimately  result  in  death. 

We  thus  reach  an  essentially  statistical  standpoint,  and  this 
aspect  of  the  subject  of  nutrition,  which  has  been  designated  by 
some  writers  as  "The  Statistics  of  Nutrition,"  forms  the  subject  of 
the  succeeding  pages.  The  topic  naturally  divides  itself  into  two 
distinct  although  closely  related  parts,  viz. : 

1.  The  income  and  expenditure  of  matter. 

2.  The  income  and  expenditure  of  energy. 


4  PRINCIPLES  OF  ANIMAL   NUTRITION. 

These  topics  will  be  considered  in  the  above  order,  it  being 
assumed  that  the  reader  is  already  familiar  with  the  general  nature 
of  the  nutritive  processes  included  under  the  general  heads  of 
digestion,  resorption,  circulation,  respiration,  and  excretion. 


PART  I. 
THE  INCOME  AND  EXPENDITURE  OF  MATTER. 


CHAPTER  I. 
THE  FOOD. 

The  supply  of  matter  to  the  body  is,  of  course,  contained  in  the 
food,  including  water  and  the  oxygen  taken  up  from  the  air.  In 
a  more  limited  and  familiar  sense,  the  term  food  is  employed  to 
signify  the  supply  of  solid  matter,  or  dry  matter,  to  the  animal. 
It  is  proposed  here  simply  to  recall  certain  familiar  facts  relative 
to  the  composition  and  digestibility  of  the  food  in  this  narrower 
sense,  taking  up  the  subject  in  the  barest  outline. 

Composition. — While  a  vast  number  of  individual  chemical 
compounds  are  found  in  common  feeding-stuffs,  the  conventional 
scheme  for  their  analysis  unites  these  substances  into  groups  and 
regards  feeding-stuffs  as  composed,  aside  from  water  and  mineral 
matter,  essentially  of  protein,  carbohydrates  and  related  bodies, 
and  fats.  Or,  setting  aside  the  mineral  ingredients,  the  "  organic  " 
ingredients  may  be  divided  into  the  nitrogenous,  comprised  under 
the  term  protein,  and  the  non-nitrogenous,  including  the  fats  and 
carbohydrates. 

Protein. — The  name  "protein"  originated  with  Mulder,  who 
used  it  to  designate  what  he  supposed  to  be  a  common  ingredient 
of  all  the  various  proteids,  but  it  has  since  come  to  be  employed  as  a 
group  name  for  the  nitrogenous  ingredients  both  of  feeding-stuffs 
and  of  the  animal  body. 

The  amount  of  protein  in  feeding-stuffs  we  have  at  present  no 

S 


6  RINCIPLES    OF  ANIMAL   NUTRITION. 

means  of  determining  directly,  but  it  is  commonly  estimated  from 
the  amount  of  nitrogen  upon  two  assumptions:  first,  that  all  the 
substances  of  the  protein  group  contain  16  per  cent,  of  nitrogen,  and 
second,  that  all  the  nitrogen  of  feeding-stuffs  exists  in  the  proteid 
form.  On  the  basis  of  these  assumptions,  protein  is,  of  course, 
equal  to  total  nitrogen  X  6.25. 

Although  it  was  never  claimed  that  this  method  of  estimating 
protein  was  strictly  accurate,  it  was  for  a  long  time  assumed  that 
the  two  sources  of  error  involved  were  not  serious.  Later  investi- 
gations, however,  have  dispelled  this  pleasing  illusion.  Further 
investigations  of  the  true  proteids,  notably  those  of  Ritthausen  and 
of  Osborne,  have  shown  a  very  considerable  variation  in  the  per- 
centage of  nitrogen  contained  in  them,  while,  on  the  other  hand,  the 
researches  of  Scheibler,  E.  Schulze,  Kellner,  and  others  have  shown 
the  presence  in  many  feeding-stuffs  of  relatively  large  amounts  of 
nitrogenous  matters  of  non-proteid  nature.  The  results  of  these 
latter  investigations  have  made  it  necessary  to  subdivide  the  total 
nitrogenous  matter  of  feeding-stuffs  into  two  groups,  called  respec- 
tively "proteids"  and  " non-proteids,"  while  the  name  "protein" 
has  been  retained  in  the  sense  of  total  nitrogen  X  6.25  or  other  con- 
ventional factor.  For  various  classes  of  human  foods,  Atwater  and 
Bryant  *  propose  the  following  factors,  based  on  the  results  in- 
dicated in  the  next  two  paragraphs,  for  the  computation  of  protein 
from  nitrogen: 

Animal  foods 6 .  25 

Wheat,  rye,  barley,  and  their  manufactured  products     5.70 
Maize,  oats,  buckwheat  and  rice,  and  their  manufactured 

products 6 .  00 

Dried  seeds  of  legumes 6 .  25 

Vegetables 5 .  65 

Fruits 5.80 

Proteids. — In  the  absence  of  any  adequate  knowledge  regarding 
the  very  complex  molecular  structure  of  the  proteids,  both  the 
classification  and  the  terminology  of  these  bodies  are  in  a  very  con- 
fused state.      For  convenience,  however,  we  may  adopt  here  those 

*  Storrs  (Conn.)  Ag.  Ex.  St.,  Rep.  12,  79. 


THE  FOOD. 


tentatively  recommended  by  the  Association  of  American  Agri- 
cultural Colleges  and  Experiment  Stations,*  viz. : 


f  Albuminoids  ■{ 
al  f  Proteids  1 

I  Co: 

Extractives, 


Albumins, 
f  Simple     \  Globulins, 


nitrogen  com- 
pounds 


and  allies. 
Derived. 
Compound. 


Proteids  -j  [  Modified 

[  Collagens  or  gelatinoids 

at  „*  m„  i  Extractives, 

[  Non-proteids  j  Amides>  amido.acidS)  etc. 

It  is  not  necessary  for  our  present  purpose  to  enter  into  any  dis- 
cussion either  of  the  properties  of  the  proteids  as  a  whole  or  of  the 
differences  between  the  different  classes  of  proteids.  One  point, 
however,  is  of  particular  importance,  namely,  the  elementary  com- 
position of  these  bodies.  As  noted  above,  this  has  been  found  to  be 
more  variable  than  was  supposed  earlier.  In  particular  the  per- 
centage of  nitrogen  has  been  found  to  have  a  somewhat  wide  range. 
"Recent  investigations  with  perfected  methods  show  percentages 
of  nitrogen  in  the  numerous  single  proteid  substances  found  in  the 
grains  ranging  from  15.25  to  18.78.  These  are  largest  in  certain 
oil  seeds  and  lupines  and  smallest  in  some  of  the  winter  grains. 
Ritthausen,f  a  prominent  German  authority,  concedes  that  the 
factor  6.25  should  be  discarded,  and  suggests  the  use  of  5.7  for  the 
majority  of  cereal  grains  and  leguminous  seeds,  5.5  for  the  oil  and 
lupine  seeds,  and  6.00  for  barley,  maize,  buckwheat,  soja-bean,  and 
white  bean  (Phaseolus)  rape,  and  other  brassicas.  Nothing  short 
of  inability  to  secure  greater  accuracy  justifies  the  longer  contin- 
uance of  a  method  of  calculation  which  is  apparently  so  greatly 
erroneous."     (Jordan.) 

Non-proteids. — This  term  is  used  as  a  convenient  designation 
for  all  the  nitrogenous  materials  of  feeding-stuffs  which  are  not 
proteid  in  their  nature.  It  is  an  abbreviated  form  of  non-pro teid 
nitrogenous  bodies.  The  substances  of  this  class  found  in  plants 
are  chiefly  the  organic  bases,  amides,  amido-acids,  and  similar 
bodies  which  are  produced  by  the  cleavage  of  the  proteid  molecule 
under  the  action  of  digestive  and  other  ferments  or  of  hydrating 
agents.  They  appear  to  exist  in  the  plant  partly  as  intermediate 
stages  in  the  synthesis  of  the  proteids  and  partly  as  products  of 

*U.  S.  Dept.  Agr.,  Ofl&ce  of  Experiment  Station,  Bui.,  65,  p.  117. 
fLandw.  Vers.  Stat.,  74,  391. 


8  PRINCIPLES   OF  ANIMAL   NUTRITION. 

their  subsequent  cleavage  in  the  metabolism  of  the  plant.  They 
are  chiefly  soluble,  crystalline  bodies.  The  most  common  of  them 
is  asparagin,  which  has  been,  to  a  certain  extent,  regarded  as 
typical  of  the  group. 

The  non-proteids  are  commonly  determined  by  determining  as 
accurately  as  possible  the  non-proteid  nitrogen  and  multiplying  the 
latter  by  the  factor  6.25.  In  the  case  of  asparagin,  however,  which 
contains  21.2  per  cent,  of  nitrogen,  the  proper  factor  obviously 
should  be  4.7,  while  the  factor  would  vary  for  the  different  forms  of 
non-proteids  which  have  been  observed  in  plants.  It  is  no  simple 
matter,  therefore,  either  to  determine  directly  the  amount  of  non- 
proteids  or  to  decide  upon  the  proper  nitrogen  factor  in  any  partic- 
ular case.  For  the  present,  however,  the  factor  4.7  would  seem 
to  be  at  least  a  closer  approximation  to  the  truth  than  6.25. 

In  the  animal  body  the  group  of  non-proteids  is  represented  by 
the  so-called  "extractives"  or  "flesh  bases"  of  the  muscle,  chiefly 
creatin  and  creatinin. 

Fats. — The  fats  of  the  plant,  like  those  of  the  animal,  consist 
chiefly  of  glycerin  compounds  of  the  so-called  "fatty  acids,"  or  of 
similar  bodies.  These  are  accompanied  in  the  plant,  however,  by 
other  materials — wax,  chlorophyl,  etc. — which  are  extracted  along 
with  the  fat  by  the  common  method  of  determination  and  consti- 
tute part  of  the  "crude  fat"  or  "ether-extract."  The  results, 
therefore,  which  have  been  obtained  in  feeding  experiments  with 
pure  fats  cannot  be  used  with  safety  as  a  basis  for  estimating  the 
nutritive  value  of  the  so-called  "  fat "  of  feeding-stuffs,  particularly 
in  the  case  of  coarse  fodders. 

Carbohydrates. — The  well  -  characterized  group  of  carbo- 
hydrates makes  up  a  large  proportion  of  the  organic  matter  of  our 
more  common  feeding-stuffs.  This  group  of  substances  may  be  sub- 
divided on  the  basis  of  molecular  structure  into  hexosans  and  their 
derivatives  (hexoses,  bioses,  trioses,  etc.),  on  the  one  hand,  whose 
molecules  contain  six  atoms  of  carbon  or  a  multiple  of  that  number, 
and  the  pentosans  and  pentoses,  or  five-carbon  series,  on  the  other. 
In  the  grains  and  other  common  concentrated  feeding-stuffs,  and 
particularly  in  the  food  of  man,  the  hexose  group  largely  predomi- 
nates, including  starch,  dextrin,  the  common  sugars,  and  more  or 
less  cellulose.     In  the  coarse  fodders  consumed  by  our  domestic 


THE  FOOD.  9 

herbivorous  animals,  while  the  hexose  group  is  also  largely  repre- 
sented it  is  accompanied  by  no  inconsiderable  quantities  of  carbo- 
hydrates belonging  to  the  pentose  group.  The  individual  members 
of  this  latter  group  are  both  less  abundant  and  less  well  known 
chemically  than  the  hexoses,  and  at  present  our  knowledge  of  their 
actual  nutritive  value  is  somewhat  scanty.  Since  the  methods  for 
their  determination  are  based  upon  the  fact  that  they  yield  furfural 
upon  boiling  with  dilute  hydrochloric  acid,  some  recent  analysts 
have  proposed  the  term  "  furfuroids  "  as  a  more  appropriate  desig- 
nation of  these  substances  as  determined  by  present  methods. 

In  the  conventional  scheme  for  the  analysis  of  feeding-stuffs,  the 
carbohydrates  are  subdivided,  not  upon  the  basis  of  their  chemical 
structure  but  upon  the  basis  of  their  solubility.  Those  members 
of  the  group  which  can  be  brought  into  solution  by  boiling  dilute 
acids  and  alkalies  under  certain  conventional  conditions  are  grouped 
together  as  "Nitrogen-free  extract,"  while  those  ingredients 
which  resist  solution  under  these  conditions  are  designated  as 
"Crude  fiber."  The  more  common  hexose  carbohydrates,  such  as 
starch,  sugars,  etc.,  are  included  in  the  nitrogen-free  extract,  while 
the  larger  part,  although  not  all,  of  the  cellulose  is  included  under 
the  crude  fiber.  At  the  same  time,  more  or  less  of  the  pentose  carbo- 
hydrates or  "  furfuroids  "  are  found  in  both  these  groups,  while  the 
crude  fiber  of  coarse  fodders  contains  also  a  variety  of  other  ill- 
known  compounds,  somewhat  roughly  grouped  together  under  the 
general  designation  of  ligneous  material. 

Digestibility. — A  part  of  nearly  all  common  food  materials  is 
incapable  of  digestion  and  is  rejected  in  the  feces.  In  the  food  of 
man  and  that  of  carnivorous  animals  this  indigestible  portion  is 
usually  small  and  may  disappear  entirely.  In  the  food  of  herbivora, 
on  the  other  hand,  there  are  contained  relatively  large  amounts  of 
substances  which  are  incapable  of  solution  in  the  digestive  tract, 
while  varying  proportions  of  materials  which  in  themselves  are 
capable  of  being  digested  may  escape  actual  digestion  under  some 
circumstances.  In  the  latter  aniroals,  therefore,  it  becomes  par- 
ticulary  important  to  determine  the  digestible  portion  of  the  food. 
The  digestibility  of  a  feeding-stuff  is  estimated  indirectly  by  deter- 
mining as  accurately  as  possible  the  undigested  matter  eliminated 
from  the  body  in  the  feces  and  subtracting  it  from  the  total  amount 


io  PRINCIPLES   OF  ANIMAL    NUTRITION. 

contained  in  the  food.  This  method  may  of  course  be  applied 
either  to  the  dry  matter  or  the  organic  matter  of  the  food  as  a 
whole  or  to  any  single  determinable  ingredient. 

Metabolic  Products. — The  digestive  tract  of  an  animal,  how- 
ever, not  only  serves  as  a  mechanism  for  the  digestion  of  food  but 
has  excretory  functions  as  well,  and  the  rejected  matter  contains, 
besides  the  undigested  portion  of  the  food,  these  excreta  and  the 
metabolic  products  of  intestinal  action.  In  the  case  of  food  largely 
or  completely  digestible,  these  substances  may  make  up  the  larger 
portion  or  even  the  whole  of  the  feces,  while,  on  the  other  hand, 
they  constitute  but  a  small  proportion  of  the  bulky  excreta  of 
herbivora. 

It  is  obvious  that  these  products  must  be  taken  account  of  if  it  is 
desired  to  learn  the  actual  digestibility  of  the  food.  Unfortunately, 
however,  we  have  at  present  no  trustworthy  method  for  their  deter- 
mination. In  the  past  it  has  been  customary  to  designate  the 
difference  between  food  and  feces  as  digestible  and,  in  the  case  of 
domestic  animals  at  least,  to  assume  that  the  error  involved  is  not 
serious. 

Apparent  Digestibility — Availability. — Even  with  herbivo- 
rous animals,  however,  the  presence  of  the  so-called  metabolic 
products  in  the  feces  may  give  rise  to  serious  errors  in  the  deter- 
mination of  the  real  digestibility  of  some  ingredients  of  the  food, 
notably  fat  and  protein.  With  carnivora,  or  with  the  human 
subject,  the  case  is  for  obvious  reasons  still  worse,  and  it  is 
scarcely  possible  to  determine  the  digestibility  of  the  food  in  the 
strict  sense  of  the  word. 

The  difference  between  food  and  feces  does  represent,  however, 
the  net  gain  of  matter  to  the  organism  resulting  from  the  digestion 
of  the  food.  To  express  this  conception,  the  use  of  the  word  avail- 
able has  been  proposed  by  Atwater.*  The  "  available  nutrients  "  of 
a  food,  according  to  him,  are  the  actually  digestible  nutrients  minus 
the  metabolic  products  contained  in  the  feces  and  which  may  be 
regarded  as  representing  the  expenditure  of  matter,  in  the  form  of 
residues  of  digestive  fluids,  intestinal  mucus,  epithelium,  etc., 
necessarily  incident  to  the  digestion  of  the  food.   The  term  has  been 

*  Storrs  (Conn.)  Agr'l  Expt.  St.,  Rep.,  12,  69. 


THE  FOOD.  II 

used  chiefly  in  connection  with  human  nutrition.  In  discussions  of 
animal  nutrition  the  terms  digestible  and  digestibility  have  become 
so  firmly  established  that  it  may  be  questioned  whether  the  intro- 
duction now  of  a  new  term  would  not  create  more  confusion  than  it 
would  prevent,  and  whether  it  is  not  preferable,  when  strict  accuracy 
of  expression  is  required,  to  attach  a  modifying  word  and  designate 
the  difference  between  food  and  feces  as  apparently  digestible,  in 
distinction  from  the  real  digestibility,  which  we  cannot  as  yet  deter- 
mine. 

Determination  of  Apparent  Digestibility. — The  determi- 
nation of  the  apparent  digestibility  of  the  nutrients  of  a  feeding- 
stuff  in  the  above  sense,  or  of  their  "  digestibility  "  in  the  older  sense, 
consists  simply  in  determining  the  amount  of  the  feces  or  of  their 
separate  ingredients  and  comparing  them  with  the  correspond- 
ing amounts  in  the  food. 

Aside  from  ordinary  analytical  precautions,  the  chief  condition 
of  accurate  results  is  that  the  feces  correspond  to  the  food  consumed. 
In  animals  with  a  comparatively  simple  digestive  canal,  like  man 
and  the  carnivora,  this  is  readily  brought  about  by  the  ingestion  of 
a  small  amount  of  some  substance  like  powdered  charcoal  or  infu- 
sorial earth,  which  is  in  itself  indigestible  and  which  serves  to  sepa- 
rate the  feces  of  two  successive  periods.  In  the  case  of  herbivora, 
on  the  other  hand,  the  undigested  residues  of  the  food  become  mixed 
to  a  large  extent  with  those  of  the  previous  period.  In  this  case, 
therefore,  it  is  essential  that  a  preliminary  feeding  be  continued  for 
a  sufficient  length  of  time  to  remove  the  residues  of  previous  foods 
from  the  digestive  organs,  and  further  that  the  experiment  itself 
extend  through  a  number  of  days  in  order  to  eliminate  the 
influence  of  irregularity  of  excretion. 

Significance  of  Results. — It  is  plain  from  what  has  just 
been  said  that  what  the  results  of  such  an  experiment  actually 
show  is  that  a  certain  amount  of  material  has  disappeared  from 
the  food  during  its  transit  through  the  alimentary  canal.  This 
fact  of  itself,  however,  does  not  necessarily  show  that  the  missing 
material  has  been  digested  in  any  true  sense.  In  the  case  of  animals 
possessing  a  relatively  short  and  simple  digestive  apparatus,  we  are 
probably  justified  in  assuming  that  the  difference  between  food  and 
undigested   matter   represents   material   that   has   actually   been 


12  PRINCIPLES  OF  ANIMAL   NUTRITION. 

digested.  In  the  long  and  complicated  digestive  apparatus  of 
herbivora,  however,  there  is  the  possibility  that  a  variety  of  proc- 
esses may  go  on  aside  from  a  simple  solution  of  nutrients  by  the 
digestive  fluids.  In  particular,  it  has  been  shown,  as  will  appear  in 
greater  detail  later,  that  extensive  fermentations,  particularly  of  the 
carbohydrates,  occur,  and  that  relatively  large  amounts  of  these 
bodies  may  be  destroyed  in  this  way. 

Furthermore,  with  our  present  conventional  scheme  for  fodder 
analysis,  we  have  to  take  account  of  the  possibility  of  the  conversion 
of  members  of  one  group  of  nutrients  into  those  of  another.  For 
example,  it  seems  not  improbable  that  a  portion  of  the  crude  fiber 
of  feeding-stuffs  may  be  so  modified  in  the  digestive  tract,  without 
bemg  actually  dissolved,  that,  in  the  feces,  it  is  determined  as 
nitrogen-free  extract,  thus  diminishing  the  apparent  digestibility 
of  the  latter  group  and  increasing  that  of  the  crude  fiber.* 

Composition  of  Digested  Food. — The  proteids  during  the 
process  of  digestion  are  largely  converted  into  proteoses  and  pep- 
tones, while  the  trypsin  of  the  pancreatic  juice,  at  least  outside  the 
body,  carries  the  cleavage  of  the  proteid  molecule  still  further  and 
gives  rise  to  comparatively  simple,  crystalline  bodies.  It  is  not 
altogether  clear  to  what  extent  this  degradation  of  the  proteids 
occurs  in  natural  digestion,  but  the  probability  appears  to  be  that 
it  does  not  play  a  large  part,  and  it  has  been  generally  believed  that 
the  proteids  are  resorbed  chiefly  as  proteoses  and  peptones. 

The  non-proteids  being  largely  crystalline  bodies  and  readily 
soluble,  we  may  presume  that  they  are  resorbed  without  material 
change  except  so  far  as  they  may  serve  as  nitrogenous  food  for  the 
micro-organisms  of  the  digestive  tract. 

The  fat  of  the  food  does  not  undergo  any  profound  change  in 
digestion,  but  appears  to  be  resorbed  largely  in  the  form  of  an 
emulsion.  A  part  of  it,  however,  is  undoubtedly  saponified  by 
the  bile,  although  the  extent  to  which  this  process  takes  place  is  a 
disputed  point,  while  in  some  cases  at  least  a  cleavage  into  glycerin 
and  free  fatty  acids  appears  to  take  place. 

The  carbohydrates,  particularly  the  easily  soluble  members  of 
the  hexose  group,  are  in  the  case  of  man  and  the  carnivora,  and 

*Cf.  Fraps,  Jour.  Am.  Chem.  Soc,  22,  543. 


THE   FOOD.  13 

probably  also  to  a  large  extent  in  the  swine  and  horse,  converted 
into  sugars  and  resorbed  in  that  form. 

Fermentations. — Reference  has  already  been  made  to  the  fermen- 
tations taking  place  in  the  digestive  tract.  In  the  herbivora,  and 
especially  in  ruminants,  these  fermentations  play  an  important 
part  in  the  solution  of  the  carbohydrates  which  make  up  so  large 
a  portion  of  the  food  of  these  animals.  These  bodies  undergo  a 
fermentation  which  was  first  studied  by  Tappeiner  *  in  the  case  of 
cellulose,  but  which  has  since  been  shown  by  G.  Kuhn  |  to  extend 
also  to  the  more  soluble  carbohydrates.  The  products  of  this 
fermentation  appear  to  be  methane,  carbon  dioxide,  and  organic 
acids,  chiefly,  according  to  Tappeiner,  acetic  and  butyric.  Of  these 
products,  only  the  organic  acids  at  best  can  be  supposed  to  be  of 
any  value  to  the  animal  organism,  and  obviously  it  makes  a  very 
serious  difference  in  our  estimate  of  the  nutritive  value  of  starch, 
for  example,  whether  it  is  resorbed  chiefly  or  entirely  in  the  form  of 
sugar  or  whether  in  a  ruminant  more  than  half  of  it,  as  in  some  of 
Kiihn's  experiments,  is  fermented. 

*  Zeit.  f.  Biol.,  20,  52.  f  Landw.  Vers.  Stat.,  44,  569. 


CHAPTER  II. 
METABOLISM. 

General  Conception. — By  the  various  processes  of  digestion 
and  resorption  the  epithelium  of  the  alimentary  canal  extracts  from 
the  crude  materials  eaten  those  ingredients  which  are  fitted  to 
nourish  the  animal  and  transmits  them  more  or  less  directly  to  the 
general  circulation  which  carries  them  to  all  the  tissues  of  the  body. 
While  these  ingredients  are  many  in  number  and  diverse  in  charac- 
ter, yet  the  vast  mass  of  them,  aside  from  the  water  in  which  most 
of  them  are  dissolved,  may  be  grouped  under  six  heads,  viz.,  ash 
ingredients,  albuminoids  or  bodies  related  to  the  albuminoids, 
amides  and  other  crystalline  nitrogenous  substances,  fats,  carbo- 
hydrates, and  organic  acids,  and  these,  together  with  relatively 
small  amounts  of  other  materials,  may  be  regarded  as  constituting 
the  real  food  of  the  organism. 

As  was  pointed  out  in  the  Introduction,  the  cells  of  which  the 
living  tissues  of  the  animal  body  are  composed  are  the  seat  of  con- 
tinual chemical  change.  On  the  one  hand,  the  digested  ingredients 
of  the  food  which  are  brought  to  them  by  the  circulation  are  being 
built  up  into  the  structure  of  the  body.  On  the  other  hand,  the 
material  of  the  cells  is  undergoing  a  continual  process  of  breaking 
down  and  oxidation,  uniting  with  the  oxygen  supplied  by  the  blood 
to  form  the  waste  products  which  are  removed  from  the  body 
through  the  organs  of  excretion.  These  excretory  products  are 
substantially  carbon  dioxide,  water,  and  urea  and  similar  nitroge- 
nous substances. 

The  general  term  Metabolism  is  commonly  used  to  designate  the 
totality  of  the  chemical  and  physical  changes  which  the  materials 
of  the  resorbed  food,  or  of  the  tissues  formed  from  them,  undergo  in 
being  converted  into  the  excretory  products.  Similarly,  we  may 
speak  in  a  more  restricted  sense  of  the  metabolism  of  a  single  ingre- 

14 


METABOLISM.  15 

dient  of  the  food,  as  of  the  proteids,  carbohydrates,  or  fats.  Thus 
proteid  metabolism  signifies  the  chemical  changes  undergone  by  the 
proteids  of  the  food  in  their  conversion  into  the  corresponding 
excretory  products.  In  ordinary  usage  the  chemical  reactions 
undergone  by  the  ash  ingredients  of  the  food  are  not  included,  the 
word  metabolism  being  practically  used  to  designate  the  chemical 
changes  in  the  organic  matter  of  food  or  tissue. 

Metabolism  a  Process  of  Oxidation. — The  process  of  met- 
abolism as  a  whole  is  one  of  oxidation.  While  we  must  beware 
of  being  misled  by  analogy  into  regarding  as  a  simple  burning  of 
food-materials  that  which  is  in  reality  a  highly  complex  action  of 
the  living  cells  of  the  organism,  still  the  final  result  is  much  the 
same  in  both  cases.  Starting  with  more  or  less  complex  organic 
substances  and  oxygen,  we  end  either  with  the  completely  oxidized 
compounds  carbon  dioxide  and  water  or  with  nitrogenous  sub- 
stances like  urea  more  highly  oxidized  than  the  protein  from  which 
they  are  derived. 

The  oxidative  character  of  the  total  metabolism  is  most  simply 
illustrated  by  a  comparison  of  the  percentage  of  oxygen  contained 
in  the  most  prominent  ingredients  of  the  food,  on  the  one  hand,  and 
in  the  chief  excretory  products,  on  the  other  hand,  as  in  the  follow- 
ing statement: 

Percentage  of  Oxygen. 
In  food: 

Protein  (average) 23.00 

Fats 11.50 

Dextrose 53.33 

In  excreta: 

Urea 26.67 

Carbon  dioxide 72.72 

Water 88.89 

Metabolism  an  Analytic  Process. — From  a  slightly  different 
point  of  view,  metabolism  may  be  described  as  an  analytic  process. 
The  molecules  of  the  food  constituents  are  highly  complex.  The 
molecule  of  dextrose  or  lsevulose,  the  forms  in  which  the  carbo- 
hydrates are  chiefly  resorbed,  contains  24  atoms;  the  molecules  of 


1 6  PRINCIPLES   OF  ANIMAL    NUTRITION. 

the  three  most  common  fats,  respectively  155,  167,  and  173  atoms. 
The  molecular  structure  of  the  proteids  has  not  yet  been  made  out, 
but  it  is  highly  complex.*  The  molecules  of  the  excretory  prod- 
ucts, on  the  contrary,  are  comparatively  simple,  those  of  carbon 
dioxide  and  water  containing  but  three  atoms  each,  that  of  urea 
eight,  and  even  that  of  hippuric  acid  but  twenty-two. 

In  metabolism,  in  other  words,  the  complex  molecules  of  the 
carbohydrates,  fats,  proteids,  etc.,  which  have  been  built  up  in  the 
plant,  by  means  of  the  energy  contained  in  the  sun's  rays,  out  of 
carbon  dioxide,  water,  and  nitric  acid  or  ammonia,  gradually  break 
down  again  into  simpler  compounds,  their  atoms  reuniting  with 
the  oxygen  from  which  they  were  separated  in  the  plant. 

Metabolism  a  Gradual  Process. — The  chemical  changes  in- 
cluded under  the  term  metabolism  take  place  gradually.  As  has 
already  been  indicated,  metabolism  is  not  a  simple  oxidation  of 
nutrients,  like  the  burning  of  fuel  in  a  stove,  but  the  nutrients  enter, 
to  a  large  extent  at  least,  into  the  structure  of  the  cells  of  which  the 
various  tissues  are  composed.  Metabolism  is  really  the  sum  of  the 
chemical  actions  through  which  the  nutrition  and  life  of  these  cells 
is  manifested.  These  actions,  however,  differ  from  tissue  to  tissue 
and  from  cell  to  cell,  and  even  in  the  same  cell  from  time  to  time, 
and  the  resulting  metabolic  products  are  correspondingly  varied. 
Between  the  nutrients  supplied  to  the  cells  by  the  blood  and  the 
final  products  of  metabolism  as  excreted  from  the  body  there  are 
innumerable  intermediate  products,  a  few  of  which  we  know  but 
concerning  most  of  which  we  are  still  ignorant.  We  know  the  first 
and  last  terms  of  the  series  and  thus  are  able  to  measure,  as  it 
were,  the  algebraic  sum  of  the  changes,  but  of  the  single  factors 
making  up  this  sum.  as  well  as  of  the  specific  tissues  concerned  in 
the  changes,  we  are  largely  ignorant,  although  we  know  that  they 
are  numerous. 

Anabolism  and  Katabolism. — While  the  process  of  metab- 
olism as  a  whole  is  one  of  analysis  and  oxidation,  with  liberation 
of  energy,  it  must  not  be  supposed  that  each  single  step  in  the 
process  is  of  this  nature.  As  has  been  already  pointed  out,  the 
chemical  activities  of  the  tissues  possess  a  dual  character.     By  the 

*  Osborne  (Zeit.  physio! .  Chem.,  33,  240)  has  recently  obtained  the 
number  14,500  as  the  approximate  molecular  weight  of  edestin. 


METABOLISM.  1 7 

various  processes  of  nutrition,  ingredients  of  the  food  are  first  incor- 
porated into  the  tissues  of  the  body,  to  be  subsequently  broken 
down  and  oxidized.  In  this  building-up  process  changes  undoubt- 
edly occur  in  the  direction  of  greater  complexity  of  molecular  struc- 
ture, involving  the  temporary  absorption  of  energy.  Thus  it  is 
known  that  fats  may  be  formed  from  carbohydrates  in  the  body. 
Many  physiologists  hold  that  the  metabolism  in  the  quiescent  muscle 
results  in  the  building  up  of  a  complex  "contractile  substance," 
whose  breaking  down  furnishes  the  energy  for  muscular  work.  In 
general,  we  may  regard  it  as  highly  probable  that  the  molecules  of 
the  living  substance  of  the  body  are  much  more  complex  than  those 
of  the  nutrients  of  the  food,  and  that  the  former  are  built  up  out  of 
the  latter  by  synthetic  processes,  carried  on  at  the  expense  of  energy 
derived  from  the  breaking  down  of  other  molecules.  Such  changes 
as  this  are  called  anabolic  and  the  process  anabolism,  while  the 
changes  in  the  direction  of  greater  simplicity  of  molecular  structure 
are  called  katabolic,  and  the  process  katabolism.  The  metabolism 
of  the  living  body,  then,  consists  of  both  anabolism  and  katabolism. 
By  the  former  the  food  nutrients  are  built  up  into  body  material; 
by  the  latter  they  are  broken  down,  yielding  finally  the  compara- 
tively simple  excretory  products.  On  the  whole,  however,  the 
katabolism  prevails  over  the  anabolism,  so  that  metabolism  as  a 
whole  is,  as  already  stated,  an  analytic  and  oxidative  process. 
Neither  the  anabolism  of  tissue  production  nor  the  minor  anabolic 
changes  which  seem  to  occur  in  various  tissues  alter  the  main  direc- 
tion of  the  metabolic  changes  in  the  body,  but,  from  the  standpoint 
of  the  statistics  of  nutrition,  are  simply  eddies  in  the  main  current 

§  1.  Carbohydrate  Metabolism. 

HEXOSE    CARBOHYDRATES. 

The  hexose  carbohydrates  of  the  food  appear  to  be  resorbed 
chiefly  by  the  capillary  blood-vessels  of  the  intestines.  For  the 
most  part,  they  reach  the  blood  in  the  form  of  dextrose,  with  smaller 
amounts  of  lsevulose  and  with  greater  or  less  quantities  of  acetic, 
butyric,  lactic,  and  other  acids  derived  chiefly  from  the  fermenta- 
tion of  the  carbohydrates  in  the  digestive  tract.  In  the  genera] 
circulation  only  dextrose  is  found. 


1 8  PRINCIPLES   OF  ANIMAL    NUTRITION. 

The  percentage  of  dextrose  in  the  blood  is  small,  but  remarkably 
constant,  the  limits  of  variation  being  from  about  0.11  to  about  0.20 
per  cent.,  and  the  average  about  0.15  per  cent.  Its  amount 
varies  but  slightly  in  different  regions  of  the  body,  and  in  different 
classes  of  animals,  and  is  scarcely  at  all  affected  by  the  nature  or 
amount  of  the  food.  Not  only  so,  but  any  excess  of  dextrose  in  the 
blood  is  promptly  gotten  rid  of.  It  is  a  striking  fact  that  if  any  con- 
siderable amount  of  this  substance,  which  forms  so  large  a  part  of 
the  resorbed  nutriment,  be  injected  directly  into  the  blood  it  is 
treated  as  an  intruder  and  at  once  excreted  through  the  kidneys. 
Evidently  it  is  of  the  greatest  importance  to  the  organism  that  the 
supply  of  this  substance  to  the  tissues  shall  be  constant. 

Under  ordinary  conditions,  however,  the  influx  of  sugar  from  the 
digestive  tract  is  more  or  less  intermittent.  After  a  meal  rich  in 
easily  digestible  carbohydrates,  an  abundant  supply  of  it  is  taken 
up  by  the  intestinal  capillaries,  while  on  a  diet  poor  in  carbohydrates 
or  in  prolonged  fasting,  the  supply  sinks  to  a  minimum.  This  is,  of 
course,  especially  true  of  animals  like  man  and  the  carnivora  in 
which  the  process  of  digestion  is  comparatively  rapid,  but  even  in 
herbivorous  animals,  with  their  more  complicated  digestive  appara- 
tus, the  rate  of  resorption  of  dextrose,  and  still  more  its  absolute 
amount,  must  be  more  or  less  fluctuating.  Evidently  there  must  be 
some  regulative  apparatus  which  holds  back  from  the  general  circu- 
ation  any  excess  of  dextrose,  on  the  one  hand,  and  prevents  its 
being  excreted  unused,  and  on  the  other,  supplements  any  lack 
resulting  from  a  deficiency  of  the  food  in  carbohydrates.  This 
regulation  is  accomplished  by  the  liver. 

Functions  of  the  Liver. 

The  functions  of  the  liver  in  this  regard  appear  to  be  twofold : 
First,  it  manufactures  dextrose  and  supplies  it  to  the  general  circu- 
lation ;  and  second,  it  serves  as  a  reservoir,  or  a  place  of  deposit,  for 
any  excess  of  carbohydrates  supplied  by  the  digestive  apparatus. 

The  Liver  as  a  Source  of  Dextrose. — The  blood  as  it 
comes  from  the  intestinal  capillaries,  bearing  the  digested  carbo- 
hydrates and  proteids,  enters  the  liver  through  the  portal  vein  and 
is  distributed  by  means  of  the  capillary  blood-vessels  into  which  this 
vein  divides  through  all  parts  of  that  organ,  reaching  the  general 


METABOLISM.  19 

circulation  again  through  the  hepatic  vein.  In  its  passage  through 
the  capillaries  of  the  liver,  the  blood  is  subjected  to  the  action  of  the 
cells  of  the  liver  (hepatic  cells) .  Our  knowledge  of  the  exact  nature 
of  this  action  is  still  more  or  less  conjectural,  in  spite  of  a  vast 
amount  of  experimental  investigation,  but  certain  general  facts  are 
pretty  clearly  established. 

In  the  first  place,  the  hepatic  cells  appear  to  serve  as  a  source  of 
dextrose  when  no  carbohydrates  are  supplied  in  the  food.  If  a 
carnivorous  animal  be  given  a  diet  as  free  as  possible  from  carbo- 
hydrates, as,  for  instance,  prepared  lean  meat,  consisting  substan- 
tially of  proteids,  its  blood  still  contains  a  normal  amount  of  dex- 
trose and  the  blood  in  the  hepatic  vein  is  found  to  be  richer  in 
dextrose  than  that  of  the  portal  vein,  showing  that  this  substance 
is  being  formed  in  the  liver.  Moreover,  while  the  percentage  of 
dextrose  in  the  blood  is  small,  the  total  amount  thus  manufactured 
is  very  considerable.  Seegen  *  estimates  it  at  about  one  per  cent, 
of  the  weight  of  the  body  in  twenty-four  hours.  This  is  regarded 
by  many  physiologists  as  an  overestimate,  the  considerable  differ- 
ences in  sugar  content  between  the  portal  and  hepatic  blood  found 
by  Seegen  being  regarded  as  in  part  the  effect  of  the  necessary 
operation.  Indeed,  it  is  questioned  by  some  whether  any  actual 
difference  in  sugar  content  between  the  portal  and  hepatic  blood 
under  normal  conditions  has  been  satisfactorily  established  analyti- 
cally, but  the  indirect  evidence  at  least  seems  strongly  in  its  favor. 

In  the  second  place,  the  same  outflow  of  dextrose  from  the  liver 
appears  to  take  place  when  the  animal  consumes  a  mixed  diet  con- 
taining carbohydrates.  In  this  case  also,  except  shortly  after  a 
meal  containing  much  carbohydrates,  the  blood  of  the  hepatic  vein 
shows  an  excess  of  dextrose  over  that  of  the  portal  vein.  The 
amount  of  dextrose  thus  introduced  into  the  circulation  is  sub- 
stantially the  same  as  in  the  first  case,  and  its  percentage  in  the 
blood  is  not  perceptibly  altered.  The  source  of  this  dextrose,  how- 
ever, is  not  so  simple  a  question,  since  it  is  possible  that  all  or  a 
considerable  portion  of  it  may  be  supplied  directly  or  indirectly 
by  the  dextrose  resorbed  by  the  intestinal  capillaries. 

Granting  the  continual  production  of  sugar  by  the  liver,  sub- 

*  Die  Zuckerbildung  im  Thierkorper,  p.  115. 


20  PRINCIPLES   OF  ANIMAL   NUTRITION. 

stantially  two  suppositions  are  open:  On  the  one  hand,  we  may 
consider  that  the  resorbed  carbohydrates  of  the  food,  after  being 
temporarily  stored  up  in  the  liver,  as  described  below,  are  given  off 
again  without  radical  change  and  that  the  sugar-forming  power  of 
the  hepatic  cells  is  limited  to  the  transformation  of  the  proteids  and 
perhaps  the  fats  of  the  food.  Or,  on  the  other  hand,  we  may  sup- 
pose that  the  nutrients  brought  to  the  liver  by  the  portal  blood 
enter  into  the  constitution  of  the  protoplasm  of  the  hepatic  cells, 
and  that  the  vital  activity  of  this  protoplasm  gives  rise  to  the  dex- 
trose found  in  the  blood,  to  the  glycogen  found  in  the  liver,  and  to 
other  products  of  whose  nature  we  are  largely  ignorant.  The 
evidence  at  hand  is  doubtless  insufficient  for  a  final  decision  between 
these  alternatives,  but  the  latter  hypothesis  would  seem  more  in 
accord  with  our  general  knowledge  of  cell  activity.  As  relates  to 
the  carbohydrates,  it  is  supported  by  the  fact  that  while  various 
sugars  besides  dextrose  (lsevulose,  mannose,  galactose,  sorbinose, 
and,  as  Miinch  *  has  shown,  certain  artificial  hexoses)  may  be  con- 
verted into  glycogen,  the  resulting  glycogen  is  always  the  same  and 
the  product  of  its  hydration  is  always  dextrose. f  In  other  words, 
the  molecular  structure  of  these  sugars  is  altered  in  a  manner  sug- 
gesting an  assimilation  by  the  hepatic  cells  rather  than  anything 
resembling  an  enzyme  action.  The  subject  can  be  more  intelli- 
gently considered,  however,  in  the  light  of  a  discussion  of  the  second 
function  of  the  liver. 

The  Liver  as  a  Reservoir  of  Carbohydrates. — When  the 
food  is  rich  in  carbohydrates,  the  supply  of  dextrose  to  the  blood 
through  the  intestinal  capillaries  is  more  or  less  intermittent.  As 
a  means  of  regulating  this  intermittent  supply,  the  hepatic  cells 
have  the  power  of  arresting  the  dextrose  brought  to  them  by  the 
portal  vein  and  converting  it  into  an  insoluble  carbohydrate  called 
"  glycogen  "  or  "  animal  starch  "  which  is  stored  up  in  the  liver.  On 
the  other  hand,  when  the  supply  of  carbohydrate  food  is  cut  off, 
and  especially  if  all  food  be  withdrawn,  the  glycogen  of  the  liver 
rapidly  diminishes,  being  apparently  reconverted  into  dextrose. 
This  latter  phenomenon  may  be  readily  observed  in  the  liver  of  a 
freshly  killed  animal.     If  the  fresh  liver,  after  removal  from  the 

*Zeit.  physiol  Chem  .  29,  493. 

f  Compare  Neumeister.  Physiologische  Chemie,  p.  326 


METABOLISM.  21 

body,  be  washed  out  by  water  injected  through  the  portal  vein  till 
all  sugar  is  removed,  and  if  then,  after  standing  for  a  time,  the  wash- 
ing be  renewed,  the  first  portions  of  water  that  pass  contain  sugar. 
The  same  process  may  be  repeated  several  times. 

What  is  known  as  the  glycogenic  function  of  the  liver  was  dis- 
covered by  Claude  Bernard  in  1853,  and  has  been  the  subject  of  a 
bewildering  amount  of  discussion  and  controversy,  both  as  to  the 
origin  of  glycogen,  its  final  fate,  and  its  relations  to  the  production 
of  dextrose  by  the  liver.  Certain  facts,  however,  may  be  regarded 
as  established  with  at  least  a  high  degree  of  probability : 

First — The  liver  produces  glycogen  from  dextrose  and  other 
(not  all)  carbohydrates,  as  above  described. 

Second — The  liver  seems  also  to  form  glycogen  from  prdteids, 
since  this  substance  is  found  in  considerable  quantity  in  the  livers 
of  animals  fed  exclusively  on  meat. 

Third — Glycogen  largely  disappears  from  the  liver  during  fast- 
ing, and  to  a  considerable  degree  also  in  the  absence  of  carbo- 
hydrates from  the  food. 

Fourth — The  liver  produces  dextrose  at  an  approximately  con- 
stant rate,  largely  independent  of  the  food-supply  or  the  variations 
in  the  store  of  glycogen. 

These  facts  seem  to  point  unmistakably  to  the  sugar-producing 
function  of  the  liver  as  the  primary  factor  in  the  whole  matter.  The 
general  metabolism  of  the  body  requires  a  constant  proportion  of 
dextrose  in  the  blood,  and  as  this  dextrose  is  consumed  the  liver 
furnishes  a  fresh  supply.  This  supply  it  manufactures  from  the 
materials  brought  to  it  by  the  blood  of  the  portal  vein.  When 
carbohydrates  are  lacking  in  this  blood,  it  apparently  has  the  power 
of  breaking  down  the  proteids  and  perhaps  the  fats,  thus  supplying 
the  needful  dextrose.  Some  authorities  claim  that  the  same  process 
goes  on  when  carbohydrates  are  present,  and  it  seems  not  unlikely 
that  this  is  true,  but  when  the  food-supply  consists  so  largely  of 
carbohydrates  as  it  does  in  the  case  of  our  domestic  herbivorous 
animals,  the  conclusion  seems  unavoidable  that  at  least  a  consider- 
able part  of  the  dextrose  consumed  in  the  body  must  be  derived 
from  these  substances.  As  already  suggested,  a  very  plausible  view 
of  the  matter  is  to  regard  the  resorbed  nutrients  of  the  portal  blood 
as  serving  to  feed  the  protoplasm  of  the  hepatic  cells  and  to  look 


2  2  PRINCIPLES   OF  ANIMAL   NUTRITION. 

upon   the  dextrose  as  one  of  the  products  of  the  metabolism  of 
those  cells. 

Since,  however,  the  demands  of  the  organism  for  dextrose  and 
the  supply  of  it,  or  of  the  materials  for  its  manufacture,  in  the  food 
do  not  keep  pace  with  each  other,  sometimes  one  and  sometimes  the 
other  being  in  excess,  the  liver  has  a  second  function.  When  the 
food-supply,  of  whatever  kind,  is  in  excess,  instead  of  continuing  to 
produce  dextrose  the  metabolism  in  the  liver  takes  a  slightly  differ- 
ent form  and  produces  the  insoluble  glycogen,  or  perhaps  the  dex- 
trose of  the  portal  blood  is  simply  converted  into  glycogen  without 
entering  into  the  structure  of  the  hepatic  protaplasm.  When,  on 
the  other  hand,  the  food-supply  is  deficient,  the  stored-up  glyco- 
gen is  converted  into  dextrose;  whether  by  some  sort  of  enzyme 
action  or  by  again  serving  as  food  for  the  hepatic  protoplasm  is 
uncertain. 

Fate  o]  the  Dextrose  of  the  Blood. 

The  fact  that  the  proportion  of  dextrose  in  the  blood  is  approxi- 
mately constant,  notwithstanding  the  continual  supply  which  is 
received  from  the  liver,  shows  that  there  must  be  a  continual  abstrac- 
tion of  dextrose  from  the  blood,  which  is  as  continually  made  good 
by  the  activity  of  the  hepatic  cells.  In  fact,  the  dextrose  of  the 
blood  appears  to  play  a  very  prominent  part  in  the  animal  economy, 
and  the  function  of  the  liver  in  preparing  it  from  other  ingredients 
of  the  food  is  a  most  important  one. 

Consumption  in  the  Muscles. — From  the  point  where  it  leaves 
the  liver,  our  knowledge  of  the  metabolism  of  the  dextrose  of  the 
blood  is  scanty,  but  a  large  proportion  of  it  undoubtedly  takes 
place  in  the  muscles.  It  was  early  shown  by  Chauveau  that 
the  proportion  of  dextrose  in  the  blood  diminishes  in  its  passage 
through  the  capillaries  of  the  body,  so  that  the  arterial  blood  con- 
tains more  of  this  substance  than  the  venous.  In  conjunction  with 
Kaufmann  *  he  has  subsequently  shown  more  specifically  that  in  its 
passage  through  the  muscular  capillaries  and  through  those  of  the 
parotid  gland  the  blood  is  impoverished  in  dextrose,  and  to  a  much 
greater  extent  in  the  active  than  in  the  quiescent  muscle.     Coin- 

*Comptes  rend.,  103.  974  and  1057;  104,  1126  and  1352. 


METABOLISM.  23 

■cident  with  this  disappearance  of  dextrose,  there  is  an  increase  in 
the  carbon  dioxide  of  the  blood  and  a  decrease  of  its  oxygen. 

The  relations  of  the  dextrose  of  the  blood  to  the  evolution  of 
heat  and  work  in  the  muscles  and  other  tissues,  so  far  as  they  are  at 
present  understood,  will  be  considered  in  a  subsequent  chapter. 
For  our  present  purpose  it  suffices  to  note  the  fact  that  it  disappears 
in  the  capillaries  with  the  ultimate  production  of  carbon  dioxide 
and  water.  That  the  dextrose  is  immediately  oxidized  to  carbon 
dioxide  and  water,  however,  is  extremely  unlikely.  It  has  been 
suggested  that  the  lactic  acid  which  is  found  in  the  muscle  after 
muscular  contraction  is  one  of  the  intermediate  products  of  the 
oxidation.  Several  considerations,  however,  seem  to  render  it 
more  probable  that  the  dextrose  first  enters  in  some  way  into  the 
constitution  of  the  muscles,  or  in  other  words,  that  a  synthetic  or 
anabolic  process  precedes  the  katabolic  one. 

Muscular  Glycogen. — Another  fact,  of  much  interest  in  this 
connection,  is  that  the  muscles  (and  other  tissues  also),  as  well  as 
the  liver,  contain  glycogen.  Moreover,  the  muscular  glycogen 
diminishes  or  disappears  during  work  and  reappears  again  after 
rest.  It  would  appear,  then,  that  the  muscular  tissue  shares  with 
the  liver  the  ability  to  form  glycogen.  As  in  the  case  of  the  former 
organ,  the  simplest  supposition  is  that  this  glycogen  is  produced 
from  the  dextrose  supplied  in  the  blood,  and  Ktiltz  *  and  others 
have  shown  that  subcutaneous  injections  of  sugar  give  rise  to  a 
formation  of  muscular  glycogen  in  frogs  whose  livers  have  been 
removed.  On  the  other  hand,  of  course,  the  considerations  pre- 
sented above  relative  to  the  sources  of  the  liver  glycogen  apply, 
ceteris  paribus,  to  the  formation  of  glycogen  in  the  muscles.  Neither 
the  source  nor  the  exact  functions  of  the  muscular  glycogen  are 
yet  beyond  controversy,  but  the  facts  just  stated  strongly  suggest 
a  storing  up  of  reserve  carbohydrates  during  rest  to  be  drawn  upon 
when  there  is  a  sudden  demand  for  rapid  metabolism. 

Fat  Production. — In  addition  to  its  important  relation  to  the 
muscles,  the  dextrose  of  the  blood  likewise  supplies  nourishment 
for  the  fat  tissues  of  the  body.  Hitherto  we  have  spoken  as  if  the 
supply  of  dextrose  to  the  blood  were  determined  substantially  by 

*  Neumeister,  Physiologische  Chemie,  p.  322. 


24  PRINCIPLES  OF  ANIMAL    NUTRITION. 

the  demands  of  the  general  metabolism  for  material  to  produce  heat 
and  motion.  Plainly,  however,  the  capacity  of  the  muscles  and 
the  liver  to  store  up  carbohydrates  is  limited,  and  if  the  food-supply 
is  permanently  greater  than  the  demands  of  the  organism,  some 
other  provision  must  be  made  for  the  excess.  Under  these  circum- 
stances the  superfluous  dextrose  which  finds  its  way  into  the  blood 
gives  rise  to  a  production  of  fat,  which  is  stored  up  as  a  reserve  in 
special  tissues  and  apparently  does  not  enter  again  into  the  general 
metabolism  until  a  permanent  deficiency  in  the  food-supply  occurs. 
The  experimental  evidence  of  the  production  of  fat  from  carbo- 
hydrates, as  well  as  the  quantitative  relations  of  the  process  so  far  as 
they  are  known,  will  be  considered  subsequently.  In  its  relations 
to  the  economy  of  the  organism  the  process  is  analogous  to  the 
formation  of  glycogen  in  the  liver,  except  that  the  storage  capacity 
of  the  fat  tissues  is  vastly  greater,  but  as  compared  with  the  forma- 
tion of  glycogen  it  is  distinctively  an  anabolic  process,  the  fat 
molecule  being  more  complex  and  containing  more  potential  energy 
than  that  of  dextrose.  Hanriot,*  assuming  the  formation  of  olein, 
stearin,  and  palmitin  in  molecular  proportions,  represents  the 
process  by  the  equation: 

13C6H1206  =  C55H104O6  +  23C02  +  26H20. 

PENTOSE    CARBOHYDRATES. 

The  facts  of  the  foregoing  paragraphs  relate  primarily  to  the 
hexose  carbohydrates,  particularly  starch  and  sugar,  and  to  a  con- 
siderable extent  to  the  metabolism  of  carnivorous  animals.  The 
food  of  herbivora,  however,  contains  a  great  variety  of  carbohy- 
drates and  especially  considerable  quantities  of  the  pentose  or  five- 
carbon  carbohydrates.  That  these  substances  are  in  part  digest- 
ible, or  that  at  least  a  considerable  proportion  of  them  disappears 
from  the  food  during  its  transit  through  the  alimentary  canal,  was 
first  shown  by  Stone,f  and  has  since  been  fully  confirmed  by  the 
investigations  of  Stone  &  Jones  \  and  of  Lindsey  &  Holland, S 
but  of  their  further  fate  in  the  body  relatively  little  is  known. 

*  Archives  de  Physiol.,  1893,  248.  J  Agricultural  Science,  5,  6. 

t  Amer.  Chem.  Jour.,  14,  9.  %Ibid.t  8,  172. 


PROPERTY  LIBRARY 
N.  C  State  College 


METABOLISM.  25 

Ebstein,*  who  was  the  first  to  investigate  this  subject,  showed 
qualitatively  the  presence  of  pentose  carbohydrates  in  the  urine  of 
man  after  the  ingestion  of  arabinose  and  xylose  even  in  very  small 
doses,  and  concluded  that  these  sugars  are  not  assimilable. 

Salkowski  f  shortly  afterward  observed  the  appearance  of  pen- 
toses in  the  urine  of  rabbits  given  arabinose  after  five  or  six  days  of 
fasting.  He  found  in  the  urine,  however,  only  about  one-fifth  of 
the  amount  ingested,  together  with  small  amounts  in  the  blood  and 
larger  ones  in  the  muscles,  but  there  was  a  considerable  increase  of 
the  glycogen  of  the  liver.  From  the  latter  fact  Salkowski  con- 
cludes that  arabinose  may  be,  either  directly  or  indirectly,  a  source 
of  glycogen.  The  glycogen  found  in  his  experiment  was  the  ordi- 
nary six-carbon  glycogen. 

Subsequent  investigations  by  Cremer,  J  Munk,§  FrentzelJ  Linde- 
mann  &  May,f  Fr.  Voit,**  Jacksch,ff  Munch,!!  Salkowski,§§ 
and  others  have  been  directed  largely  to  two  questions,  viz., 
whether  the  pentose  carbohydrates  are  oxidized  in  the  body  and 
whether  they  serve  as  a  source  of  glycogen. 

Pentoses  Oxidized  in  the  Body. — As  the  general  result  of 
these  investigations,  it  may  be  stated  that  pentoses  (in  particular 
arabinose  and  xylose),  whether  administered  by  the  stomach  or 
injected  into  the  blood,  are  at  least  partially  oxidized  in  the  body. 
In  the  human  organism  the  power  of  oxidizing  the  pentoses,  which 
do  not  normally  constitute  any  considerable  portion  of  its  food, 
appears  to  be  quite  limited,  and  even  when  they  are  given  in  small 
quantities  a  portion  (not  all)  is  excreted  in  the  urine.  In  the  rabbit 
the  pentoses  seem  to  be  more  vigorously  oxidized,  only  about 
twenty  per  cent,  being  excreted  unaltered,  even  when  compara- 
tively large  doses  are  given. 

In  these  experiments  the  pentose  sugars  were  administered  in 
considerable  amounts  at  once,  and  the  excretion  of  a  portion  unal- 
tered would  seem  to  be  a  phenomenon  similar  to  the  temporary 


*Virchow's  Archiv,  129,  401;  132,  368.  If  Arch.  klin.  Med.,  56,  283. 

fCentralbl.  med.  Wiss.,  1893,  p.  193.  **Ibid.,  58,  524. 

JZeit.  f.  Biol.,  29,  536;  42,  428.  ft  Zeit.  f.  Heilk.,  20,  195. 

§Centralbl.  med.  Wiss.,  1894,  p.  83.  jjzeit.  physiol.  Chem.,  29,  493. 

||  Arch.  ges.  Physiol.,  56 ,  273.  %%Ibid.,  32,  393. 


26  PRINCIPLES  OF  ANIMAL   NUTRITION. 

glycosuria  caused  by  large  doses  of  the  common  sugars.  The  pen- 
tose carbohydrates  in  the  ordinary  food  of  herbivora,  however,  are 
largely  or  entirely  the  comparatively  insoluble  pentosans.  As 
already  stated,  these  bodies  are  partially  digested — that  is,  they  do 
not  reappear  in  the  feces.  As  to  the  manner  of  their  digestion  we 
are  ignorant.  If  we  are  justified  in  assuming  that  the  digested 
portion  is  converted,  wholly  or  partially,  into  pentoses,  then  the 
conditions  differ  from  those  of  the  experiments  above  mentioned 
in  that  the  production  and  assimilation  of  the  pentoses  is  gradual. 
Under  these  circumstances  we  might  be  justified  in  anticipating 
a  more  complete  oxidation  of  these  bodies.  To  what  extent  this 
is  true  it  is  at  present  impossible  to  say.  Weiske,*  in  connection 
with  his  investigations  upon  the  digestibility  of  the  pentosans,  states 
that  the  urine  of  the  sheep  and  rabbits  experimented  upon  gave 
only  a  slight  reaction  for  pentoses.  The  writer  has  not  been  able 
to  find  any  records  of  other  tests  of  the  urine  of  domestic  animals 
for  pentoses. 

Pentoses  as  a  Source  of  Glycogen. — Most,  although  not 
all,  investigators  have  found  an  increase  in  the  glycogen  of  the  liver 
consequent  upon  the  ingestion  of  pentoses,  but  in  every  case  it  has 
been  the  ordinary  six-carbon  glycogen.  This  has  been  commonly 
and  most  naturally  interpreted  as  showing  that  the  pentoses  are  not 
themselves  converted  into  glycogen  in  the  body,  but  are  simply 
oxidized  in  the  place  of  some  other  material  which  is  the  true  source 
of  the  observed  gain  of  glycogen.  In  the  light  of  known  facts 
regarding  the  apparent  power  of  the  liver  to  produce  glycogen  from 
very  diverse  hexoses  (see  p.  20)  it  would  seem,  however,  that  the 
possibility  of  an  actual  assimilation  of  the  pentoses  by  the  hepatic 
cells  should  at  least  be  borne  in  mind. 

THE   ORGANIC    ACIDS. 

In  addition  to  such  quantities  of  the  organic  acids,  free  and  com- 
bined, as  are  contained  in  their  food,  relatively  large  amounts  of 
these  substances  are,  in  the  case  of  herbivorous  animals  and  par- 
ticularly of  ruminants,  produced  by  the  fermentation  of  the  carbo- 
hydrates in  the   alimentary  canal.      For  this  reason  their  meta- 

*Zeit.  physiol.  Chem.,  20,  489. 


METABOLISM.  27 

bolism  may  properly  be  considered  in  connection  with  that  of  the 
carbohydrates  themselves. 

But  little  is  known  of  the  metabolism  of  the  organic  acids,  how- 
ever, beyond  the  fact  that  they  are  oxidized  in  the  body,  a  portion 
of  the  resulting  carbon  dioxide  appearing  in  the  urine,  in  combina- 
tion with  sodium  and  potassium,  rendering  that  fluid  alkaline. 
Wilsing  *  and  v.  Knieriem  f  have  shown  that  organic  acids  such  as 
result  from  the  fermentation  of  carbohydrates  are  not  found  to  any 
appreciable  extent  in  the  excreta,  while  the  researches  of  Munk  J 
and  Mallevre,§  which  will  be  considered  more  particularly  in 
another  connection,  have  shown  that  the  sodium  salts  of  butyric 
and  acetic  acids  when  injected  into  the  blood  are  promptly  oxi- 
dized, and  Nencki  &  Sieber  ||  have  shown  that  lactic  acid  is 
readily  oxidized,  even  by  a  diabetic  patient. 

NON-NITROGENOUS    MATTER    OF   THE    URINE. 

It  has  been  implied  in  the  foregoing  pages  that  the  digested 
carbohydrates  of  the  food,  whatever  the  intermediate  stages  through 
which  they  may  pass,  are  ultimately  oxidized  to  carbon  dioxide  and 
water.  Of  the  ordinary  hexose  carbohydrates  this  is  doubtless 
true,  but  with  some  of  the  large  variety  of  substances  ordinarily 
grouped  together,  by  the  conventional  scheme  of  feeding-stuffs  analy- 
sis, as  "carbohydrates  and  related  bodies,"  or  as  "crude  fiber" 
and  "nitrogen-free  extract,"  the  case  appears  to  be  otherwise. 

It  has  been  shown  that  the  urine,  in  addition  to  the  nitrogenous 
products  of  proteid  metabolism  which  will  be  considered  in  a 
subsequent  section,  contains  also  non-nitrogenous  materials,  pre- 
sumably metabolic  in  their  nature.  In  the  urine  of  man  and  of  the 
carnivora  these  non-nitrogenous  substances  are  chiefly  or  wholly 
such  as  might  be  derived  from  the  metabolism  of  proteids  (phenols 
and  other  compounds  of  the  aromatic  series),  and  their  amount  is 
comparatively  small.  In  the  urine  of  herbivora,  particularly  of 
ruminants,  however,  their  quantity  is  relatively  very  considerable, 
and  it  seems  impossible  to  regard  any  large  portion  of  them  as 
derived  from  the  proteid  metabolism. 

*Zeit,  f.  Biol.,  81,  625.  J  Arch.  ges.  Physiol.,  46,  322. 

tlbid.,  21,  139.  %Ibid.,  49,  460.' 

||  Jour.  pr.  Chem...  N.  F.,  26,  32. 


28  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Henneberg  *  found  that  from  26.7  to  30.0  per  cent,  of  the  organic 
matter  of  sheep  urine  was  neither  urea  nor  hippuric  acid,  while  from 
95  to  100  per  cent,  of  the  total  nitrogen  was  contained  in  these  two 
substances.  G.  Kiihn  in  his  extensive  respiration  experiments  on 
oxen,  as  reported  by  Kellner,f  assuming  that  all  the  nitrogen  of 
the  urine  was  in  the  form  either  of  hippuric  acid  or  urea,  found  that 
from  40.05  to  67.64  per  cent,  of  the  total  carbon  of  the  urine  was 
present  in  non-nitrogenous  substances.  The  more  recent  investi- 
gations of  Kellner,J  as  well  as  those  of  Jordan  §  and  of  the  writer,  || 
have  fully  confirmed  this  fact. 

Apparently  these  non-nitrogenous  organic  substances  are  de- 
rived in  some  way  largely  from  the  coarse  fodders.  Their  propor- 
tion in  the  urine  is  relatively  large  when  the  ration  consists  exclu- 
sively of  coarse  fodder,  and  the  addition  of  such  fodders  to  a  basal 
ration  causes  a  marked  increase  in  their  amount,  while,  on  the 
other  hand,  such  concentrated  feeding-stuffs  as  have  been  inves- 
tigated do  not  produce  this  effect  in  any  very  marked  degree. 
Furthermore,  their  amount  seems  to  bear  no  fixed  relation  to  the 
protein  of  the  coarse  fodder.  When  the  amount  of  the  latter 
ingredient  is  small,  the  total  organic  matter  of  the  urine  has  in 
some  cases  exceeded  the  maximum  amount  that  could  have  been 
derived  from  the  protein  of  the  food,  thus  demonstrating  that  a 
portion  at  least  of  the  non-nitrogenous  urinary  constituents  must 
have  had  some  other  source.  As  the  proportion  of  protein  in  the 
food  increases,  the  amount  of  nitrogenous  products  in  the  urine 
likewise  increases,  while  that  of  the  non-nitrogenous  products 
appears  to  be  more  constant,  so  that  the  ratio  of  urinary  nitrogen 
to  carbon  increases.  The  most  plausible  explanation  of  these  facts 
seems  to  be  that  the  substances  in  question  are  derived  from  some  of 
the  non-nitrogenous  ingredients  of  the  coarse  fodders,  but  from  what 
ones,  or  what  is  the  nature  of  the  products,  we  are  still  ignorant. T 

*Neue  BeitrSge,  etc.,  p.  119. 
t  Landw.  Vers.  Stat.,  44,  34S,  404,  474,  529. 
\lbicl,  47,  275;  50,  245;  53,  1. 
§  New  York  State  Expt.  Station,  Bull.  197,  p.  27. 
||  Penna.  Expt.  Station,  Bull.  42,  p.  150. 

T[A  further  discussion  of  this  subject  in  its  relations  to  the  energy  of 
the  food  will  be  found  in  Part  II. 


METABOLISM.  29 


§2.  Fat  Metabolism. 

Scarcely  a  tissue  or  portion  of  the  animal  body  can  be  named 
in  which  more  or  less  fat  is  not  found.  The  muscular  fibers,  the 
epithelium,  the  nerves  and  ganglia,  etc.,  all  contain  cells  in  which 
globules  of  fat  may  be  recognized,  so  that  the  capacity  to  produce 
or  store  up  fat  seems  to  be  common  to  almost  all  the  cells  of  the 
body.  It  is  particularly  in  certain  cells  of  the  connective  tissue, 
however,  that  the  large  accumulations  of  visible  fat  in  the  body 
take  place.  At  the  outset  these  cells  present  no  special  characters, 
but  in  a  well-nourished  animal  globules  of  fat  begin  to  accumulate 
in  them,  the  cells  enlarge,  the  globules  of  fat  coalesce  into  larger 
ones,  and  finally  the  cell  substance  is  reduced  to  a  mere  envelope, 
the  nucleus  being  pushed  to  one  side  and  almost  the  whole  volume 
of  the  cell  occupied  by  fat.  Masses  of  connective  tissue  thus  loaded 
with  fat  constitute  what  is  called  adipose  tissue.  Large  deposits 
of  adipose  tissue  are  met  with  surrounding  various  organs,  particu- 
larly the  kidneys,  but  the  largest  deposit  of  fat  is  usually  in  the 
connective  tissue  underlying  the  skin.  In  milk  production,  too, 
large  amounts  of  .fat  appear  in  the  epithelial  cells  of  the  milk  glands. 

Fat  Manufactured  in  the  Body. — The  older  physiologists  held 
that  all  the  ingredients  of  the  body  pre-existed  in  the  food.  Specifi- 
cally, animal  fat  was  regarded  as  simply  vegetable  fat  which  had 
escaped  oxidation  in  the  body  and  been  deposited  in  the  tissues. 
But  while  there  is  no  doubt  that  the  fat  of  the  food  can  contribute 
to  the  fat  supply  of  the  body,  the  food  of  herbivorous  animals 
usually  contains  a  relatively  small  quantity  of  fat  and  the  amount 
produced  by  a  rapidly  fattening  animal  or  by  a  good  dairy  cow  is 
usually  much  greater  than  that  consumed  in  the  food. 

Deferring  to  subsequent  pages  a  discussion  of  the  sources  of 
animal  fat,*  we  may  content  ourselves  here  with  anticipating  the 
general  results  of  the  great  amount  of  experimental  inquiry  which 
has  been  expended  upon  this  question.  These  results  may  be 
briefly  summarized  in  the  following  statements : 

*  For  a  very  complete  review  of  the  literature  of  fat  production  up  to  1894, 
see  Soskin,  Journ.  f.  Landw.,  42,  157. 


30  PRINCIPLES   OF  ANIMAL   NUTRITION. 

1.  The  animal  body  produces  fat  from  other  ingredients  of  its 

food. 

2.  The  carbohydrates  and  related  bodies  of  the  food  serve  as 

sources  of  fat. 

3.  It  is  probable  that  the  proteids  also  serve  as  sources  of  fat. 

So  far,  then,  as  that  portion  of  the  fat  which  is  actually  pro- 
duced in  the  body  from  other  substances  is  concerned,  we  may  most 
readily  conceive  of  its  formation  as  consisting  essentially  of  a 
manufacture  of  fat  by  the  protoplasm  of  the  fat  cells,  which  are 
nourished  by  the  carbohydrates,  proteids,  and  other  materials 
brought  to  them  by  the  circulation. 

Functions  of  the  Food  Fat. — The  fat  which  is  manufactured 
in  the  body  from  other  ingredients  of  the  food,  however,  often  con- 
stitutes the  larger  portion  of  the  total  fat  production,  while  but 
a  relatively  small  proportion  at  most  can  be  derived  from  the  fat 
of  the  food.  The  question  naturally  arises  whether  this  smaller 
portion  contained  in  the  food  is  simply  deposited  mechanically,  so 
to  speak,  in  the  fat  cells,  or  whether  it  too,  like  the  carbohydrates 
and  proteids,  serves  to  nourish  the  fat  cells  and  supply  raw  material 
out  of  which  they  may  manufacture  fat. 

At  first  thought  the  former  alternative  might  seem  more  prob- 
able. The  fat  of  the  food,  so  far  as  we  are  able  to'  trace  it,  does  not 
undergo  any  considerable  chemical  changes,  such  as  the  proteids 
do,  e.g.,  in  the  process  of  digestion,  but  is  largely  resorbed  in  the 
form  of  apparently  unaltered  fat.  Moreover,  resorption  of  fat  takes 
place  largely  through  the  lacteals  and  the  resorbed  fat  reaches  the 
general  circulation  without  being  subjected  like  the  carbohydrates 
to  the  action  of  the  liver. 

Deposition  of  Foreign  Fats. — The  view  just  indicated  is 
supported  to  a  considerable  extent  by  the  results  of  experiments 
upon  the  fate  of  foreign  fats  introduced  into  the  body. 

Experiments  by  Radziejewsky  *  and  Subbotin  f  were  indecisive, 
but  Lebedeff  |  was  later  successful  in  obtaining  positive  re- 
sults. Two  dogs,  after  prolonged  fasting,  received  small  amounts 
of  almost  fat-free  meat  together  with,  in  the  one  case,  linseed  oil, 

*Virchow's  Archiv.,  56,  211 ;  43.  268.  fZeit.  f.  Biol...  6,  73. 

JThier.  Chem.  Ber..  12,425;  Zeit.  Physiol.  Chem..  6.  149:  Centralbl.  med. 
Wiss..  1882..  129. 


METABOLISM.  31 

and  in  the  other,  mutton  tallow.  After  three  weeks,  during  which 
the  animals  recovered  their  original  weights,  the  adipose  tissue  was 
found  to  contain,  in  the  one  case,  fat  fluid  at  0°  C.  and  agreeing  very 
closely  with  linseed  oil  in  its  chemical  behavior,  while  in  the  other 
case  the  fat  had  a  melting-point  of  over  50°  C,  and  was  almost 
identical  with  mutton  fat.  On  the  other  hand,  the  same  author 
in  experiments  with  tributyrine  failed  to  obtain  any  noteworthy 
deposition  of  this  substance. 

Munk  *  fed  large  amounts  of  rape  oil  to  a  previously  fasted  dog 
for  seventeen  days  and  found  in  the  body  considerable  amounts 
of  fat  differing  markedly  in  appearance  and  properties  and  in  the 
proportion  of  olein  to  solid  fats  from  normal  dog  fat.  He  likewise 
succeeded  in  isolating  from  the  fat  eruic  acid,  the  characteristic 
ingredient  of  rape  oil.  In  a  second  experiment  f  the  fatty  acids 
prepared  from  mutton  tallow  were  fed  with  similar  results,  the 
proportion  of  stearin  and  palmitin  to  olein  being  approximately 
reversed  as  compared  with  normal  dog  fat.  The  latter  experiment 
is  also  of  interest  as  showing  that  the  fatty  acids  may  be  synthesized 
to  fat  in  the  body,  the  change  taking  place,  according  to  Munk,  in 
the  process  of  resorption. 

More  recently  Winternitz  \  has  experimented  with  the  iodine 
addition  products  of  fats.  He  observed  the  retention  of  a  con- 
siderable proportion  of  iodine  in  the  body  (of  hens  and  dogs)  in 
organic  form  and  also  found  iodine  in  the  fat  of  the  body  at  the  close 
of  the  experiment.  Similar  experiments  on  a  milking  goat  §  showed 
that  at  least  6  per  cent,  of  the  fat  fed  passed  into  the  milk. 

Henri ques  and  Hansen  ||  fed  two  three-months-old  pigs  for  about 
nine  months  with  ground  barley,  to  which  was  added,  in  one  case 
linseed  oil  and  in  the  other  cocoanut  oil,  while  in  the  succeeding 
three  months  the  rations  were  exchanged.  Samples  of  the  sub- 
cutaneous fat  of  the  back  were  taken  (with  the  aid  of  cocaine)  at 
four  different  times  and  the  fat  of  the  carcasses  at  the  close  of  the 
experiment  was  also  examined.  The  results  showed  an  abundant 
deposition  of  the  linseed  oil  (and  cocoanut  oil?).     On  the  other 

*Thier.  Chem.  Ber.,  14,  411;  Virchow's  Archiv.,  95,  407. 
f  Archiv.  f.  (Anat.  u.)  Physiol.,  1883,  p.  273. 
%  Zeit.  physiol.  Chem  .  24,  425. 
§  Thier.  Chem.  Ber.,  27,  293. 
II  Ibid.,  29. 68. 


32  PRINCIPLES   OF  ANIMAL   NUTRITION. 

hand,  experiments  with  cows  failed  to  show  any  passage  of  linseed 
oil  as  such  into  the  milk. 

Leube  *  made  subcutaneous  injections  of  melted  butter  on  two 
dogs  and  found  an  abundant  deposit  of  butter  fat  especially  under 
the  skin  of  the  abdomen,  the  Reichert-Meissl  number  of  the  fat 
being  20.46  in  the  first  case  and  15.3  in  the  second.  Rosenfelt  f 
fed  fasted  dogs  with  mutton  fat  and  observed  a  large  deposit 
of  this  fat  in  all  parts  of  the  body. 

Influence  of  Feeding  on  Composition  of  Fat. — In  addition 
to  the  more  purely  physiological  experiments  just  cited,  there  are 
on  record  a  not  inconsiderable  number  of  feeding  experiments, 
especially  upon  swine,  in  which  the  feeding  appears  to  have 
sensibly  influenced  the  appearance,  firmness,  melting-point,  or 
composition  of  the  body  fat. 

While  it  is  not  impossible,  however,  that  in  some  cases  the 
peculiar  fats  of  the  food  (e.g.,  the  fat  of  maize  or  of  the  oil-meals) 
may  have  been  deposited  in  the  adipose  tissue  unchanged,  it  must 
be  borne  in  mind  that  these  experiments  were  made  on  mixed  rations 
and  that  undoubtedly  there  was  a  considerable  production  of  fat  in 
the  body  from  other  ingredients  of  the  food.  This  being  the  case, 
we  are  left  in  doubt  as  to  whether  the  effect  observed  is  due  direct ly 
to  the  fat  of  the  food  or  is  to  be  explained  as  an  effect  of  the  food  as 
a  whole,  or  of  some  unknown  ingredients  of  it,  in  modifying  the 
nature  of  the  metabolism  in  the  fat  cells.  That  such  an  explana- 
tion is  at  least  possible  would  seem  to  be  indicated  by  the  well- 
established  fact  that  marked  changes  of  food  do  modify  the 
metabolism  in  the  milk  gland  sufficiently  to  materially  affect  the 
proportion  of  volatile  fatty  acids  in  butter  fat. 

A  striking  example  of  the  possibility  of  such  an  effect  upon  the 
metabolism  of  the  fat  cells  is  afforded  by  the  recent  investigations  of 
Shutt  \  into  the  causes  of  "soft"  pork.  On  the  average  of  a  con- 
siderable number  of  animals,  he  finds  that  the  shoulder  and  loin  fat 
of  pigs  fed  exclusively  on  maize  shows  a  very  low  melting-point 
and  a  high  iodine  absorption  number,  indicating  a  large  percentage 
of  olein,  and  inclines  to  attribute  this  effect  to  the  oil  of  the  maize. 
When,  however,  he  fed  skim  milk  with  the  maize,  he  obtained  pork 

*  Thier.  Chem.  Ber.,  25,  45.  t  I^d.,  25,  44. 

X  Canada:  Dominion  Experiment  Station,  Bull.  38. 


METABOLISM.  33 

of  good  quality,  the  fat  having  a  melting-point  and  iodine  number 
not  widely  different  from  those  obtained  with  the  most  approved 
rations.  While  it  is  possible  that  part  of  this  effect  was  due  to  a 
reduced  consumption  of  maize  oil,  so  that  more  fat  was  produced 
from  the  other  ingredients  of  the  food,  the  conclusion  seems  justified 
that  the  principal  factor  was  the  influence  of  the  skim  milk  upon 
the  nutrition  of  the  fat  cells.  This  influence  may  with  some  degree 
of  probability  be  ascribed  to  its  protein,  and  it  is  worthy  of  notice 
that  in  Shutt's  experiments  the  rations  which  produced  the  highest 
grade  of  pork  were  composed  of  materials  rich  in  protein. 

Another  fact  warns  us  to  be  cautious  in  our  interpretation  of 
the  results  of  this  class  of  feeding  experiments.  Such  experiments 
in  most  cases  involve  a  comparison  of  the  composition  of  the  fat 
from  animals  differently  fed.  Albert  *  has  found  that  both  with 
swine  and  sheep  the  composition  of  the  body  fat  is  subject  to  very 
considerable  individual  variations  as  to  melting-point,  refractive 
index,  and  iodine  number,  the  differences  being,  in  his  experiments, 
greater  than  the  average  differences  which  could  be  ascribed  to  the 
feeding. 

Moreover,  the  fat  of  the  same  individual  has  not  the  same  com- 
position in  different  parts  of  the  body.  This  point  has  recently 
been  the  subject  of  an  elaborate  investigation  by  Henriques  & 
Hansen, f  whose  results  show  a  higher  melting-point  and  a  lower 
iodine  number  in  the  inner  as  compared  with  the  outer  layers  of 
fat.  This  difference  they  ascribe  to  the  difference  in  the  tempera- 
ture of  the  tissues  and  support  this  view  by  an  experiment  with 
three  pigs.  One  animal  was  kept  in  a  stall  heated  to  about  30°  C. 
for  two  months,  while  the  others  were  exposed  to  a  temperature  of 
0°  C,  one  unprotected  and  the  other  partially  enveloped  in  a  sheep- 
pelt.  At  the  close  of  the  experiment  the  fat  immediately  under 
the  skin  gave  the  following  figures: 

Iodine  Solidifying 

Number.  Point. 

Kept  at  30°-35°  C 69.4  24.6°  C. 

Kept  at  0°,  in  sheep  pelt : 

Part  under  the  pelt 67.0  25.4°  C. 

Part  exposed 69.4  24.1°  C. 

Kept  at  0°,  unprotected 72.3  23.3°  C. 

*  Landw.  Jahrb.,  28,  961,  986. 

t  Bied.  Centr.  Blatt.  Ag.  Ch.,  30,  182. 


54 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Towards  the  interior  of  the  body  the  differences  became  grad- 
ually less. 

It  is  evident,  then,  that  the  sources  of  possible  error  in  ex- 
periments upon  the  influence  of  food  on  the  composition  of  body 
fat  are  considerable,  and  that  not  only  is  great  care  necessary  to 
secure  representative  samples  of  fat  for  examination,  but  the  effect 
of  individuality  must  be  eliminated  so  far  as  possible  by  the  use 
of  a  considerable  number  of  animals.  When  we  add  to  this  the 
other  fact  that  the  fat  production  of  herbivorous  animals  is 
largely  at  the  expense  of  other  nutrients  than  fat,  we  shall  hardly 
incline  to  give  the  results  of  such  investigations  much  weight  as 
regards  the  question  of  the  functions  of  food  fat. 

Quantitative  Relations. — Some  further  light  upon  the  point 
under  discussion  may  perhaps  be  obtained  from  a  consideration  of 
the  quantitative  relations  of  food  fat  to  fat  production  shown  by 
respiration  experiments  and  which  will  be  considered  more  fully 
on  subsequent  pages  (compare  Chapter  V).  In  scarcely  any  of  these 
experiments  has  the  food  fat  been  deposited  quantitatively  in 
the  tissue.  In  three  out  of  five  experiments  by  Rubner  in  which 
fat  was  given  to  a  previously  fasting  animal,  from  65.82  to  91.89 
per  cent,  of  the  fat  supplied  in  excess  of  the  amount  metabolized 
during  fasting  was  stored  up  in  the  body.  Similarly,  in  the  ex- 
periments of  Pettenkofer  &  Voit,  in  which  the  fat  was  added  to  a 
ration  already  more  than  sufficient  for  maintenance,  on  the  average 
87.86  per  cent,  of  the  fat  of  the  food  was  deposited  in  the  tissues. 

Kellner,*  among  his  extensive  respiration  experiments  upon 
cattle,  reports  the  results  of  three  in  which  peanut  oil  was  added  to  a 
basal  ration  more  than  sufficient  for  maintenance.  The  amounts 
of  fat  consumed  in  excess  of  the  basal  ration  and  the  resulting  gains 
by  the  animals  were  as  follows,  the  slight  variations  in  the  amounts 
of  the  other  nutrients  being  neglected: 


Additional  Fat 

Digested, 

Grams. 

Gain  by  Animal. 

Gain  of  Fat 

Animal. 

Protein, 
Grams. 

Fat, 
Grams. 

Digested. 

D 

F 
G 

677 
542 

458 

8 
86 
44 

239 
205 

279 

35.30 
37.83 
60.91 

Landw.  Vers.  Stat..  53.  112,  124.  199.  214. 


METABOLISM.  35 

Computations  of  the  proportion  of  the  energy  of  the  added  fat 
which  was  recovered  in  the  total  gain  of  flesh  and  fat  (compare 
Chapter  XIII,  §  1)  showed,  according  to  the  method  of  computa- 
tion employed,  a  loss  of  from  31  to  48  per  cent. 

The  comparatively  small  losses  observed  in  Rubner's  and  in 
Pettenkofer  &  Voit's  experiments  may  well  be  ascribed  to  a  con- 
sumption of  energy  in  the  work  of  digestion  (compare  Chapter 
XI),  but  it  hardly  seems  possible  to  account  in  this  way  for 
the  large  losses  observed  by  Kellner.  Apparently  the  peanut 
oil  in  these  experiments,  after  its  digestion  and  resorption,  must 
have  been  subjected  to  extensive  molecular  changes  involving  a 
considerable  expenditure  of  potential  energy,  and  if  this  be  true, 
the  suggestion  of  an  assimilation  by  the  fat  cells  and  a  construction 
of  animal  fat  from  the  oil  is  obvious. 

Constancy  of  Composition  of  Fats. — The  relatively  constant 
and  characteristic  composition  of  the  fat  of  the  same  species  of 
animal,  notwithstanding  differences  in  the  food,  has  been  urged  in 
favor  of  the  view  that  the  fat  of  the  animal  is  a  product  of  the 
protoplasmic  activity  of  the  fat  cells.  "The  fat  of  a  man  differs 
from  the  fat  of  a  dog,  even  if  both  feed  on  the  same  food,  fatty  or 
otherwise"  (M.  Foster).  The  steer  produces  beef  fat  and  the  sheep 
mutton  fat  on  identical  rations.  Unless,  however,  we  are  prepared 
to  discredit  the  experimental  results  above  cited,  it  would  appear 
that  this  general  and  approximate  uniformity  of  composition  is 
largely  due  to  a  general  uniformity  of  food,  and  that  marked  changes 
in  the  nature  of  the  latter  may  result  in  altering  the  former.  To  this 
must  be  added,  as  already  insisted  upon,  the  fact  that  much  of  the 
fat  found  in  the  body,  especially  in  the  herbivora,  is  undoubtedly 
produced  in  the  organism.  We  may  fairly  presume  that  this  fat 
will  be  the  characteristic  fat  of  the  species.  If  we  may  suppose 
further  that  a  considerable  share  of  the  food  fat  is  oxidized  directly, 
and  if  we  take  into  consideration  the  general  uniformity  of  diet  of 
our  domestic  animals  and  the  relativety  small  total  amount  of  fat 
which  it  often  contains,  we  have  at  least  a  plausible  explanation  of 
the  observed  facts  and  one  which  does  not  preclude  a  direct  deposi- 
tion of  food  fat  in  the  body  and  a  consequent  effect  upon  the  com- 
position of  the  body  fat. 

The  Katabolism  of  Fat. — The  proportion  of  the  food  fat  which 


36  PRINCIPLES  OF  ANIMAL   NUTRITION. 

serves  to  increase  the  store  of  fat  in  the  body  depends  largely  upon 
the  total  food-supply.  When  the  latter  is  more  than  sufficient  to 
balance  the  total  metabolism  of  the  organism,  the  excess  may  give 
rise  to  a  storage  of  fat,  and  under  these  circumstances  the  food  fat 
or  a  part  of  it  may,  as  we  have  seen,  contribute  to  the  increase 
of  adipose  tissue.  On  the  other  hand,  when  the  food-supply  is  in- 
sufficient, not  only  is  its  fat  in  common  with  its  other  ingredients 
in  effect  consumed  to  support  the  vital  processes,  but  the  fat  pre- 
viously stored  in  the  adipose  tissue  is  drawn  upon  to  make  up  the 
deficiency.  Under  these  circumstances  the  fat  disappears  more  or 
less  rapidly  from  the  fat  cells,  passing  away  gradually  either  into 
the  lymphatics  or  the  blood-vessels  in  some  manner  not  as  yet  fully 
understood. 

Fat,  then,  whether  derived  immediately  from  the  food  or  drawn 
in  the  first  instance  from  the  adipose  tissue  of  the  body,  passes  into 
the  circulation  and  serves  to  supply  the  demands  of  the  body 
for  oxidizable  material  and  energy,  the  final  products  of  its  oxida- 
tion being  carbon  dioxide  and  water.  Of  the  intermediate  steps 
in  this  katabolic  process  we  are  comparatively  ignorant,  but  one 
hypothesis  regarding  it  has  acquired  so  much  importance  in  its 
bearings  on  the  availability  of  the  potential  energy  of  the  food  as  to 
require  mention  here. 

Formation  of  Dextrose  from  Fat. — This  hypothesis  is,  in 
brief,  that  the  first  step  in  the  katabolism  of  fat  takes  place  in  the 
liver  and  consists  in  its  conversion  into  sugar.  In  other  words,  it  is 
held  that  the  fat  of  the  food  or  that  drawn  from  the  adipose  tissue 
of  the  body  supplies  the  liver  with  part  of  the  material  for  its  func- 
tion of  sugar  production  described  in  the  previous  section. 

This  hypothesis  is  advocated  especially  by  those  physiologists 
who,  like  Seegen  in  Vienna  and  Chauveau  and  his  associates  in  Paris, 
look  upon  the  carbohydrates,  and  particularly  dextrose,  as  the  im- 
mediate source  of  the  energy  exerted  in  muscular  contraction  or 
in  the  various  other  forms  of  physiological  work.  The  evidence 
upon  which  this  view  is  based  will  be  considered  in  subsequent 
chapters.  For  the  present  it  suffices  to  point  out  that,  if  we  admit 
its  truth,  then  the  general  metabolism  of  the  body  is  essentially  a 
carbohydrate  metabolism.  Whether  we  consider  the  case  of  a 
fasting  animal,  living  upon  its  store  of  protein  and  fat,  or  that  of  an 


METABOLISM.  37 

animal  receiving  food,  the  liver  breaks  down  the  proteids  and  fat 
supplied  to  the  blood  either  by  the  food  or  from  the  tissues,  pro- 
ducing dextrose.  This  dextrose,  like  that  derived  from  the  carbo- 
hydrates of  the  food,  is  then,  as  indicated  in  the  previous  section, 
oxidized  in  the  tissues  either  directly  or  with  previous  conversion 
into  glycogen. 

As  regards  the  katabolism  of  fat,  in  particular,  Nasse  *  has 
brought  forward  reasons  for  believing  that  the  liver  is  concerned  in 
it.  Seegen  |  submitted  fat  to  the  action  of  finely  chopped,  freshly 
excised  liver  suspended  in  defibrinated  blood  at  a  temperature  of 
35-40°  C,  in  a  current  of  air  and  observed  a  considerable  formation 
of  sugar  in  five  to  six  hours  as  compared  with  a  control  experiment 
without  the  fat.  He  likewise  found  \  in  experiments  upon  dogs 
fed  on  fat  with  little  or  no  meat  that  the  blood  of  the  hepatic  vein 
was  much  richer  in  sugar  than  that  of  the  portal  vein.  On  the  basis 
of  the  probable  amount  of  blood  circulating  through  the  liver,  he 
computes  that  the  total  amount  of  sugar  thus  produced  was  much 
greater  than  could  have  been  supplied  by  the  glycogen  stored  in 
the  liver  and  the  amount  of  proteids  metabolized  (as  measured 
by  the  urinary  nitrogen),  and  hence  concludes  that  at  least  the 
difference  was  produced  from  fat.  As  was  pointed  out  in  the 
preceding  section,  however,  many  physiologists  regard  the  large 
differences  between  the  dextrose  content  of  the  portal  and  the 
hepatic  blood  observed  by  Seegen  as  being  in  large  part  the  result  of 
the  necessary  operation  and  thus  abnormal,  and  the  production  of 
glycogen  or  dextrose  from  fat  is  not  regarded  as  proven  by  the 
majority  of  physiologists. §  Thus  Girard  ||  and  Panormow  1"  found 
the  post-mortem  formation  of  sugar  in  the  liver  to  be  strictly  pro- 
portional to  the  disappearance  of  glycogen,  and  similar  results 
were  obtained  by  Cavazzani  and  Butte.** 

Kaufmann,ft  who  has  developed  this  hypothesis  in  considerable 

*  v.  Noorden,  Pathologie  des  Stoffwechsels,  p.  85. 

f  Die  Zuckerbildung  im  Thierkorper,  p.  151. 

%  Ibid.,  p.  171. 

§  Cf.  Neumeister,  Physiologische  Chemie,  p.  368. 

H  Arch.  ges.  Physiol.,  41,  294. 

IT  Thier.  Chem.  Ber.,  17.  304. 

**  Ibid.,  24,  391  and  394. 

ft  Archives  de  Physiol.,  1896,  p.  331. 


3§  PRINCIPLES   OF  ANIMAL   NUTRITION. 

detail,  represents  the  two  supposed  stages  in  the  katabolism  of  fat 
by  the  two  following  equations,  proposed  by  Chauveau :  * 

First  Stage  :      2(C57H110O6)  +  6702  =  16(C6H1206)  +  18C02+ 14H20. 

Second  Stage:  16(C6H1206)  +9602  =  96C02  +  96H20. 

Even,  however,  if  we  admit  the  formation  of  dextrose  from  fat 
in  the  body,  it  may  fairly  be  doubted  whether  the  process  is  as 
simple  as  these  equations,  even  if  regarded  as  simply  schematic, 
would  imply. 

§  3.  Proteid  Metabolism. 

ANABOLISM. 

Digestive  Cleavage. — The  digestion  of  the  proteids  is  essen- 
tially a  process  of  cleavage  and  hydration  under  the  influence  of 
certain  enzyms.  By  this  process  the  complex  proteid  molecules 
are  partially  broken  up  into  simpler  ones.  By  the  action  of  pepsin 
in  acid  solution  we  obtain  albumoses  and  peptones,  while  the 
trypsin  of  the  pancreatic  juice,  at  least  outside  the  body,  carries 
the  cleavage  still  further,  producing  crystalline  nitrogenous  bodies  of 
comparatively  simple  constitution.  Opinions  are  still  more  or  less 
divided  as  to  how  far  these  processes  of  cleavage  and  hydration  are 
carried  in  the  actual  process  of  digestion,  where  the  products  of  the 
action  are  constantly  being  resorbed,  but  there  are  not  wanting  in- 
dications that  it  is  both  less  extensive  and  less  rapid  than  in  arti- 
ficial digestion.  It  likewise  seems  to  have  been  demonstrated  that 
some  soluble  proteids  are  capable  of  direct  resorption  without 
change,  while  others  are  not  and  some,  notably  casein,  are  promptly 
coagulated  by  the  rennet  ferment,  apparently  expressly  in  order 
that  they  may  be  subjected  to  the  action  of  the  digestive  ferments. 
In  a  general  way,  the  statement  appears  to  be  justified  that  the 
larger  share  of  the  proteid  material  of  the  food  is  resorbed  as 
albumoses  and  peptones. 

Purpose  of  the  Cleavage. — The  fact  just  mentioned  that, 
on  the  one  hand,  some  soluble  proteids  appear  capable  of  direct  re- 
sorption, while,  on  the  other  hand,  some,  like  casein,  are  at  once 
rendered  insoluble  as  the  first  step  in  digestion,  plainly  necessitates 
a  material  modification  of  the  old  view  that  the  object  of  the  cleav- 
*  La  Vie  et  l'Energie  chez  l'Animale. 


METABOLISM.  39 

age  and  hydration  of  the  proteids  in  digestion  is  to  render  them 
soluble.  Undoubtedly  this  is  an  important  function  of  the  digestive 
fluids,  but  the  fundamental  object  lies  deeper  and  is  found  in  the 
constitution  of  the  proteids  themselves. 

Nature  of  the  Proteids. — While  we  are  still  very  far  removed 
from  any  adequate  knowledge  of  the  molecular  structure  of  the 
proteids,  a  study  of  the  action  upon  them  of  various  hydrolytic 
agents,  and  particularly  of  the  proteolytic  enzyms  of  the  digestive 
fluids,  has  shown  that  they  undergo  cleavage  along  certain  definite 
lines,  giving  rise  to  two  series  of  products  known  as  the  hemi-  and 
the  anti-series.  The  primary  products  are  the  proteoses,  or  albu- 
moses  (hemi  and  anti).  By  further  action  of  the  ferment  these 
give  rise  to  the  secondary  or  deutero-proteoses,  and  these  in  turn 
to  peptones,  while  the  peptones  of  the  hemi-series,  by  the  further 
a,ction  of  trypsin,  are  broken  up,  as  noted,  into  simpler  bodies  such 
as  aspartic  acid,  glutaminic  acid,  and  notably  tyrosin  and  leucin. 
The  two  latter  bodies  belong  to  the  aromatic  series  and  contain  the 
phenyl  radicle,  which  is  thus  shown  to  be  present  in  the  bodies  of 
the  hemi  group,  while  it  is  absent  from  the  anti  group.  Without 
pursuing  the  subject  further,  enough  has  been  said  to  show  that 
the  general  result  of  the  digestive  proteolysis  is  to  break  up  the  pro- 
teid  molecule  into  a  considerable  number  of  unlike  fragments.* 

Differences  in  Proteids. — Turning  now  to  another  phase  of  the 
subject,  it  is  a  familiar  fact  that  the  numerous  proteids  which  have 
been  studied  differ  quite  markedly  from  each  other  in  properties 
and  in  composition.  To  instance  but  a  single  characteristic  differ- 
ence, the  investigations  of  Osborne  and  his  associates  at  the  Connec- 
ticut Agricultural  Experiment  Station  have  shown  in  detail  what 
was  to  a  certain  extent  known  before,  viz.,  that  the  nitrogen  con- 
tent of  the  vegetable  proteids  is  notably  higher  than  that  of  the 
animal  proteids.  We  can  only  interpret  these  differences  in  com- 
position and  properties  as  the  results  of  differences  in  molecular 
structure.  We  may  fairly  suppose  that  these  differences  in  struc- 
ture are  brought  about,  in  part  at  least,  by  differences  in  the  relative 
proportions  in  the  proteid  molecule  of  the  several  molecular  group- 
ings whose  presence  is  indicated  to  us  by  the  results  of  proteolysis. 

*For  a  full  treatment  of  the  subject,  compare  Chittenden,  Digestive  Pro- 
teolysis, 1894. 


40  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Food  Proteids  and  Body  Proteids. — What  is  especially  to  be 
noted  in  this  connection  is  that  the  food  proteids  are  not  identical 
with  the  body  proteids.  This  is  especially  true  of  the  vegetable 
proteids  in  the  food  of  the  herbivora,  and  of  the  casein  of  milk,  but  is 
measurably  true  in  all  cases.  A  simple  resorption  of  unaltered 
protein,  therefore,  would  not  serve  the  purposes  of  the  organism. 
The  food  proteids  must  be  changed  to  body  proteids.  This  means, 
however,  that  the  proportions  of  those  molecular  groupings  which 
have  just  been  spoken  of  must  be  changed — that  is,  the  molecules 
of  the  food  proteid  must  be  so  far  broken  down  into  their  constituent 
molecular  groupings  as  to  permit  of  a  rearrangement  and  repropor- 
tioning  of  the  latter  into  molecules  of  body  proteid. 

Such  a  partial  breaking  down  of  proteid  material  takes  place  in 
digestion,  and  indeed,  as  has  been  indicated  above,  it  is  the  study  of 
digestive  proteolysis  which  has  given  us  our  general  conception  of 
the  structure  of  the  proteid  molecule.  The  products  of  proteid 
digestion,  then,  as  they  are  presented  to  the  resorbent  organs  of  the 
digestive  tract,  are  no  longer  proteids,  but  the  constituent  molecular 
groupings  out  of  which  body  proteids  may  be  built  up. 

Rebuilding  of  Proteids. — But  while  the  proteids  of  the  food  are 
resorbed  in  the  form  of  cleavage  products,  apparently  largely  as  pep- 
tones, no  trace  of  these  bodies  is  found  in  the  blood  or  in  the  lymph, 
nor  even  in  the  walls  of  the  digestive  canal.  Still  further,  peptones 
when  injected  into  the  blood  are  treated  by  the  organism  as  foreign 
substances  and  excreted  as  rapidly  as  possible,  while  if  added  in  any 
considerable  amount  they  act  as  poisons.  The  reconstruction  of 
the  proteid  molecule  from  the  fragments  produced  by  the  digestive 
process  has  been  thought  to  take  place  in  the  epithelial  cells  of  the 
intestines,  the  first  product  being  probably  serum  albumen,  so  that 
we  may  say  that  the  first  step  in  proteid  metabolism  is  anabolic. 

Recently,  however,  Okunew,  working  in  Danilewsky's  labora- 
tory, has  announced  the  discovery  that  the  enzym  of  rennet  (chy- 
mosin)  has  the  power  of  synthesizing  peptones  to  proteids,  and 
Sawjalow  *  has  published  further  studies  on  the  same  subject. 
The  latter  investigator  finds  the  product  to  be  a  gelatinizing  pro- 
teid which  is  identical  whatever  the  original  source  of  the  peptones 
and  which  he  calls  plastein.     He  considers  that  this  plastein  is 

*Arch.  ges.  Physiol.,  85,  171. 


METABOLISM.  41 

formed  in  the  digestive  canal  and  is  the  form  in  which  the  proteids 
of  the  food  are  resorbed,  and  points  out  that  this  hypothesis  accounts 
for  the  hitherto  puzzling  fact  of  the  occurrence  of  the  milk-curdling 
ferment  in  animals,  such  as  birds,  fishes,  and  amphibia,  which  never 
consume  milk.  In  further  support  of  this  view,  Winogradow  * 
finds  the  formation  of  chymosin  in  the  stomach  to  be  most  active 
at  the  height  of  the  .digestive  process,  when  peptones  are  being 
formed  most  freely. 

The  proteid  or  proteids  first  formed  from  the  albumoses  and 
peptones,  whether  in  the  epithelial  cells  or  by  the  action  of 
chymosin,  is  subject  to  still  further  changes  in  other  portions  of  the 
body,  inasmuch  as  all  the  various  nitrogenous  tissues  of  the  body 
are  formed  from  it.  Some  of  these  changes  may  be  slight,  but 
others,  as,  e.g.,  the  formation  of  the  collagens,  must  be  profound, 
while  the  formation  of  the  compound  proteids  like  haemoglobin, 
mucin,  the  nucleins,  etc.,  is  clearly  synthetic  and  anabolic.  As 
to  where  and  how  these  changes  and  syntheses  take  place,  we  are 
largely  ignorant.  We  simply  know  the  general  fact  that  the  food 
proteids  are  first  partially  broken  down  in  the  process  of  digestion 
and  then  that  the  fragments  are  built  up  again  into  body  proteids; 
first,  probably,  into  some  single  form  and  later  into  still  more  com- 
plex bodies  in  the  various  tissues. 


katabolism. 

Final  Products. — The  anabolic  processes  which  have  just  been 
indicated  might  be  characterized  in  general  terms  as  a  preparation 
of  the  food  proteids  for  their  diverse  functions  in  the  body.  In  the 
performance  of  those  functions  they,  like  all  the  organic  ingredients 
of  the  body,  undergo  katabolic  changes,  liberating  the  energy  which 
was  originally  contained  in  them  or  which  may  have  been  tem- 
porarily added  in  the  preliminary  anabolic  changes.  We  have 
every  reason  to  believe  that  the  katabolism  of  proteids  is  a  gradual 
process,  passing  through  many  intermediate  stages,  but  we  have 
very  little  actual  knowledge  of  the  steps  which  intervene  between 
the  proteids  and  bodies  which  are  either  excretory  products 
themselves  or  closely  related  to  them.     Such  information  as  has 

*Arch.  ges.  Physiol.,  87,  170. 


42  PRINCIPLES   OF  ANIMAL    NUTRITION. 

thus  far  been  acquired  upon  this  subject  has  resulted  chiefly  from 
attempts  to  trace  back  the  excretory  products  to  their  antecedents. 

The  products  of  the  complete  breaking  down  and  oxidation  of 
proteids  in  the  body  are  carbon  dioxide  and  water,  excreted  through 
the  lungs,  skin,  and  kidneys,  and  urea  and  a  number  of  other  com- 
paratively simple  crystalline  nitrogenous  compounds  found  in  the 
urine.  To  these  are  to  be  added  the  nitrogenous  metabolic  prod- 
ucts of  the  feces,  the  sulphuric  and  phosphoric  acids  resulting  from 
the  oxidation  of  the  sulphur  of  the  proteids  and  the  phosphorus  of 
the  nucleo-proteids,  and  the  relatively  minute  amounts  of  nitroge- 
nous matter  found  in  the  perspiration. 

Excretion  of  Free  Nitrogen. — The  question  whether  any 
portion  of  the  nitrogen  of  the  proteids  is  excreted  as  free  gaseous 
nitrogen  is  one  which  has  been  the  subject  of  no  little  investigation 
and  controversy  in  the  past,  the  especial  champions  being,  on  the 
affirmative,  Seegen  in  Vienna  and,  on  the  negative,  Voit  in  Munich. 
It  would  lead  us  too  far  aside  from  our  present  purpose,  however,  to 
attempt  even  to  outline  the  evidence,  and  it  must  suffice  to  say  that 
the  great  majority  of  physiologists  regard  it  as  established  that  there 
is  no  excretion  of  gaseous  nitrogen  as  a  result  of  the  katabolism  of 
proteids,  but  that  all  the  proteid  nitrogen  is  excreted  in  the  urine 
and  feces  with  the  exception  of  small  amounts  in  the  perspiration. 
In  accordance  with  this  view,  we  shall  assume  in  subsequent  pages 
that  the  urinary  nitrogen  (together  with,  strictly  speaking,  the 
metabolic  nitrogen  of  the  feces  and  perspiration)  furnishes  a  meas- 
ure of  the  total  proteid  katabolism  of  the  body. 

A  brief  consideration  of  some  of  the  principal  nitrogenous 
products  of  proteid  katabolism  will  serve  to  indicate  some  of  the 
main  features  of  the  process,  so  far  as  they  have  been  made  out. 

Urea. — Urea,  or  dicarbamid,  CON2H4,  is  the  chief  nitrogenous 
product  of  proteid  metabolism  in  the  carnivora  and  omnivora.  In 
the  urine  of  man,  e.g.,  from  82  to  86  per  cent,  of  the  nitrogen  is  in 
the  form  of  urea.* 

Antecedents  of  Urea. — A  vast  amount  of  study  has  been  expended 
upon  this  question  without  as  yet  leading  to  any  general  unanimity 
of  views.     It  appears,  however,  to  be  fairly  well  made  out  that  at 

*v.  Noorden,  Pathologie  des  Stoffwechsels,  p.  45. 


METABOLISM.  43 

least  a  considerable  part  if  not  all  of  the  urea  is  formed  in  the  liver, 
and  that  its  immediate  antecedent  is  ammonium  carbonate,  to 
which  it  is  closely  related  chemically.  This  theory  of  Schmiede- 
berg's  is  supported  by  the  facts: 

1st.  That  ammonium  salts,  and  also  the  amid  radicle  NH2  in 
the  amido  acids  of  the  fatty  series,  when  administered  in  the  food 
are  converted  into  urea. 

2d.  That  ammonium  carbonate  or  formiate  injected  into  the 
portal  vein  is  converted  in  the  liver  into  urea  which  appears  in  the 
blood  of  the  hepatic  vein. 

3d.  That  the  administration  of  inorganic  acids  to  the  dog  and  to 
man  results  in  the  excretion  of  ammonium  salts  in  the  urine,  it 
being  supposed  that  the  acid  displaces  the  weaker  carbonic  acid 
and  that  the  resulting  ammonium  salt  is  incapable  of  conversion 
into  urea  in  the  liver. 

4th.  Severe  disease  of  the  liver  has  been  observed  to  result  in 
a  decreased  production  of  urea  and  an  excretion  of  ammonium  salts 
in  the  urine. 

Later  investigations  by  Minkowski  *  and  others  have  followed 
the  process  of  the  formation  of  urea  one  step  further  back  and  ren- 
dered it  highly  probable  that  the  ammonium  salts  out  of  which  urea 
is  formed  reach  the  liver  in  the  form  of  ammonium  lactate.  It  has 
been  shown  that  sarcolactic  acid  is  one  of  the  products  of  the  meta- 
bolism of  the  muscles.  It  would  appear  that  this  acid  unites  with 
the  ammonium  radicle  derived  from  the  proteids  to  form  ammonium 
lactate,  and  that  the  latter  on  reaching  the  liver  is  first  oxidized  to 
the  carbonate,  which  is  then  converted  into  urea.  If,  by  disease 
or  surgical  interference,  this  action  of  the  liver  is  prevented,  ammo- 
nium lactate  appears  in  the  urine,  and  the  same  effect  may  even  be 
produced  by  excessive  stimulation  of  the  proteid  metabolism,  so 
that  the  production  of  ammonium  lactate  exceeds  the  capacity  of 
the  liver  to  convert  it. 

Uric  Acid. — Uric  acid  is  contained  in  small  amounts  in  the 
urine  of  mammals.  With  birds  it  constitutes  the  chief  nitrogenous 
product  of  the  proteid  metabolism.  Of  its  antecedents  in  the 
organism  scarcely  anything  is  known.  One  theory  regards  it  as  a 
specific  product  of  the  metabolism  of  the  nucleins,  but  this  cannot 

*Cf.  Neumeister,  Physiologische  Chemie,  pp.  313-318. 


44  PRINCIPLES   OF  ANIMAL   NUTRITION. 

be  regarded  as  established,  and  appears  difficult  to  reconcile  with  its 
relation  to  the  proteid  metabolism  of  birds.  Others  regard  it  as  an 
intermediate  product  in  the  production  of  urea,  a  small  portion  of 
which  escapes  further  oxidation  by  being  excreted  by  the  kidneys. 

Hippuric  Acid. — This  substance  is  a  normal  ingredient  of  the 
urine  of  mammals,  but  in  that  of  man  and  the  carnivora  is  found  in 
but  very  small  amounts.  In  the  urine  of  herbivora,  on  the  other 
hand,  it  occurs  abundantly. 

Light  was  thrown  upon  its  origin  by  the  well-known  discovery 
by  Wohler,  in  1824,  that  it  is  also  found  in  large  amount  in  the  urine 
of  man  or  of  carnivora  after  the  administration  of  benzoic  acid. 
Chemically,  hippuric  acid  is  benzamido-acetic  acid,  or  benzoyl 
glycocol.  When  the  food  contains  benzoic  acid  the  latter  unites 
with  glycocol  resulting  from  the  metabolism  of  the  proteids  and 
forms  hippuric  acid,  while  otherwise  the  glycocol  would  be  further 
oxidized  to  simpler  nitrogenous  products.  The  synthesis  of  hip- 
puric acid  has  been  shown  to  occur  only  in  the  kidneys  in  the  dog, 
but  in  the  case  of  the  rabbit  and  frog  they  appear  to  share  this 
capacity  with  other  organs. 

In  this  action  of  benzoic  acid  we  have  the  most  familiar  demon- 
stration of  the  formation  of  metabolic  products  intermediate  be- 
tween the  proteids  and  the  comparatively  simple  nitrogenous  sub- 
stances found  in  the  urine.  Glycocol  has  never  been  detected  in  the 
body,  obviously  because  as  fast  as  it  is  formed  it  is  again  decom- 
posed. Benzoic  acid  reveals  its  presence  by  seizing  upon  it  and 
converting  it  into  a  compound  which  is  incapable  of  further  oxida- 
tion, and  is  therefore  excreted.  Other  less  familiar  examples  of 
the  same  fact  might  be  cited  did  space  permit. 

The  normal  presence  of  small  quantities  of  hippuric  acid  in  the 
urine,  even  when  no  benzoic  acid  is  contained  in  the  food,  arises 
from  the  fact  that  the  putrefaction  of  the  proteids  in  the  intestines 
yields  aromatic  compounds,  containing  the  benzoyl  radicle,  which 
are  resorbed  and  combine  with  glycocol  to  form  hippuric  acid. 
The  origin  of  the  large  quantities  of  hippuric  acid  ordinarily  ex- 
creted by  herbivora,  however,  or  rather  of  its  benzoyl  radicle, 
is  still  more  or  less  of  a  puzzle,  notwithstanding  the  consider- 
able amount  of  investigation  which  has  been  devoted  to  its 
study.     The  most  natural  supposition  would  be  that  the  food  of 


METABOLISM.  45 

these  animals  contains  substances  of  the  aromatic  series  capable 
of  yielding  benzoic  acid  or  its  equivalent  in  the  body,  but  in  none 
of  the  feeding-stuffs  known  to  be  efficient  in  causing  an  excretion 
of  hippuric  acid  have  such  compounds  been  discovered  in  quantity 
even  remotely  sufficient  to  account  for  the  hippuric  acid  produced. 

On  the  other  hand,  the  hypothesis  that  the  benzoyl  radicle  of 
the  hippuric  acid  is  derived  to  any  large  extent  from  the  proteids 
of  the  food  appears  to  be  decisively  negatived  by  several  facts: 
First,  the  quantity  of  proteids  in  the  ordinary  rations  of  herbivora 
is  relatively  small,  and  even  if  it  all  underwent  putrefaction  the 
amount  of  aromatic  products  which  could  be  formed,  on  any  reason- 
able estimate,  would  account  for  only  a  small  fraction  of  the  hip- 
puric acid  actually  found.*  Second,  in  several  instances  it  has 
been  observed  that  variations  in  the  extent  of  the  putrefactive 
processes  in  the  intestines,  as  measured  by  the  amount  of  con- 
jugated sulphuric  acid  in  the  urine  (compare  p.  46),  bore  no  rela- 
tion to  the  variations  in  the  production  of  hippuric  acid.  Third, 
the  addition  of  pure  proteids  or  of  foods  very  rich  in  proteids  to  a 
ration  does  not  increase  the  production  of  hippuric  acid,  and  in  at 
least  one  casej  was  found  to  diminish  it  and  even  stop  it  alto- 
gether. 

Apparently  we  must  regard  the  non-nitrogenous  ingredients  of 
feeding-stuffs  as  the  chief  source  of  hippuric  acid  formation,  but  be- 
yond this  our  knowledge  is  rather  vague.  It  is  well  established 
that  the  coarse  fodders  are  the  chief  producers  of  hippuric  acid, 
while  the  concentrated  feeding-stuffs  give  rise  to  little  or  none,  and 
may  even  reduce  the  amount  previously  produced  on  coarse  fodder, 
as  may  also  starch.  Among  the  coarse  fodders,  the  graminege  give 
rise  to  a  markedly  greater  production  of  hippuric  acid  than  the 
leguminosae.  This  effect  of  the  coarse  fodders  naturally  led  to  the 
suspicion  that  the  crude  fiber  contained  in  them  in  large  amounts 
might  be  the  source  of  the  hippuric  acid,  and  in  fact  numerous 
experiments  seem  to  show  that  some  relation  exists  between  the 
two,  although  the  results  of  various  investigators  are  far  from  con- 
cordant. 

Finally,   the    investigations    of    Goetze   &   Pfeiffer,  \   and   of 
*  Compare  Salkowski,  Zeit.  physiol.  Chem.,  9,  234. 
f  Henneberg  and  Pfeiffer,  Jour.  f.  Landw.,  38,  239. 
JLandw.  Vers.  Stat.,  47,  59. 


46  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Pfeiffcr  &  Eber.*  have  shown  with  a  high  degree  of  probability 
that  the  pentose  carbohydrates  of  the  feed  have  some  connection 
with  the  production  of  hippuric  acid.  The  former  investigators 
observed  a  marked  increase  in  the  production  of  hippuric  acid  by 
a  sheep  after  the  administration  of  cherry  gum  (impure  araban)  and 
of  arabinose,  and  the  latter  obtained  the  same  effect,  although  in  a 
less  marked  degree,  by  feeding  cherry  gum  to  a  horse.  They  also 
call  attention  to  the  differences  in  the  behavior  of  the  pentose  carbo- 
hydrates in  the  organism  of  the  herbivora  and  in  that  of  man  and 
the  carnivora,  but  do  not  attempt  to  give  a  final  solution  of  the 
problem  of  the  origin  of  the  hippuric  acid  in  the  former  case,  while 
they  freely  admit  that  it  is  difficult,  if  not  impossible,  to  explain 
some  of  the  facts  already  on  record  on  the  hypothesis  that  the  pen- 
toses are  the  chief  source  of  hippuric  acid. 

Creatin  and  Creatinin. — Among  other  nitrogenous  constit- 
uents of  the  urine  of  man  and  the  carnivora  may  be  mentioned 
creatinin.  This  body  is  the  anhydride  of  creatin,  and  the  two 
together  constitute  the  principal  part  of  the  so-called  flesh  bases 
which  are  contained  in  considerable  quantity  in  muscular  tissue. 
When  meat  is  consumed,  its  creatin  is  converted  into  creatinin  and 
excreted  quantitatively  in  the  urine,  the  creatinin  content  of  which 
may  be  thus  considerably  increased.  As  to  the  physiological  signifi- 
cance of  the  creatin  of  muscular  tissue  opinions  are  divided,  but 
good  authorities  are  inclined  to  regard  it  as  an  intermediate, 
product  of  the  metabolism  of  the  proteids  which  is  ultimately  con- 
verted into  urea,  and  to  urge  that  the  fate  of  creatin  taken  into  the 
stomach  is  not  necessarily  the  same  as  that  of  the  creatin  produced 
in  the  muscles. 

Aromatic  Compounds. — Besides  the  benzol  radicle  of  hippuric 
acid,  small  amounts  of  other  aromatic  compounds  are  also  found  in 
the  urine.  These  bodies,  belonging  chiefly  to  the  phenol  and  indoi 
groups,  owe  their  origin  exclusively  to  the  putrefactive  processes 
already  mentioned  as  taking  place  in  the  intestines,  and  are  found  in 
the  urine  almost  entirely  in  combination  with  sulphuric  acid  as  the 
so-called  conjugated  sulphuric  acids,  so  that  the  amount  of  the 
latter  is  employed  as  a  measure  of  the  extent  of  these  putrefactive 
processes. 

*Landw.  Vers.  Stat.,  49.  97. 


METABOLISM.  47 

Metabolic  Products  in  Feces. — As  already  stated  in  Chapter 
I,  the  feces  contain,  in  addition  to  undigested  residues  of  the  food, 
certain  materials  derived  from  the  body  of  the  animal.  This  fact 
was  early  recognized  as  true  of  both  carnivora*  and  herbivora.f 
Of  more  recent  investigations  may  be  noted  especially  those  of 
Miiller;J  Rieder,§  and  Tsuboi  ||  on  carnivora,  those  of  Prausnitz^f  and 
his  associates  on  man,  and  those  of  Kellner,**  Stutzer,ff  Pfeiffer,J| 
and  Jordan  §§  on  herbivora. 

These  "metabolic  products"  appear  to  consist  of  unresorbed 
or  altered  residues  of  the  digestive  fluids  and  of  mucus  and  other 
materials  excreted  or  otherwise  thrown  off  by  the  walls  of  the  intes- 
tines. Their  production  goes  on  even  when  the  digestive  tract  is 
void  of  food,  producing  the  so-called  fasting  feces  which  constitute 
a  true  excretory  product.  The  consumption  of  highly  digestible 
food — e.g.,  lean  meat — does  not  seem  to  materially  increase  their 
amount,  but  when  food  containing  indigestible  matter  is  eaten  it  is 
believed  that  they  increase  in  quantity. 

It  is  presumed  that  these  substances  are  largely  nitrogenous  in 
character,  and  it  is  known  at  any  rate  that  not  inconsiderable 
amounts  of  nitrogen  may  leave  the  body  by  this  channel.  In  other 
words,  these  nitrogenous  substances,  derived  from  the  proteids 
of  the  body,  instead  of  undergoing  complete  conversion  into  the 
ordinary  crystalline  products  have  their  katabolism  interrupted 
as  it  were  at  an  intermediate  stage. 

Many  attempts  have  been  made  to  determine  the  amount  of 
these  metabolic  products,  or  of  their  nitrogen,  in  the  feces,  but 
without  much  success,  and  it  may  fairly  be  said  that  at  present 
we  have  no  method  which  can  be  depended  upon  to  distinguish 
sharply  between  the  nitrogen  of  undigested-food  residues  and  that 
of  metabolic  products. 

*  Bischoff  and  Voit,  Die  Ernahrung  des  Fleischfressers,  p.  291. 

|Henneberg,  Beitrage,  etc.,  1864,  p.  7. 

JZeit.  f-  Biol.,  20,  327. 

$Ibid.,  20,  378. 

|j  Ibid.,  35,68. 

^Ibid.,  35,  2S7;  39,  277;  42,  377. 

**Landw.  Vers.  Stat.,  24,  434;  Bied.  Centralbl.,  9,  763. 

ffZeit.  physiol.  Chem.,  9,211. 

jt  Jour.  f.  Landw.,  31,  221;  33,  149;  Zeit.  physiol.  Chem.,  10,  561. 

§§  Maine  Expt.  Station  Rep.,  1888,  p.  196. 


48  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Nitrogen  in  Perspiration. — The  perspiration  of  such  animals 
as  secrete  this  fluid  must  be  regarded  as  one  of  the  minor  channels  by 
which  nitrogen  is  excreted.  In  human  perspiration  there  have  been 
found,  in  addition  to  small  amounts  of  proteids,  urea,  uric  acid, 
creatinin,  and  other  nitrogenous  products  of  the  proteid  meta- 
bolism. In  a  recent  investigation,  Camerer  *  found  about  34  per 
cent,  of  the  total  nitrogen  of  human  perspiration  to  be  in  the  form 
of  urea,  about  7.5  per  cent,  existed  as  ammonium  salts,  and  the 
remainder  in  undetermined  forms,  including  uric  acid  and  traces 
of  albumen. 

The  total  quantity  of  nitrogen  excreted  in  the  insensible  perspi- 
ration appears  to  be  insignificant.  Atwater  &  Benedict  t  found 
it  to  amount  to  0.048  gram  per  day  for  an  adult  man  in  a  state 
of  rest.  Rubner  &  Heubner  \  obtained  from  the  clothing  of  an 
infant  2.83  mgrs.  of  ammonia  and  0.0205  mgr.  of  urea  per  day 
and  estimated  the  total  nitrogen  of  the  perspiration  at  39  mgrs. 

When  the  secretion  of  sweat  is  stimulated  by  work  or  a  high 
external  temperature  the  amount  of  nitrogen  excreted  may  be  con- 
siderably increased  as  compared  with  a  state  of  rest,  although  its 
absolute  amount  is  still  small.  Atwater  &  Benedict,!  in  a  work  ex- 
periment, observed  an  excretion  of  0.220  gram  of  nitrogen  per  day 
in  the  perspiration  of  man. 

The  Non-nitrogenous  Residue  of  the  Proteids. — The  various 
nitrogenous  products  found  in  the  urine  and  other  excreta,  the  most 
important  of  which  have  been  noticed  above,  are  believed  to  con- 
tain all  the  nitrogen  of  the  metabolized  proteids.  This  does  not 
imply,  however,  that  a  quantity  of  proteids  equivalent  to  this  nitro- 
gen, or  even  to  that  of  the  urine,  has  been  completely  oxidized  to  the 
final  products  of  metabolism,  viz.,  carbon  dioxide,  water,  and  urea 
and  its  congeners. 

A  comparison  of  the  ultimate  composition  of  the  proteids  with 
that  of  the  nitrogenous  products  of  their  metabolism  reveals  the 
fact  that  an  amount  of  the  latter  sufficient  to  account  for  all  the 
nitrogen  of  the  proteids  contain  but  a  relatively  small  part  of  their 
carbon,  hydrogen,  and  oxygen.  Taking  urea  as  the  chief  and 
*Zeit.  f.  Biol.,  41,271. 

fU.  S.  Dcpt.  Agr.,  Office  of  Expt.  Stations,  Bull.  69,  73. 
tZeit.  f.  Biol.,  36,34. 
§Loc.  cit.,  p.  53. 


METABOLISM.  49 

typical  metabolic  product,  and  using  average  figures  for  the  com- 
position of  animal  proteids,  we  have,  omitting  the  sulphur  of  the 
proteids,  the  following: 

Proteids.  Urea.  Residue. 

Carbon 53.0  6.86  46.14 

Hydrogen 7.0  2.29  4.71 

Oxygen 24.0  9.14  14.86 

Nitrogen 16.0  16.00 

100.0  34.29  65.71 

After  abstracting  the  elements  of  urea,  we  have  left  considerably 
over  half  the  hydrogen  and  oxygen  of  the  proteid  and  the  larger 
part  of  its  carbon.  A  substantially  similar  result  is  reached  in  case 
of  the  other  nitrogenous  metabolic  products.  The  splitting  off  of 
these  products  from  the  proteids  leaves  a  non-nitrogenous  residue. 

Fate  of  the  Non-nitrogenous  Residue.— The  foregoing 
statements  and  comparison  must  not  be  understood  to  mean  that 
the  proteids  split  up  in  the  body  into  two  parts,  viz.,  urea,  etc.,  on 
the  one  hand,  and  an  unknown  non-nitrogenous  substance  or  sub- 
stances on  the  other.  As  we  have  already  seen,  the  processes  of 
proteid  metabolism  are  far  more  complicated  than  such  a  simple 
cleavage.  Neither  are  we  to  assume  that  any  substance  or  group 
of  substances  corresponding  in  composition  to  the  "  residue  "  of  the 
above  computation  exists.  The  figures  mean  simply  that  while 
the  nitrogenous  bodies  of  the  urine  contain  all  the  nitrogen  of  the 
proteids  they  do  not  account  for  all  of  the  other  elements,  but  that 
part  of  the  latter  must  be  sought  elsewhere. 

Ultimately,  of  course,  the  elements  of  this  non-nitrogenous 
residue  are  converted  into  carbon  dioxide  and  water.  The  conver- 
sion into  these  final  products,  however,  is  necessarily  a  process  of 
oxidation,  presumably  yielding  energy  to  the  organism.  It  is  a 
matter  of  some  interest,  then,  to  trace  the  steps  of  the  transforma- 
tion bo  far  as  this  is  at  present  possible. 

Formation  of  Sugar. — In  discussing  the  functions  of  the  liver  in 
§  1  of  this  chapter,  we  have  seen  reason  to  believe  that  this  organ 
continues  to  produce  sugar  when  the  diet  consists  largely  or  exclu- 
sively of  proteids.  In  this  case  we  are  forced  to  the  conclusion  that 
this  sugar  is  manufactured  from  the  elements  of  the  non-nitrogenous 
residue. 


5©  PRINCIPLES   OF  ANIMAL   NUTRITION. 

This  conclusion,  based  on  what  appears  to  be  the  normal  func- 
tion of  the  liver,  is  further  strengthened  by  a  large  number  of  ex- 
periments and  observations  upon  the  metabolism  in  diabetes. 
This  disease,  whether  arising  spontaneously  or  provoked  artificially, 
is  characterized  by  the  presence  of  large  amounts  of  sugar  in  the 
urine.  It  has  been  shown  that  this  production  of  sugar  continues 
when  all  carbohydrates  are  withdrawn  from  the  diet,  and  further- 
more, that  the  amount  of  sugar  excreted  bears  a  quite  constant 
relation  to  the  amount  of  proteids  metabolized,  thus  clearly  in- 
dicating the  latter  as  the  source  of  the  sugar.  It  is  true  that  the 
formation  of  sugar  from  proteids  is  denied  by  some  physiologists,* 
but  by  the  majority  it  seems  to  be  accepted  as  a  well-established 
fact  that  sugar  is  one  of  the  intermediate  products  of  proteid 
metabolism. 

Of  the  steps  of  the  process,  as  well  as  of  its  quantitative  rela- 
tions, we  are  ignorant.  In  effect,  it  is  a  process  of  oxidation  and 
hydration,  since  a  residue  of  the  composition  computed  above 
would  require  the  addition  of  both  hydrogen  and  oxygen  to  con- 
vert it  into  sugar,  but  that  it  is  as  simple  a  process  as  this  state- 
ment would  make  it  appear,  or  that  the  conversion  is  a  quantitative 
one,  may  well  be  doubted. 

In  conclusion  it  may  be  stated  that  while  recent  investigations 
have  shown  the  presence  of  a  carbohydrate  radicle  in  numerous 
(although  by  no  means  all)  proteids,  it  does  not  appear  that  this 
fact  stands  in  any  direct  relation  to  the  physiological  production  of 
sugar  from  these  substances.  In  the  first  place,  the  carbohydrate 
radicle  constitutes  a  much  smaller  proportion  of  these  proteids  than 
corresponds  to  the  amount  of  sugar  which  they  are  apparently 
capable  of  yielding  in  the  body,  and  in  the  second  place  it  appears 
to  be  a  well-established  (although  not  undisputed)  fact  that  the 
organism  can  produce  sugar  from  proteids  which  do  not  contain 
the  carbohydrate  radicle. 

Formation  of  Fat. — Whether  fat  is  formed  from  the  elements 
of  proteids  in  the  animal  body  is  at  present  a  subject  of  controversy, 
but  this  question  will  be  more  profitably  considered  in  a  subsequent 
chapter.  It  is  sufficient  to  remark  here  that  while  much  of  the 
earlier  evidence  bearing  upon  this   point  has  been  shown  to  be 

*Cf.  Schondorf,  Arch.  ges.  Physiol.,  82,  60. 


METABOLISM.  51 

inconclusive,  the  formation  of  fat  from  proteids  has  not  yet  been 
disproved  and  has  weighty  direct  evidence  in  its  favor,  while  the 
facts  that  sugar  may  be  formed  from  proteids,  and  that  carbohy- 
drates are  certainly  a  source  of  fat  to  the  animal  organism  are 
strong  additional  arguments  in  favor  of  its  possibility. 

Schematic  Equations. — Chauveau  and  his  associates  *  whose 
views  regarding  the  functions  of  the  carbohydrates  in  the  body 
have  already  been  mentioned,  regard  the  katabolism  of  the  proteids 
as  taking  place  in  three  stages.  The  first  consists  of  the  splitting 
off  of  urea  with  production  of  carbon  dioxide,  water,  and  fat,  accord- 
ing to  the  equation : 

4(C72HU2N18022S)  +  13902 

(Stearin) 

=  2(C57Hn  0O6)  +  36CON2H4  +  138C02  +  42H20  +  2S2. 

The  resulting  fat  is  then,  according  to  Chauveau,  further  oxi- 
dized in  the  liver,  yielding  dextrose,  in  accordance  with  the  equation 
already  given  on  p.  38,  viz., 

2C57H110O6  +  67O2=  16C6H1206+ 18C02+ 14H20, 

and  the  dextrose  is  finally  oxidized  to  carbon  dioxide  and  water. 
\nother  equation  representing  the  katabolism  of  proteids  is  that- 
proposed  by  Gautier,  which  regards  the  first  step  in  the  process  as  a 
combined  hydration  and  cleavage  with  the  production  of  urea,  fat, 
dextrose,  and  carbon  dioxide,  as  follows: 

2(C72H112N18022S)+28H20 

(Tripalmitin) 

=  18CON2H4  +  2C51H9806 + C6H1206  +  18C02 + Sj. 

It  may  be  assumed  that  these  authors  regard  the  above  equa- 
tions simply  as  schematic  representations  of  the  general  course  of 
proteid  metabolism  and  do  not  intend  to  imply  that  there  are  no 
intermediate  stages  in  the  process.  Interpreting  them  in  this 
sense,  we  have  good  reasons  for  believing  that  the  facts  which  they 
represent  are  qualitatively  true.  A  chemical  equation,  however, 
expresses  not  merely  qualitative  but  quantitative  results.  If  the 
above  equations  have  any  significance  beyond  that  of  the  mere 
verbal  statement  that  fat  and  sugar  are  products  of  proteid  meta- 

*Cf.  Kaufmann,  Archives  de  Physiol.,  1896,  p.  341. 


5 2  PRINCIPLES   OF  ANIMAL   NUTRITION. 

bolism,  they  mean  that  from  100  grams  of  proteids  there  is  pro- 
duced, according  to  the  first  scheme,  27.61  grams  of  fat,  and  that 
from  this,  by  the  addition  of  oxygen,  44.67  grams  of  sugar  are 
formed.  Some  of  the  evidence  by  which  these  equations  are  sup- 
ported will  be  considered  in  another  connection,  but-  may  be  antici- 
pated here  in  the  statement  that,  in  the  judgment  of  the  writer,  it 
is  far  from  sufficient  to  establish  them  as  quantitative  statements. 

THE  NON-PROTEIDS. 

Under  this  comprehensive  but  somewhat  vague  term  have  been 
grouped  all  those  numerous  nitrogenous  constituents  of  the  food 
which  are  not  proteid  in  their  nature,  the  name  being  a  contraction 
of  non-proteid  nitrogenous  substances.  It  includes  the  extractives 
of  meat,  and  in  vegetable  foods  several  groups  of  substances,  of 
which,  however,  the  amides  and  amido-acids  are  most  abundant. 
Various  substances  of  this  class  are  produced  by  the  splitting  up  of 
the  reserve  proteids  in  the  germination  of  seeds  and  apparently 
also  to  some  extent  in  the  translocation  of  proteids  in  the  growing 
plant,  while  some  at  least  of  them  appear  to  be  produced  syntheti- 
cally from  inorganic  materials  and  to  be  the  forerunners  of  pro- 
teids. In  young  plants  a  considerable  proportion  of  the  so-called 
crude  protein  (N  X  6.25)  often  consists  of  these  non-proteids,  and 
considerable  interest,  therefore,  attaches  to  their  transformations 
in  the  body. 

Amides  Oxidized  in  the  Body. — It  has  been  shown  by  numer- 
ous investigators  that  various  amides  and  amido-acids  when  added 
to  the  food  are  oxidized,  giving  rise  to  a  production  of  urea. 
Shultzen  &  Nencki  *  found  that  glycocol,  leuein,  and  tyrosin  were 
thus  oxidized,  while  acetamid  apparently  was  not.  So  far  as 
glycocol  is  concerned,  this  result  is  what  would  have  been  expected, 
since,  as  we  have  seen  (p.  44),  this  body  appears  to  be  normally 
formed  in  the  body  as  an  intermediate  product  of  proteid  meta- 
bolism. Similar  results  were  obtained  by  v.  Knieriem  f  from 
trials  with  asparagin,  aspartic  acid,  glycocol,  and  leucin.  Munk  \ 
likewise  found  that  the  ingestion  of  asparagin  increased  the  pro- 

*Zeit.  f.  Biol.,  8,  124. 

flbid.,  10,  277;       ,  36. 

\  Virchow's  Archiv.  f.  path.  Anat..  94,  441. 


METABOLISM.  53 

duction  of  urea  in  the  dog,  all  the  nitrogen  of  the  asparagin  together 
with  an  excess  over  that  previously  found  in  the  urine  being  ex- 
creted. The  sulphur  in  the  urine  also  increased.  Hagemann  * 
has  more  recently  fully  confirmed  this  result.  Salkowski  f  found 
that  glycocol,  sarkosin,  and  alanin  were  oxidized  to  urea  and  caused 
no  gain  of  proteids.  Apparently,  then,  this  class  of  bodies,  like 
ammonia,  furnish  material  out  of  which  the  organism  can  con- 
struct urea. 

Can  Amides  Replace  Proteids? — Since  the  amides  yield  the 
same  end  products  of  metabolism  as  the  proteids,  it  is  natural  to 
inquire  whether  they  can  perform  any  of  the  functions  of  those 
substances. 

Amides  not  Synthesized  to  Proteids. — We  have  already  seen  that 
the  albumoses  and  peptones  resulting  from  the  cleavage  of  the 
proteids  during  digestion  are  built  up  again  into  proteids  in  the 
process  of  resorption.  The  amides  commonly  found  in  vegetable 
feeding-stuffs  are  likewise  simpler  cleavage  products  of  the  proteids, 
and  some  of  them  are  also  formed  in  digestion  by  the  proteolytic 
action  of  trypsin.  Can  proteids  be  regenerated  from  these  simpler 
cleavage  products? 

If  this  is  the  case,  then  it  should  be  possible,  under  suitable  con- 
ditions, to  cause  a  gain  of  proteids,  or  at  least  to  maintain  the 
stock  of  proteids  in  the  tissues,  on  a  food  free  from  proteids  but 
containing  amides.  Up  to  the  present  time,  however,  all  attempts 
of  this  sort  have  failed.  With  the  most  abundant  supply  of  non- 
nitrogenous  nutrients  and  ash,  the  animals  perished  when  supplied 
with  amides  (asparagin)  but  not  with  proteids. J  What  has  thus 
been  found  to  be  true  of  asparagin  we  may  regard  as  probably  true 
of  other  amides  and  say  that  there  is  no  evidence  that  the  animal 
body  can  build  proteids  from  amides. 

Partial  Replacement  of  Proteids. — But  even  if  the  amides  can- 
not serve  as  a  source  of  proteids  to  the  animal,  it  seems  not  impos- 
sible that  they  may  by  their  oxidation  perform  a  part  of  the  func- 
tions of  the  proteids,  thus  protecting  a  portion  of  the  latter  from 
oxidation  and  rendering  it  available  for  tissue  production. 

*Landw.  Jahrb.,  20,  264. 

f  Zeit.  physiol.  Chem.,  4,  55. 

^Compare  Politis,  Zeit.  f.  Biol.,  28,  492,  and  Gabriel,  lb.,  29,  115. 


54  PRINCIPLES   OF  ANIMAL    NUTRITION. 

The  earliest  investigations  upon  this  point  are  those  of  Weiske  * 
and  his  associates  upon  the  nutritive  value  of  asparagin.  The 
experiments  were  made  upon  rabbits,  hens,  geese,  sheep,  and  goats, 
and  in  the  case  of  the  two  latter  species  included  experiments  on 
milk  production.  While  the  experiments  are  open  to  criticism  in 
some  respects,  as  a  whole  they  seemed  to  show  that  asparagin, 
especially  when  added  to  a  ration  poor  in  proteids,  caused  a  gain  of 
proteids  by  the  body.  Weiske  accordingly  concluded  that  aspara- 
gin, while  not  capable  of  conversion  into  proteids,  was  capable  of 
partially  performing  their  functions  and  thus  acting  indirectly  as  a 
source  of  proteids,  and  this  view  has  been  somewhat  generally 
accepted.  Subsequent  experiments  by  Bahlmann,f  Schrodt,J 
Potthast,§  Meyer,|  and  Chomsky  ^[  upon  milch-cows,  rabbits,  and 
sheep  gave  results  which  tended  to  confirm  Weiske's  conclusions. 

Not  all  of  Weiske's  experiments,  however,  gave  positive  results 
in  favor  of  asparagin,  and  experiments  upon  carnivorous  and  omniv- 
orous animals  have  failed  to  show  any  such  effect.  In  addition 
to  the  experiments  of  Politis  and  of  Gabriel,  referred  to  above, 
Mauthner,**  Munk,f  f  and  Hagemann  \\  have  failed  to  observe  any 
gain  of  proteids  by  the  body  as  a  result  of  the  ingestion  of  asparagin, 
but  found  simply  an  increase  in  the  apparent  proteid  metabolism 
as  measured  by  the  urinary  nitrogen. 

Influence  on  Digestion. — It  can  hardly  be  assumed  that  the 
actual  processes  of  metabolism  in  the  body  tissues  are  fundamen- 
tally different  in  different  species  of  mammals,  and  investigators 
have  therefore  been  led  to  seek  an  explanation  of  the  striking  differ- 
ence in  the  effects  of  asparagin  on  herbivora  and  carnivora  in  the 
differences  in  the  digestive  processes  of  the  two  classes  of  animals. 

Digestion  in  herbivora  is  a  relatively  slow  process  and,  as  pointed 
out  in  Chapter  I,  is  accompanied  by  extensive  fermentations  par- 

*Zeit.  f.  Biol.,  15,  261-  17,  415;  30,  254. 

t  Reported  by  Zuntz,  Arch.  f.  (Anat.  u.)  Physiol.,  1882,  424. 

%  Jahresb.  Agr.  Chem.,  26,  426. 

§Arch.  ges.  Physiol.,  32,  288. 

||  Cf.  Kellner,  Zeit.  f.  Biol.,  39,  324. 

t  Ber.  physiol.  Lab.  Landw.  Inst.  Halle,  1898,  Heft  13,  p.  1. 

**Zeit.  f.  Biol.,  28,  507. 

t+Virchow's  Arch.  f.  path.  Anat.,  94,  441. 

XX  Landw.  Jahrb..  20,  264. 


METABOLISM.  55 

ticularly  of  the  carbohydrates  of  the  food,  as  is  shown  by  the  large 
amounts  of  gaseous  hydrocarbons  produced  by  these  animals.  In 
carnivora,  on  the  contrary,  digestion  is  relatively  rapid  and  the 
dog,  as  a  representative  of  this  class,  excretes,  according  to  Voit  & 
Pettenkofer,*  but  traces  of  hydrocarbons,  and  according  to  Tap- 
peiner,f  none. 

Zuntz  \  has  therefore  suggested  that  soluble  amides  introduced 
into  the  digestive  canal  of  herbivora  may  be  used  as  nitrogenous 
food  by  the  micro-organisms  there  present  in  preference  to  the  less 
soluble  proteids,  so  that  the  latter  are  to  a  certain  extent  protected, 
and  that  it  is  even  possible  that  the  amides  are  synthesized  to 
proteids  by  the  organisms.  Hagemann  §  has  added  the  suggestion 
that  the  proteids  possibly  thus  formed  may  be  digested  in  another 
part  of  the  alimentary  canal  and  thus  actually  increase  the  pro- 
teid  supply  of  the  body. 

If  this  explanation  is  correct,  we  should  expect  the  effect  of 
asparagin  to  be  more  marked  when  the  proportion  of  proteids  in 
the  food  is  small,  and  precisely  this  appears  to  be  the  case.  In 
Weiske's  first  experiments,  which  gave  the  most  decided  results, 
the  nutritive  ratio  of  the  ration  without  asparagin  was  1 :  19-20, 
while  a  later  experiment  with  a  nutritive  ratio  of  1 :9.4  showed  no 
effect  of  the  asparagin  upon  the  gain  of  protein.  Chomsky's  results, 
too,  were  obtained  with  rations  poor  in  protein  and  rich  in  carbo- 
hydrates. 

Later  experiments  on  lambs  by  Kellner  ||  have  fully  confirmed 
this  anticipation.  In  his  first  experiment  two  yearling  lambs  were 
fed  with  a  mixture  of  hay,  starch,  and  cane-sugar,  having  a  nutri- 
tive ratio  of  1 :  28,  until  nitrogen  equilibrium  was  reached,  when 
fifty  grams  of  the  starch  was  replaced  by  asparagin.  The  result 
was  a  gain  of  protein  by  both  animals  as  compared  with  a  loss  in 
the  first  period.  In  the  third  experiment  asparagin  was  substi- 
tuted for  starch  in  a  ration  having  a  nutritive  ratio  of  1  : 7.9,  and 
caused  with  one  animal  a  slight  gain  and  with  the  other  a  slight 
loss  of  protein.     In  the  fourth  experiment  it  was  added  to  a  ration 

*Zeit.  f.  Biol.,  7,  433;  9,  2  and  438. 

flbid.,  19,  318. 

JArch.  ges.  Physiol.,  49,  483. 

§  Landw.  Jahrb.,  20,  264. 

||  Zeit.  f.  Biol.,  39,  313. 


56  PRINCIPLES   OF  ANIMAL   NUTRITION. 

having  a  nutritive  ratio  of  1 : 7.7,  and  caused  neither  a  gain  nor 
a  loss  of  any  consequence. 

Particular  interest  attaches  to  Kellner's  second  experiment  in 
which  ammonium  acetate  was  added  to  a  ration  poor  in  protein 
(1:19),  followed  in  a  third  period  by  a  quantity  of  asparagin  con- 
taining the  same  amount  of  nitrogen.  The  average  amounts  of 
protein  (N  X  6.25)  gained  per  day  and  head  by  the  two  lambs 
were  as  follows: 

Basal  ration 4. 12  grins. 

"        "      +  ammonium  acetate 15.56     " 

"        "      +  asparagin 15.69     " 

Although  it  is  impossible  to  suppose  that  the  ammonium  acetate 
is  capable  of  performing  any  of  the  functions  of  proteids  in  the 
body,  it  nevertheless  caused  as  great  a  gain  of  protein  by  the  body 
as  did  the  asparagin.  The  only  obvious  explanation  is  that  both 
these  substances  acted  in  the  manner  suggested  by  Zuntz  to  protect 
the  small  amount  of  protein  in  the  food  from  the  attacks  of  the 
organized  ferments  of  the  digestive  tract.  Accepting  this  explana- 
tion, we  must  suppose  that  when  the  contents  of  the  alimentary 
canal  contain  a  normal  amount  of  proteids  the  micro-organisms 
find  an  abundant  supply  of  nitrogenous  food  in  their  cleavage 
products  and  reach  their  normal  development,  so  that  an  addition 
of  soluble  nitrogenous  substances  is  a  matter  of  indifference.  When, 
on  the  other  hand,  the  amount  of  protein  present  is  abnormally 
low,  as  in  Weiske's  and  Kellner's  experiments,  the  organisms  are 
limited  in  their  food-supply  and  attack  the  food  proteids  them- 
selves. 

Kellner's  results  stand  in  apparent  contradiction  to  the  earlier 
ones  of  Weiske  and  Flechsig,*  who  report  no  gain  of  proteids  as  re- 
sulting from  the  addition  on  three  days  of  a  mixture  of  ammonium 
carbonate  and  acetate  to  a  ration  poor  in  protein.  The  excretion 
of  sulphur  in  the  urine  was  likewise  unaffected.  They  assume, 
however,  a  long-continued  after  effect  of  the  ammonium  salts  on  the 
nitrogen  excretion.  If  the  comparison  be  limited  to  the  three  days 
on  which  the  ammonium  salts  were  given  and  the  next  following 
day,  a  gain  of  1.15  grams  of  nitrogen  per  day  results,  but,  as  just 
stated,  there  was  no  corresponding  gain  of  sulphur. 
♦Journ.  f.  Landw.,  38,  137. 


METABOLISM. 


57 


Kellner's  experiments  afford  indirect  evidence  that  both  the 
asparagin  and  the  ammonium  acetate  actually  did  stimulate  the 
development  of  the  ferment  organisms,  in  the  fact  that  the  apparent 
digestibility  of  the  carbohydrates  of  the  food  was  increased.  On 
the  basal  rations  starch  could  be  readily  recognized  in  the  feces, 
but  under  the  influence  of  the  two  substances  mentioned  it  dis- 
appeared. In  the  second  experiment  the  increase  in  the  amounts 
of  crude  fiber  and  of  nitrogen-free  extract  digested  was  as  follows : 

Nitrogen-free 
Crude  Fiber.  Extract. 

With  ammonium  acetate  ..  .    10.7  grms.  20.4  grms. 

With  asparagin 10.0      "  20.0     " 

Since  we  know  that  large  amounts  of  the  nitrogen-free  extract 
are  attacked  and  decomposed  by  organized  ferments  in  the  her- 
bivora,  and  that  this  is  the  chief  if  not  the  only  method  by  which 
crude  fiber  is  digested,  we  are  justified  in  interpreting  the  above 
figures  as  demonstrating  an  increased  activity  of  these  organisms 
as  a  result  of  the  more  abundant  supply  of  nitrogenous  food.  The 
bearing  of  this  result  upon  the  so-called  depression  of  digestibility 
by  starch  and  other  carbohydrates  is  obvious,  but  is  aside  from 
our  present  discussion. 

Tryniszewsky  *  experimented  upon  a  calf  weighing  about  175 
kgs.,  using  in  the  second  and  fourth  periods  (the  first  period  being 
preliminary)  a  ration  of  barley  straw,  sesame  cake,  starch  and  sugar, 
containing  a  minimum  of  non-proteids.  In  the  third  period  one- 
third  of  the  sesame  cake  was  replaced  by  a  mixture  of  asparagin, 
starch  and  sesame  oil,  computed  to  contain  an  equivalent  amount 
of  nitrogen,  carbohydrates,  and  fat.  Owing  to  differences  in  digest- 
ibility, however,  the  amounts  of  digested  nutrients,  and  particu- 
larly of  nitrogen,  varied  more  or  less.  The  results  of  the  nitrogen 
balance  per  100  kgs.  live  weight  were: 


Nitrogen  Digested. 

Nitrogen 

Metabolism, 

Grms. 

Proteid, 
Grms. 

Non-pro  teid, 
Grms. 

Total, 
Grms, 

Nitrogen, 
Grms. 

Period    II 

72.16 
67.05 
90.86 

72.16 
90.73 
90.86 

56.86 
78.43 
76.36 

15  3 

"      III 

"       IV 

23.68 

12.3 
14.5 

*Jahresb.  Ag.  Ch.,  43,  513. 


58  PRINCIPLES   OF  ANIMAL   NUTRITION. 

From  the  smaller  gain  in  Period  III,  the  conclusion  is  drawn 
that  the  asparagin  has  a  lower  nutritive  value  than  the  proteids. 
In  this  period  the  percentage  digestibility  of  the  crude  fiber 
of  the  ration  was  found  to  be  64.88,  as  compared  with  43.96  and 
33.33  in  the  second  and  fourth  periods,  an  effect  corresponding  to 
that  observed  by  Kellner,  and  which  Tryniszewsky  also  ascribes 
to  an  increase  in  the  micro-organisms  of  the  digestive  tract. 

The  results  of  the  experiments  which  have  been  cited  are,  of 
course,  valid,  in  the  first  instance,  only  for  the  particular  non- 
proteids  experimented  with.  If,  however,  the  above  interpretation 
of  the  results  is  correct,  it  is  to  be  anticipated  that  other  soluble 
nitrogenous  substances  in  the  food  will  be  found  to  produce  similar 
effects.  If  this  anticipation  proves  to  be  correct,  then  we  shall 
reach  the  following  conclusions  regarding  the  amides  and  similar 
bodies  in  feeding-stuffs. 

1.  That  they  do  not  serve  as  sources  of  proteids. 

2.  That  in  rations  very  poor  in  protein  they  have,  in  the  her- 
bivora,  an  indirect  effect  in  protecting  part  of  the  food  protein 
from  fermentation  in  the  digestive  tract. 

3.  That  in  carnivora,  and  in  herbivora  on  normal  rations,  they 
probably  have  no  effect  on  the  production  of  nitrogenous  tissue. 


CHAPTER  III. 
METHODS  OF  INVESTIGATION. 

An  essential  prerequisite  for  an  intelligent  study  of  the  income 
and  expenditure  of  matter  by  the  animal  body  is  a  knowledge  of 
the  general  nature  of  the  current  methods  of  investigation  and  of 
the  significance  of  the  results  attained  by  means  of  them.  It  is 
not  the  purpose  here  to  enter  into  technical  details;  this  is  not 
a  treatise  upon  analytical  or  physiological  methods.  The  present 
chapter  will  be  confined  to  outlining  the  general  principles  upon 
which  those  methods  are  based  and  to  pointing  out  the  logical 
value  of  their  results.  It  will  be  confined,  moreover,  mainly  to 
those  general  methods  by  which  the  balance  of  income  and  ex- 
penditure of  matter  is  determined. 

Tissue. — The  animal  body  has  already  been  characterized  as 
consisting,  from  the  chemical  point  of  view,  of  an  aggregate  of 
various  substances,  chiefly  organic,  representing  a  certain  capital 
of  matter  and  energy.  These  various  substances  are  grouped 
together  in  the  body  to  form  the  organized  structures  known  as 
tissues.  For  the  sake  of  brevity,  then,  it  may  be  permissible  to  use 
the  word  tissue  as  a  convenient  general  designation  for  the  aggre- 
gate of  all  the  organic  matter  contained  in  the  tissues  of  the  body, 
including  both  their  organized  elements  and  any  materials  present 
in  the  fluids  of  the  body  or  in  solution  in  the  protoplasm  of  the 
cells.  In  this  sense  tissue  is  equivalent  to  the  whole  capital  or 
store  of  organic  matter  in  the  body. 

Gains  and  Losses. — The  tissue  of  the  body,  as  thus  defined,  is 
in  a  constant  state  of  flux,  the  processes  through  which  the  vital 
functions  are  carried  on  constantly  breaking  it  down  and  oxidizing 
it  (katabolism),  while  the  processes  of  nutrition  are  as  constantly 
building  it  up  again  (anabolism).     If  the  activity  of  nutrition 

59 


60  PRINCIPLES   OF  ANIMAL   NUTRITION. 

exceeds  that  of  destruction,  material  of  one  sort  or  another  is  stored 
up  in  the  body,  and  such  an  addition  to  its  capital  of  matter  and 
energy  we  may  speak  of  as  a  gain  of  tissue.  Conversely,  if  the 
katabolic  processes  consume  more  material  than  the  processes  of 
nutrition  can  supply,  the  store  of  matter  and  energy  in  the  body 
is  diminished  and  a  loss  of  tissue  occurs.  A  simple  comparison  of 
the  amount  of  matter  supplied  in  the  food  (including,  of  course, 
the  oxygen  of  the  air)  with  that  given  off  in  the  solid,  liquid  and 
gaseous  excreta,  therefore,  will  show  whether  the  body  is  gaining 
or  losing  tissue. 

The  mere  fact  of  a  gain  or  loss  of  matter  by  the  body,  however, 
conveys  but  little  useful  information  unless  we  know  the  nature  of 
the  material  gained  or  lost.  This  we  have  no  means  of  determining 
directly.  The  processes  of  growth  or  decrease  are  not  accessible 
to  immediate  observation,  while  changes  in  the  weight  of  the  animal 
(even  aside  from  the  great  uncertainties  introduced,  especially  in 
the  herbivora,  by  variations  in  the  contents  of  the  alimentary 
canal)  represent  simply  the  algebraic  sum  of  the  gains  and  losses 
of  water,  ash  protein,  fats,  and  other  materials,  and  so  give  but  a 
very  slight  clue  if  any  to  the  real  nature  of  the  tissue-building.  We 
are  compelled,  therefore,  to  have  recourse  to  indirect  methods,  and 
to  base  our  conclusions  as  to  tissue-building  upon  a  comparison 
of  the  income  and  outgo  of  the  chemical  elements  of  which  the  body 
is  composed,  particularly  of  nitrogen  and  carbon. 

The  Schematic  Body. — The  basis  of  this  method  of  compari- 
son is  the  conception  of  the  schematic  body,  first  introduced  by 
Henneberg.*  This  conception  regards  the  dry  matter  of  the  body 
of  the  animal  as  composed  essentially  of  three  groups  of  substances, 
viz.,  ash,  fat,  and  protein,  with  at  most  comparatively  small  amounts 
of  carbohydrates  (glycogen),  and  assumes  that  the  vast  number  of 
other  compounds  which  it  actually  contains  are  present  in  such 
small  and  relatively  constant  proportions  as  not  to  materially 
affect  the  truth  of  this  view.  A  knowledge  of  the  ultimate  compo- 
sition of  these  three  groups  then  affords  the  basis  for  a  computation 
of  the  gain  or  loss  of  each  from  the  income  and  outgo  of  their  ele- 
ments. 

Ash. — The  ash  ingredients  of  the  body  form  a  well-defined 

*  Neue  Beitrage,  etc.,  p.  vii. 


METHODS   OF  INVESTIGATION. 


61 


group,  and  the  determination  of  the  gain  or  loss  of  each  ingredient 
from  a  comparison  of  income  and  outgo  is  in  principle  a  relatively 
simple  matter  and  calls  for  no  special  consideration  here. 

Fat. — The  elementary  composition  of  the  fat  of  the  body  has 
been  shown  to  be  remarkably  similar  not  only  in  different  animals 
of  the  same  species,  but  likewise  in  different  species.  The  classic 
investigations  of  Schulze  &  Reinecke*  upon  the  composition  of 
animal  fat  gave  the  following  results : 


No.  of 
Sam- 
ples. 

Carbon. 

Hydrogen. 

Oxygen. 

Aver- 
age 
Per 

Cent. 

Maxi- 
mum 
Per 

Cent. 

Mini- 
mum 
Per 
Cent. 

Aver- 
age 
'  Per 
Cent. 

Maxi- 
mum 
Per 
Cent. 

Mini- 
mum 
Per 
Cent. 

Aver- 
age 
Per 

Cent. 

Maxi- 
mum 
Per 

Cent. 

Mini- 
mum 
Per 
Cent. 

Beef  fat 

10 

6 

12 

76.50 
76.54 
76.61 

76.50 

76.63 
76.56 

77.07 
76.62 

76.74 

76.78 
76.85 

76.27 
76.29 
76.27 

11.91 
11.94 
12.03 

12.00 

12.05 
11.90 
11.69 
11.94 

12.11 
12.07 
12.16 

11.76 
11.86 

11.87 

11.59 
11.52 
11.36 

11.86 
11.83 
11.56 

11  15 

Pork  fat 

Mutton  fat 

11.15 
11.00 

Average 

Doo-  

28 

11.50 

11.32 
11.44 
11.24 
11.44 

Cat 

Alan 

Benedict  and  Osterberg  f  obtained  the  following  results  for  the 
composition  of  human  fat: 


Carbon, 

Hydrogen, 

Per  Cent. 

Per  Cent. 

Sample  No.  1 

76.29 

11.80 

"     2 

76.36 

11.72 

"    3 

75.85 

11.87 

"    4 

75.95 

11.85 

"     5 

75.94 

11.74 

"    6 

76.07 

11.69 

"    7 

76.13 

11.84 

"    8 

Average 

76.05 

11.81 

76.08 

11.78 

The  fat  of  the  body  has  been  commonly  regarded  as  containing 
76.5  per  cent,  of  carbon.     A  gain  of  100  parts  of  fat  by  the  body 


Landw.  Vers.  Stat.,  9,  97. 


f  Amer.  Jour.  Physiol.,  4, 


62 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


is  accordingly  equivalent  to  a  gain  of  76.5  parts  of  carbon,  and  con- 
versely, if  it  be  shown  that  the  body  has  gained  one  part  of  carbon 
in  the  form  of  fat,  this  is  equivalent  to  a  gain  of  1-f- 0.765  =1.307, 
or,  in  round  numbers,  1.3  parts  of  fat.  Benedict  &  Osterberg's 
average  corresponds  to  the  factor  1.314. 

Protein. — As  in  the  case  of  the  food,  the  term  protein  is  used 
to  signify  the  whole  mass  of  nitrogenous  material  in  the  body,  in- 
cluding, besides  the  true  albuminoids,  the  collagens  or  gelatinoids, 
the  keratin-like  bodies,  the  nitrogenous  extractives,  etc. 

Neumeister  *  gives  the  following  figures  for  the  elementary 
composition  of  the  simple  albuminoids: 


Minimum, 
Per  Cent. 

Maximum, 
Per  Cent. 

Average, 
Per  Cent. 

Carbon  

50 

6.5 
15 
19 

0.3 

55 

7.3 
17.6 
24 

2.4 

52 

7 

16 

23 

o 

100 

Some  of  the  compound  albuminoids,  particularly  the  nucleo- 
albuminoids,  do  not  vary  greatly  in  composition  from  the  above 
figures,  while  others  notably  the  mucins,  which  contain  a  carbo- 
hydrate group,  show  a  higher  percentage  of  oxygen  and  less  carbon 
and  nitrogen. 

The  gelatinoids,  likewise,  do  not  differ  greatly  in  composition 
from  the  albuminoids.  For  collagen,  Hofmeister  f  found  the  fol- 
lowing averages: 

Carbon; 50.75 

Hydrogen 6 .  47 

Nitrogen 17 .  86 

Oxygen  j 24.91 

Sulphur  ) 

100.00 
Keratin  is  distinguished  by  a  relatively  high  proportion  of 
sulphur  (3  to  5  per  cent.), but  otherwise,  according  to  Neumeister,  % 
does  not  differ  materially  in  composition  from  the  true  albuminoids. 
*Lehrbuch  der  Physiol.  Chem.,  p.  22.        fZeit.  physiol.  Chem.,  2,  322. 
JLoc  cit.,  p.  493. 


METHODS  OF  INVESTIGATION. 


63 


Hoppe-Seyler  *  quotes  the  following  figures  for  the  composition 
of  epidermis  and  some  of  the  tissues  derived  from  it : 


Epidermis 
of  Man. 

Hair. 

Nails. 

Horn  of 
Cow. 

Hoof  of 
Horse. 

50.28 

6.76 
17.21 
25.01 

0.74 

50.65 

6.36 

17.14 

20.85 

5.00 

51.00 

6.94 

17.51 

21.75 

2.80 

51.03 

6.80 

16.24 

22.51 

3.42 

51.41 

6.96 

17.46 

19  49 

4  23 

100.00 

100.00 

100.00 

100.00 

99.55(?) 

Henneberg  f  obtained  the  following  figures  for  the  composition  of 
two  samples  of  pure  and  dry  wool,  calculated  ash-free: 

I.  IT. 

Carbon 49.67  49.89 

Hydrogen 7.26  7.36 

Nitrogen 16.01  16.08 

Oxygen 23.65  23.10 

Sulphur 3.41  3.57 

100.00  100.00 

The  following  analyses  by  Rubner,J  Stohmann  &  Langbein,§ 
and  Argutinsky  ||  show  the  ultimate  composition  of  ash-free  muscular 
tissue  after  prolonged  extraction  with  ether :  1" 


Carbon, 
Per  Cent. 

Hydro- 
gen, 
Per  Cent. 

Nitrogen, 
Per  Cent. 

Sulphur, 
Per  Cent. 

Oxygen, 
Per  Cent. 

Heat  of 

Com. 

bustion 

perGram. 

Cals. 

53.40 
52.02 
52.33 

7.30 
7.30 

16.30 

16.36 

,  16.15 

24.32 
24  22 

5.6561 

Stohmann  and  Langbein. 

5.6409 

*  Physiol.  Chem.,  p.  90.  fNeue  Beitrage,  etc.,  p.  98. 

JZeit.  f.  Biol.,  21,  310.  §Jour.  f.  prakt.  Chem.,  N.  F.,  44,  364. 

||  Arch.  ges.  Physiol.,  55,  345. 

•fit  has  since  been  shown  by  Dornmeyer  (Arch.  ges.  Physiol.,  65,  90) 
that  such  material  is  not  fat- free 


04  PRINCIPLES  OF  ANIMAL   NUTRITION. 

Kohler*  has  investigated  the  elementary  composition  of  the 
muscular  tissue  of  cattle,  sheep,  swine,  horses,  rabbits  and 
hens.  The  material  was  prepared  with  much  care,  the  fat  being 
removed  as  fully  as  possible  by  prolonged  extraction  with  ether. 
The  residual  fat  which  cannot  be  removed  in  this  way  was  deter- 
mined by  Dornmeyer's  digestion  method, f  and  a  corresponding 
correction  made  in  the  analytical  results.  The  following  are  his 
averages  for  the  fat-  and  ash-free  substance: 


No.  of 
Samples. 

Carbon, 
Per  Cent. 

Hydrogen, 
Per  Cent. 

Nitrogen, 
Per  Cent. 

Sulphur, 
Per  Cent. 

Oxygen, 
Per  Cent. 

Heat  of 

Combustion 

per  Gram, 

Cals. 

Cattle 

Sheep 

Swine 

Horse 

Rabbit     .... 

4 
2 
2 
3 
2 
2 

52.54 
52.53 
52.71 
52.64 
52.83 
52.36 

7.14 
7.19 
7.17 

7.10 
7.10 
6.99 

16.67 
16.64 
16  60 
15.55 
16.90 
16.88 

0.52 
0.69 
0.59 
0.64 

23.12 
22.96 
22.95 

24.08 

5.6776 
5.6387 
5.6758 
5.5990 
5  6166 

Hen 

0.50 

23.28 

5.6173 

All  the  samples  were  tested  for  glycogen,  but  only  traces  were 
found,  except  in  the  horseflesh,  for  the  two  samples  of  which  an 
average  of  3.65  per  cent,  was  obtained,  a  result  which  accounts  for 
the  low  figure  for  nitrogen. 

In  the  classic  investigation  by  Lawes  &  Gilbert  %  into  the  com- 
position of  the  whole  bodies  of  animals,  determinations  were  made 
of  the  total  dry  matter,  the  ash,  the  fat,  and  the  total  nitrogen. 
From  these  data  Henneberg  §  has  compared  the  total  amount  of 
dry  matter  other  than  ash  and  fat  with  the  total  amount  of  nitro- 
gen. His  results  in  a  slightly  altered  form  are  given  in  the  table 
opposite. 

The  average  nitrogen  content  is  16.21  per  cent.  Lawes  &  Gil- 
bert extracted  the  fat  with  ether  and  hence,  as  above  noted,  the 
residue  was  not  absolutely  fat-free.    Kohler's  average  results  for  the 


*  Zeit.  physiol.  Chem.,  31,  479. 
t  Arch.  ges.  Physiol.,  65,  102. 
{Phil.  Trans.,  1859,  II,  493. 
§Neue  Beitrage,  etc.,  p.  x. 


METHODS   OF  INVESTIGATION. 


05 


Ox. 

Sheep. 

Swine. 

Half  Fat, 
Per  Cent. 

Fat, 
Per  Cent. 

Lean, 
Per  Cent. 

Fat, 
Per  Cent. 

Lean, 
Per  Cent. 

Fat, 
Per  Cent. 

"Water 

56.1 

43.9 

48.6 
51.4 

61.0 
39.0 

46.2 
53.8 

58.2 

41.8 

42.9 

57.1 

In  the  dry  matter  : 

Ash 

Fat 

100.00 

5.1 

20.7 

18.1 

100.00 

4.1 
31.9 

15.4 

100.00 

3.4 
19.9 

15.7 

100.00 

2.9 
37.9 

13.0 

100.00 

2.8 
24.6 

14.4 

100.00 

1.7 
44  0 

Other  organic   matter 
by  difference 

11.4 

43.9 

3.0 

16.58 

51.4 

2.4 

15.59 

39.0 
2.55 
16.24 

53.8 

2.1 

16.19 

41.8 
2.3 

15.97 

57.1 
1.9 

Per  cent,  of  nitrogen  in 
"other  organic  matter" 

16.66 

flesh  of  cattle,  sheep,  and  swine,  after  extraction  with  ether  for 

480  hours,  computed  ash-free,  were: 

Carbon 52.84 

Hydrogen 7.22 

Nitrogen 16.46 

Oxygen 22.89 

Sulphur 0.59 


100.00 


Considering  the  indirect  method  by  which  Henneberg's  result 
was  reached,  the  agreement  as  regards  nitrogen,  both  with  Kohler's 
results  and  with  those  of  Rubner,  Stohmann,  and  Argutinsky  just 
cited,  is  remarkably  close.  Henneberg  assumed  the  following 
round  numbers  to  represent  the  average  composition  of  the  total 
protein  of  the  body,  and  his  example  has  been  generally  followed 
by  subsequent  investigators: 

Carbon 53  per  cent. 

Hydrogen 7    "     " 

Nitrogen 16    "     " 

Oxygen 23    "     " 

Sulphur ...     1    "     " 

100 


66  PRINCIPLES   OF  ANIMAL    NUTRITION. 

Kdhler's  averages  for  dry,  fat-free  flesh  are: 

Carbon 52 .  60  per  cent. 

Nitrogen 16.67     "      " 

Glycogen. — Of  the  substances  other  than  ash,  fat  and  protein, 
which  are  found  in  the  animal  body,  only  glycogen  calls  for  special 
mention  here.  This  body,  as  we  have  seen,  may  be  stored  up  in 
considerable  amounts  in  the  liver,  and  is  found  also  in  the  muscles, 
although  not  in  large  proportion,  except  in  case  of  the  horse.  In 
the  aggregate,  however,  the  store  of  glycogen  in  the  body  is  not 
inconsiderable,  having  been  estimated  to  be  in  the  neighborhood 
of  300  grams  in  the  human  body.  Moreover,  changes  of  food  or 
conditions,  as  well  as  muscular  activity,  may  materially  alter  the 
store  of  glycogen  and  thus,  perhaps,  appreciably  affect  the  make- 
up of  the  schematic  body. 

So  far  as  appears,  however,  the  capacity  of  the  body  to  store  up 
glycogen  is  limited,  as  is  indicated  by  the  relatively  small  amount 
of  it  formed  after  even  the  most  abundant  feeding,  and  we  may 
fairly  assume  that,  at  least  on  a  ration  equal  to  or  exceeding  the 
maintenance  requirements,  no  long-continued  change  in  the  amount 
of  glycogen  in  the  body  is  likely  to  occur. 

Summary. — We  may  sum  up  the  foregoing  paragraphs  in  the 
brief  statement  that  for  the  purpose  of  investigating  the  statistics 
of  nutrition  we  may  consider  the  organic  part  of  the  animal  body 
as  composed  essentially  of  fat  and  protein,  with  small  amounts  of 
glycogen,  and  that  we  may  regard  the  permanent  effect  of  a  ration 
upon  the  body  as  consisting  (aside  from  its  effect  on  the  ash  ingre- 
dients) in  an  increase  or  decrease  of  its  stores  of  fat  and  protein, 
these  substances  having  the  average  compositon  indicated  above. 

The  Gain  or  Loss  of  Protein. — Since  the  term  protein  as  here 
used  is  synonymous  with  total  nitrogenous  matter,  the  gain  or  loss 
of  protein  by  the  body  is  necessarily  indicated  by  its  gain  or  loss  of 
nitrogen. 

The  supply  of  nitrogen  to  the  body  is  contained  in  the  pro- 
tein of  the  food.  The  losses  of  nitrogen  from  the  body  are 
contained — 

First,  in  that  part  of  the  protein  of  the  food  which  fails  of 
digestion  and  is  excreted  in  the  feces. 


METHODS  OF  INVESTIGATION.  67 

Second,  in  the  nitrogenous  products  of  the  proteid  metabolism, 
contained  chiefly  in  the  urine  but  including  also  the  small  quanti- 
ties of  nitrogenous  metabolic  products  contained  in  the  feces  and 
perspiration. 

The  nitrogen  of  urine  and  perspiration,  then,  together  with  the 
metabolic  nitrogen  of  the  feces,  will  indicate  the  extent  of  proteid 
katabolism,  while  the  difference  between  total  income  and  total 
outgo  of  nitrogen  will  show  whether  the  body  is  gaining  or  losing 
protein.  Finally,  since  the  losses  of  metabolic  nitrogen  in  feces  and 
perspiration  are  relatively  small,  and  often  not  readily  determinable, 
in  cases  where  the  greatest  accuracy  is  not  required,  and  particularly 
in  comparative  experiments,  we  may  regard  the  total  urinary  nitro- 
gen as  representing  with  a  fair  degree  of  accuracy  the  amount  of 
protein  broken  down  by  the  organism. 

In  the  foregoing  statements,  however,  it  has  been  tacitly  assumed 
that  the  protein  of  the  food  consists  of  true  proteids.  If,  how- 
ever, the  latter  are  accompanied  by  amides  and  other  non-proteid 
nitrogenous  bodies,  which  do  not  appear  to  contribute  to  the  forma- 
tion of  proteid  tissue  (compare  p.  53),  the  corresponding  amount 
of  nitrogen  will  appear  in  the  urine  and  be  added  to  that  derived 
from  the  actual  katabolism  of  body  or  food  proteids.  This,  how- 
ever, does  not,  of  course,  affect  any  conclusions  as  to  the  gain  or  loss 
of  protein  by  the  body. 

Factor  for  Protein. — It  is  thus  comparatively  easy  to  deter- 
mine in  terms  of  nitrogen  both  the  proteid  katabolism  and  the 
gain  or  loss  of  protein,  the  principal  precaution  necessary,  aside 
from  technical  details,  being  that  the  experiment  shall  extend  over 
a  sufficient  length  of  time  to  eliminate  the  influences  of  irregulari- 
ties in  ingestion  and  excretion. 

Knowing  approximately  the  ultimate  composition  of  the  pro- 
tein of  the  body,  we  may  take  a  step  further  and  infer  from  the 
amounts  of  nitrogen  determined  the  corresponding  amounts  of 
protein,  the  accuracy  of  the  result  depending,  of  course,  upon  the 
accuracy  of  the  figures  on  which  it  is  based.  The  composition 
commonly  assumed  for  the  body  protein  has  been  that  given  on 
page  65,  and  the  same  conventional  factor,  6.25,  has  been  used 
to  convert  nitrogen  into  protein  which  has  been  employed  in  case 
of  feeding-stuffs.      Kohler's   investigations  (p.  64)  show  that  the 


68  PRINCIPLES  OF  ANIMAL   NUTRITION. 

nitrogenous  organic  matter  of  muscular  tissue  has  a  materially 
higher  percentage  of  nitrogen,  viz.,  about  16.67  per  cent.  This 
would  reduce  the  factor  for  protein  from  6.25  to  6.00.  KOhler's 
samples,  after  extraction  with  ether  for  480  hours,  still  contained 
from  0.27  to  1.61  per  cent,  of  fat.  If  we  assume  the  ash  and 
fat-free  substance  of  Lawes  &  Gilbert's  experiments  (p.  65)  to 
have  still  contained  1  per  cent,  of  fat,  the  average  nitrogen  con- 
tent of  the  fat-free  substances  would  be  16.38  per  cent,  and  the 
corresponding  protein  factor  6.11,  while  the  factor  6.00  would  re- 
quire the  assumption  of  a  fat-content  of  2.7  per  cent. 

The  factor  6.0  has  been  used  by  Kellner  in  computing  the  results 
of  his  extensive  investigations  upon  cattle  at  Mockern.  Strictly 
speaking,  this  assumes  that  all  the  gain  of  nitrogen  takes  place 
either  in  the  form  of  muscular  tissue  or  of  material  of  the  same 
average  composition.  To  what  extent  such  an  assumption  is 
justified  it  is  difficult  to  say.  Certainly  a  part  of  the  protein  of  the 
food  is  applied  to  the  production  of  epidermis,  hair,  horns,  hoofs, 
etc.,  consisting  largely  of  keratins.  The  data  regarding  the  com- 
position of  these  tissues  given  on  p.  63  would  seem  to  show 
that  they  are,  on  the  average,  richer  in  nitrogen  than  muscular 
tissue,  a  fact  which  would  tend  to  lower  the  protein  factor,  but  on 
the  other  hand,  the  amount  of  this  growth  is  small  as  compared 
with  the  usual  protein  supply.  On  the  whole,  Kohler's  factor 
would  seem  to  afford  the  most  trustworthy  basis  of  computation 
which  is  at  present  available,  especially  in  view  of  its  close  agree- 
ment with  Lawes  &  Gilbert's  results. 

Urea  as  a  Measure  of  Proteid  Metabolism. — In  the  earlier 
investigations  upon  this  subject,  the  urea  of  the  urine,  as  deter- 
mined by  Liebig's  titration  method,  was  commonly  taken  as  the 
measure  of  proteid  metabolism,  one  part  of  urea  equaling  2.9  parts 
of  protein,  while  in  many  cases  the  metabolism  was  also  expressed 
in  terms  of  "flesh"  (muscular  tissue)  with  its  normal  water  con- 
tent and  an  average  of  3.4  per  cent,  of  nitrogen.  The  errors  inci- 
dent to  the  use  of  this  method  are  now  generally  recognized,  while 
its  inapplicability  to  herbivora  was  obvious  from  the  first,  and  with 
the  improvements  in  the  methods  of  nitrogen  determination,  the 
latter  has  almost  entirely  replaced  the  old  urea  determination  and 


METHODS   OF  INVESTIGATION.  69 

the  proteid  metabolism  is  now  almost  exclusively  expressed  in 
terms  of  either  nitrogen  or  protein. 

The  Gain  or  Loss  of  Fat. — As  the  balance  between  income 
and  outgo  of  nitrogen  serves  to  measure  the  gain  or  loss  of  protein 
by  the  schematic  body,  so  the  balance  between  income  and  outgo 
of  carbon  furnishes  the  means  for  estimating  the  gain  or  loss  of  fat. 

The  income  of  carbon  is,  of  course,  the  carbon  of  the  food. 
The  outgo  of  carbon  consists  of — 

First,  the  carbon  of  the  undigested  food  contained  in  the  feces. 

Second,  the  carbon  of  the  products  of  metabolism  contained 
in  feces,  urine,  and  perspiration. 

Third,  the  carbon  of  the  gaseous  excreta,  including  the  carbon 
dioxide  given  off  by  the  lungs  and  skin  and  the  carbon  dioxide  and 
hydrocarbons  resulting  from  fermentations  in  the  digestive  tract. 

Respiration  Apparatus. — The  carbon  of  the  visible  excreta 
is  readily  determined  by  the  ordinary  analytical  methods.  The 
determination  of  the  carbon  of  the  gaseous  excreta  requires  the  use 
of  a  special  apparatus,  commonly  called  a  respiration  apparatus. 

In  early  experiments  upon  respiration  the  animal  was  simply 
placed  in  a  known  confined  volume  of  air  which  was  analyzed  before 
and  after  the  experiment.  By  this  method,  however,  the  oxygen 
of  the  air  is  progressively  diminished,  while  the  respiratory  products 
accumulate,  both  of  which  conditions  are  liable  to  disturb  the 
normal  respiratory  exchange,  although  Kaufmann,*  who  has  re- 
cently reverted  to  this  primitive  method,  claims  to  have  secured 
accurate  results  in  rather  short  experiments. 

The  obvious  desirability  of  renewing  the  oxygen  and  removing 
the  products  of  respiration  soon  led  to  the  construction  of  more 
complicated  forms  of  apparatus  of  which  three  principal  types 
may  be  distinguished. 

The  Regnault  Apparatus. — The  oldest  of  these  is  the  Regnault  f 
or  closed  circuit  respiration  apparatus.  In  this  type  of  apparatus 
the  subject  breathes  in  a  confined  volume  of  air,  the  carbon  dioxide 
being  removed  by  suitable  absorbents  and  weighed,  while  the  oxy- 
gen consumed  is  replaced  from  a  receiver  containing  pure  oxygen, 
the  amount  admitted  to  the  apparatus  being  measured.     These 

*  Archives  de  Physiol.,  1896,  p.  329. 

f  Regnault  &  Reiset,  Ann.  de  China,  et  de  Physique,  3d  series,  26,  299. 


70  PRINCIPLES   OF  ANIMAL   NUTRITION. 

data,  with  the  addition  of  analyses  of  the  known  volume  of  air 
contained  in  the  apparatus  at  the  beginning  and  end  of  the 
experiment,  afford  the  means  of  computing  both  the  carbon  dioxide 
and  other  gases  given  off  and  the  oxygen  cousumed.* 

In  theory  this  is  the  most  complete  and  satisfactory  type  of 
respiration  apparatus,  since  it  permits  a  determination  of  the  total 
gaseous  exchange.  Serious  practical  difficulties  have  been  found 
in  its  use,  however,  especially  for  the  larger  animals,  among  them 
the  difficulty  of  maintaining  the  air  reasonably  pure,  the  difficulty 
of  securing  a  uniform  temperature  and  mixture  of  the  gases  in  a 
large  and  complicated  apparatus,  and  the  liability  to  contamination 
of  the  oxygen  used.  Seegen  &  Nowak  f  used  an  apparatus  of 
this  type  for  their  experiments  upon  the  excretion  of  gaseous  nitro- 
gen by  animals  (see  p.  42).  Laulanie  %  has  described  a  Regnault 
apparatus  for  small  animals  in  which  a  continuous  graphic  measure- 
ment of  the  oxygen  admitted  to  the  apparatus  is  made,  Hoppe- 
Seyler  §  has  constructed  at  Strasburg  an  apparatus  of  this  type 
large  enough  to  contain  a  man,  and  Bleibtreu  ||  has  recently  made 
use  of  a  small  one  to  investigate  the  formation  of  fat  in  geese,  but 
the  apparatus  has  not  come  into  general  use.l" 

The  Pettenkofer  Apparatus. — The  second  type  of  respiration 
apparatus  is  that  of  v.  Pettenkofer.  In  this  type  the  subject 
breathes  in  a  closed  chamber  through  which  a  measured  current 
of  air  is  maintained. 

Scharling  **  appears  to  have  been  the  first  to  construct  an  appa- 
ratus of  this  sort.  The  ingoing  air  was  freed  from  carbon  dioxide 
by  passing  through  potash  solution,  while  the  outcoming  air,  after 
drying,  gave  up  its  carbon  dioxide  to  a  weighed  potash  bulb.  Vari- 
ous similar  forms  of  apparatus  were  constructed,  but  it  was  found 

*  For  a  description  of  the  apparatus,  see  also  Hoppe-Seyler,  Physiol. 
Chem.,  pp.  526  and  536. 

t Sitzungsber.  Wiener  Akad.,  Math.-Naturwiss.  Classe,  71,111,  329; 
Arch.  ges.  Physiol.,  19,  349. 

t  Archives  de  Physiol.,  1890,  p.  571. 

§Zeit.  physiol.  Chem.,  19,  574. 

||  Arch.  ges.  Physiol.,  85,  366. 

"[[See  also  Pfliiger  and  Colasanti  (Arch.  ges.  Physiol.,  14,  92)  and  Schulz 
(76.,  p.  78). 

**Ann.  Chem.   Pharm.,  45,  214. 


METHODS    OF  INVESTIGATION.  71 

to  be  impossible  to  secure  complete  absorption  of  the  carbon  dioxide 
and  at  the  same  time  maintain  adequate  ventilation. 

In  1862  v.  Pettenkofer  *  introduced  the  important  improve- 
ment of  diverting  a  known  aliquot  of  both  the  ingoing  and  outcom- 
ing  air  for  analysis.  The  results  of  these  analyses,  calculated  upon 
the  whole  volume  of  air  used,  show  the  amounts  of  carbon  dioxide 
and  other  gases  added  by  the  subject  of  the  experiment. 

The  Pettenkofer  apparatus  has  the  advantage  of  placing  the 
subject  under  unquestionably  normal  conditions  as  to  purity  of 
air,  of  maintaining  a  practically  uniform  temperature  and  mixture 
of  gases  throughout  the  apparatus,  and  of  dispensing  with  the  ex- 
treme care  necessary  in  the  Regnault  apparatus  to  prevent  gaseous 
diffusion  between  the  air  outside  and  that  inside  the  apparatus. 
Its  great  drawback  is  that  it  does  not  in  practice  permit  the  deter- 
mination of  the  amount  of  oxygen  consumed.f  To  this  is  to  be 
added  the  magnification  of  experimental  errors  involved  in  com- 
puting the  results  obtained  by  the  analysis  of  small  samples  upon 
the  whole  volume  of  air  used. 

Despite  these  drawbacks,  however,  the  Pettenkofer  apparatus 
in  various  forms  has  been  widely  used,  especially  in  experiments v 
upon  domestic  animals,  and  has  shown  itself  capable  of  yielding 
very  accurate  results  within  its  scope.  Laulanie,J  by  largely  re- 
ducing the  rate  of  ventilation,  has  been  able  to  make  determinations 
of  the  oxygen  consumed  which  he  regards  as  satisfactory,  while 
Haldane  §  has  constructed  an  apparatus  for  small  animals,  in  which 
the  entire  air  current  is  passed  over  absorbents  before  entering 
and  after  leaving  the  apparatus,  which  also  permits  of  a  satisfac- 
tory indirect  determination  of  the  oxygen  consumed.  Sonden  and 
Tigerstedt  ||  have  also  constructed  a  modified  Pettenkofer  respira- 

*  Ann.  Chem.  Pharm.,  Suppl.  Bd.  II,  p.  1.  See  also  Atwater,  U.  S.  Dep. 
Agr.,  Office  of  Experiment  Stations,  Bull.  21,  p.  106. 

f  Such  a  determination  is  theoretically  possible  from  a  comparison  of  the 
oxygen  content  of  ingoing  and  outcoming  air,  but  the  delicacy  of  the  measure- 
ments and  analyses  required  is  so  great  as  to  render  the  method  impracti- 
cable, while  the  determination  by  difference  concentrates  all  the  errors  in 
this  one  quantity. 

%  Archives  de  Physiologie,  1895,  p.  619. 

§Jour.  Physiol.,  13,  419. 

I  Skand.  Arch.  Physiol.,  6,  1. 


72 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


tion  apparatus  of  very  large  dimensions.  Recently  Atwater  & 
Rosa  *  have  constructed  a  form  of  Pettenkofer  apparatus  for  use 
as  an  animal  calorimeter  in  which  the  method  of  measuring  and 
sampling  the  air  current  has  been  materially  improved  and  rendered 
more  accurate. 

When  the  Pettenkofer  apparatus  is  employed  for  experiments 
upon  herbivora,  special  provision  is  necessary  for  the  determination 
of  the  gaseous  hydrocarbons  excreted  in  considerable  quantities 
by  these  animals.  This  is  accomplished  by  passing  a  sample  of  the 
air  coming  from  the  apparatus  through  a  combustion- tube  contain- 
ing copper  oxide,  or  preferably  spongy  platinum  (platinized  kaolin), 
heated  to  redness.  The  hydrocarbons  are  thus  oxidized  and  the 
resulting  carbon  dioxide  determined. 

Pettenkofer  &  Voit,f  in  their  earlier  investigations,  deter- 
mined the  excretion  of  combustible  gases  by  a  dog,  with  the  follow- 
ing results  per  day: 


Food. 

Hydrogen, 
Grams. 

Methane, 
Grams. 

Carbon 

Meat, 
Grams. 

Fat, 
Grams. 

Starch, 
Grams. 

Dioxide, 
Grams. 

500 

200 
200 
200 

7.2 
5.2 
7.2 
6.4 
4.3 

4.1 
6.3 
4.7 
3.7 
4.5 

416.0 

500 

420  6 

500 

428.2 

500 
500 

200 
200 

417.3 
427.8 

According  to  the  above  figures,  a  trifle  less  than  3  per  cent., 
on  the  average,  of  the  total  carbon  excretion  was  in  the  form  of 
methane.  No  similar  determinations  seem  to  have  been  made  by 
Pettenkofer  &  Voit  in  their  later  experiments,  and  it  appears  to 
be  generally  assumed  that  they  are  unnecessary  in  investigations 
upon  man  and  the  carnivora. 

The  Zuntz  Apparatus. — Both  the  Regnault  and  the  Pettenkofer 
types  of  apparatus  are  calculated  for  the  determination  of  the 
total  gaseous  excreta  of  lungs,  skin,  and  digestive  tract  through 
considerable  periods  of  time,  and  their  use  enables  us  to  compare 
the  total  income  and  outgo  of  carbon. 

*U.  S.  Dept.  Agr.,  Office  of  Experiment  Stations,  Bulletins  44  and  63. 
t  Ann.  Chem.  Pharm.,  Supp.  Bd.  II,  p.  66. 


METHODS   OF  INVESTIGATION.  73 

The  third  type  of  respiration  apparatus  is  best  known  by  the 
name  of  Zuntz,*  from  the  extensive  development  given  it  by  this 
investigator,  although  it  has  assumed  various  forms  in  the  hands 
of  different  experimenters.  This  apparatus  is  radically  different 
from  the  other  two  types  in  that  it  is  intended  simply  for  the  deter- 
mination of  the  respiratory  exchange  in  the  lungs.  For  this  pur- 
pose the  expired  air  is  collected,  either  by  means  of  a  mask  or  a 
tracheal  cannula,  its  volume  measured,  and  its  content  of  carbon 
dioxide  and  of  oxygen  determined  in  an  aliquot  sample,  the  com- 
position of  the  inspired  air  being  assumed  to  be  that  of  the  normal 
atmosphere.  The  fundamental  principle  is  really  that  of  the  Petten- 
kofer  apparatus,  but,  owing  to  the  fact  that  the  excretory  gases 
are  not  diluted  with  many  times  their  volume  of  air,  the  results  are 
much  sharper  and  it  is  possible  to  determine  the  amount  of  oxygen 
consumed  as  well  as  of  the  carbon  dioxide  given  off.  In  addition 
to  this  advantage,  it  permits  the  experimenter  to  follow  the  varia- 
tions in  the  respiratory  exchange  in  comparatively  short  periods. 
It  is  thus  especially  adapted  for  investigating  such  questions  as  the 
influence  of  muscular  work  upon  metabolism,  and  it  is  in  the  study 
of  this  question  that  it  has  found  its  chief  application.  On  the 
other  hand,  it  is  impracticable  to  continue  its  use  through  long 
periods — a  day,  e.g. — and  it  takes  no  account  of  the  excretion 
through  the  skin  and  the  alimentary  canal.  Only  by  indirect 
methods,  therefore,  is  it  possible  to  compute  the  total  income  and 
outgo  of  carbon  by  its  use. 

But  while  the  Zuntz  form  of  respiration  apparatus  is  especially 
adapted  for  investigating  the  carbon  metabolism  during  short 
periods,  it  is  important  that  these  periods  be  not  made  too  short. 
What  is  actually  determined  by  the  use  of  any  form  of  respiration 
apparatus  is  the  excretion  or  absorption  of  carbon  dioxide  or  oxy- 
gen. In  an  experiment  extending  over  several  hours,  we  may 
fairly  assume  that  this  is  substantially  a  measure  of  the  actual  pro- 
duction or  consumption  of  these  gases  going  on  in  the  tissues.  In 
periods  of  a  few  minutes,  however,  there  is  always  a  possibility  of 
an  accumulation  of  oxygen  or  a  partial  retention  of  the  products 
of  metabolism  in  the  tissues  or  the  blood,  while,  on  the  other  hand, 

*Rohrig  &  Zuntz,  Arch.  ges.  Physiol.,  4,  57;  v.  Mehring  &  Zuntz,  ib.,  32, 
173;  Geppert  &  Zuntz,  ib.,  42,  189. 


74  PRINCIPLES   OF  ANIMAL   NUTRITION. 

the  products  of  previous  metabolism  may  be  added  to  those  formed 
during  the  experiment.  This  is  especially  true  of  the  carbon  diox- 
ide, particularly  in  work  experiments,  where  the  rate  and  volume 
of  respiration  are  largely  affected.  During  severe  work,  there  may 
be  more  or  less  accumulation  of  this  gas  in  the  blood,  while,  on  the 
other  hand,  the  increased  respiration  in  an  immediately  following 
period  of  rest  may  reduce  the  proportion  in  the  blood  below  the 
normal.  The  oxygen  is  thought  to  be  far  less  subject  to  this  error 
than  the  carbon  dioxide,  and  therefore  to  be  a  more  accurate  indi- 
cator of  the  total  metabolism. 

The  Respiratory  Quotient. — This  name  was  given  by  Pfliiger 
to  the  ratio  of  the  volume  of  carbon  dioxide  excreted  to  the  volume 
of  oxygen  consumed  in  the  same  time.     It  is  frequently  represented 

CO 
by  the  abbreviation  R.Q.,  or  by  the  symbol  -^~. 

It  is  obvious  that  this  ratio  will  vary  with  the  nature  of  the 
material  metabolized.  Thus  the  oxidation  of  a  carbohydrate,  e.g. 
dextrose,  will  give  rise  to  a  volume  of  carbon  dioxide  equal  to  that  of 
the  oxygen  consumed,  since,  as  the  following  equation  shows,  each 
molecule  of  oxygen  gives  rise  to  a  molecule  of  carbon  dioxide: 

C„H1206 + 602  =  6CO,  +  6H20. 

In  this  case  the  respiratory  quotient  is  equal  to  unity.  On  the 
other  hand,  when  fat  is  oxidized,  a  portion  of  the  oxygen  combines 
with  the  hydrogen  of  the  fat  to  form  water,  and  the  volume  of  car- 
bon dioxide  produced  is  less  than  that  of  the  oxygen  employed. 
Representing  the  process  by  the  equation  used  by  Chauveau,*  viz., 

2C57H110O6+ 16302=  114C02+  110H,O, 

114 

the  respiratory  quotient  is  -^  =  0.6993.    Computed  from  the  aver- 
luo 

age   percentage   composition  of  animal  fat  as   given  on  p.  61,  it 

equals  0.7069. 

The  proteids  of  the  food,  as  we  have  seen,  are  not  completely 

oxidized  in  the  body,  a  portion  of  their  carbon,  along  with  all  their 

nitrogen,  being  excreted  in  the  form  of  urea  and  other  organic 

*  La  Vie  et  l'Energie  chez  FAnimale. 


METHODS  OF  INVESTIGATION.  75 

compounds  in  the  urine.  Chauveau  &  Kaufmann,*  starting  with 
an  empirical  formula  for  albumin,  represent  its  complete  meta- 
bolism in  the  body  by  the  equation 

2C72H112N18022S2  + 15102  =  18CH4N20  +  126C02 + 76H20  +  S2, 

thus  obtaining  the  respiratory  quotient  —  =0.8344,     neglecting 

lot 

the  oxygen  required  to  oxidize  the  sulphur. 

The  urine,  however,  always  contains  greater  or  less  quantities 
of  nitrogenous  compounds  richer  in  carbon  than  urea,  and  in  herbiv- 
orous animals  in  particular  such  compounds  are  abundant.  The 
respiratory  quotient  of  the  proteids  is  therefore  variable,  depend- 
ing upon  the  extent  to  which  their  carbon  is  completely  oxidized. 
Thus  Zuntz  and  Hagemann  f  in  an  experiment  upon  the  horse 
in  which  approximately  15  per  cent,  of  the  total  nitrogen  of  the 
urine  was  contained  in  hippuric  acid,  compute  it  at  0.765. 

Deductions  from  Respiratory  Quotient. — The  value  of  a  determi- 
nation of  the  respiratory  quotient  lies  in  the  clue  which  it  affords  to 
the  nature  of  the  substances  which  are  being  oxidized  in  the  body. 
Assuming  that  the  materials  available  for  oxidation  in  the  schematic 
body  are  substantially  proteids,  carbohydrates  and  fat  it  is  evi- 
dent that  when  the  quotient  approaches  1.0  the  material  consumed 
must  consist  largely  of  carbohydrates,  while  if  it  falls  to  the  neigh- 
borhood of  0.7  it  is  clear  that  the  oxygen  is  combining  chiefly  with 
fat.  An  intermediate  value,  on  the  other  hand,  would  be  more  am- 
biguous, since  it  might  result  from  the  oxidation  of  proteids,  carbo- 
hydrates and  fat  in  several  proportions. 

If,  however,  the  amounts  of  oxygen  consumed  and  of  carbon 
dioxide  produced  in  the  oxidation  of  any  one  of  the  three  groups  be 
known,  it  is  a  simple  matter  to  compute  the  proportion  in  which 
the  other  two  enter  into  the  reaction.  For  the  amount  of  proteids 
metabolized,  we  have  an  approximate  measure  in  the  total  urinary 
nitrogen.  If  we  can  also  determine  the  amounts  of  carbon,  hydro- 
gen and  oxygen  contained  in  these  nitrogenous  urinary  products, 
we  can  compute  the  quantity  of  oxygen  required  to  oxidize  the  non- 
nitrogenous  residue  of  the  proteids  and  the  amount  of  carbon  diox- 
ide resulting  from  it  upon  the  assumption  of  complete  oxidation. 

♦Compare  p.  51.  fLandw.  Jahrb.,  27,  Supp.  Ill,  240. 


76  PRINCIPLES   OF  ANIMAL   NUTRITION. 

As  a  matter  of  fact,  however,  it  is  not  easy  to  determine  satis- 
factorily the  proportion  of  the  respiratory  exchange  due  to  the 
proteids,  both  because  the  nitrogenous  products  of  their  meta- 
bolism are  numerous  and  occur  in  varying  proportions  in  the  urine, 
and  because  we  may  not  always  be  justified  in  assuming  complete 
oxidation  of  the  non-nitrogenous  residue.  Computations  of  the 
nature  indicated  above,  therefore,  must  be  accepted  with  some 
reserve. 

A  simpler  case,  and  one  which  has  been  extensively  investigated, 
is  the  nature  of  the  increased  metabolism  arising  from  muscular 
exertion.  As  we  shall  see  in  a  succeeding  chapter,  such  exertion 
causes  a  marked  increase  in  the  respiratory  exchange  while  pro- 
ducing at  most  but  a  slight  effect  upon  the  proteid  metabolism. 
If  we  neglect  altogether  this  latter  effect,  the  ratio  between  the  in- 
crements of  carbon  dioxide  and  oxygen  will  indicate  whether  the 
additional  material  consumed  during  the  performance  of  the  work 
consisted  of  fat  or  carbohydrates  or  a  mixture  of  the  two,  of  course 
on  the  same  assumption  as  before,  viz.,  that  substantially  only 
these  two  classes  of  substances  are  available  in  the  schematic  body. 

For  example,  in  an  investigation  by  Zuntz,  cited  on  a  subsequent 
page,  the  performance  of  one  kilogram-meter  of  work  of  draft 
by  a  dog  caused  the  following  increments  in  the  respiratory  ex- 
change : 

Oxygen 1 .  6704  c.c. 

Carbon  dioxide 1.4670    " 

Respiratory  quotient 0 .  878 

Assuming,  as  above,  that  these  amounts  arise  from  the  oxida- 
tion of  fat  and  carbohydrates  only,  let  x  equal  the  amount  of  oxy- 
gen consumed  in  the  oxidation  of  fat  and  1.6704  —  x  the  amount 
consumed  in  the  oxidation  of  carbohydrates.  Since  the  respira- 
tory quotient  of  fat  is  0.7069,  the  x  cubic  centimeters  of  oxygen 
would  yield  0.7069a:  cubic  centimeters  of  carbon  dioxide,  while  the 
1.6704  —  x  cubic  centimeters  of  oxygen  used  to  oxidize  the  carbohy- 
drates would  yield  an  equal  volume  of  carbon  dioxide.  We  there- 
fore have — 

0 .  7069ar+  ( 1 .  6704  -  x)  =  1 .  4670, 
whence  x= 0.6939. 


METHODS  OF  INVESTIGATION.  77 

The  division  of  the  increments  of  the  respiratory  gases  was  accord- 
ingly— 

Oxygen  Carbon  Dioxide 

Consumed.  Produced. 

Byfat 0.6939  c.c.         0.4905  c.c. 

By  carbohydrates 0 .  9765    "  0 .  9765    " 

<< 

Total 1.6704    "  1.4670    " 

From  these  data  the  actual  amounts  of  fat  and  carbohydrates 
metabolized  can  be  readily  computed,  one  gram  of  fat  requiring  for 
its  oxidation  2.8875  grams  (2.028  liters)  of  oxygen  and  producing 
1.434  liters  of  carbon  dioxide,  while  one  gram  of  a  carbohydrate  of 
the  composition  of  starch  requires  1.185  grams  (0.832  liter)  of 
oxygen  and  produces  the  same  volume  of  carbon  dioxide. 

Computation  of  Fat  from  Carbon  Balance. — While  the  use 
of  the  Zuntz  type  of  respiration  apparatus  may  afford  invaluable 
information  regarding  the  nature  of  the  chemical  changes  going 
on  in  the  body,  a  satisfactory  determination  of  the  gain  or  loss  of 
carbon  by  the  body  usually  requires  the  employment  of  one  of  the 
other  types  of  apparatus.*  Having  by  such  means  added  a  deter- 
mination of  the  carbon  balance  to  that  of  the  nitrogen  balance,  we 
have  the  data  necessary  for  computing  the  gain  or  loss  of  fat  as  well 
as  of  protein  by  the  schematic  body. 

For  this  purpose  we  first  compute  the  gain  or  loss  of  protein 
in  the  manner  already  described.  Using  Kohler's  factor  for  pro- 
tein (p.  67),  a  gain  of  16.67  grams  of  nitrogen  is  equivalent  to  a 
gain  of  100  grams  of  protein.  This  100  grams  of  protein  will 
contain,  according  to  Henneberg,  53  grams,  or  according  to  Kohler, 
52.6  grams  of  carbon.  Any  gain  of  carbon  in  excess  of  this  amount 
must  therefore  be  in  the  form  of  non-nitrogenous  organic  matter, 
while  if  less  than  this  amount  of  carbon  has  been  gained  the  non- 
nitrogenous  matter  of  the  body  must  have  been  drawn  upon  to 
supply  the  difference.  The  only  non-nitrogenous  organic  substance 
assumed  to  be  present  in  the  schematic  body,  however,  is  fat,  con- 
taining on  the  average  76.5  per  cent,  of  carbon   (p.  61).     Neces- 

*For  a  direct  comparison  of  results  obtained  upon  the  horse  by  the  Zuntz 
and  the  Pettenkofer  forms  of  apparatus,  see  Lehmann,  Zuntz,  &  Hagemann, 
Landw.  Jahrb.,  23,  125. 


78  PRINCIPLES  OF  ANIMAL   NUTRITION. 

sarily,  then,  on  this  assumption,  each  gram  of  carbon  gained  in 
excess  of  that  stored  in  the  form  of  protein  will  represent  1.3  grams 
of  fat  stored. 

Formation  of  Glycogen. — Granting  the  substantial  accuracy  of 
the  computation  of  the  gain  or  loss  of  protein,  the  only  serious 
criticism  to  which  the  above  method  of  computing  the  gain  or  loss 
of  fat  is  subject  is  that  it  does  not  take  account  of  the  possible  stor- 
age of  carbon  in  other  forms,  and  particularly  as  glycogen.  In 
other  words,  it  may  be  contended  that  the  schematic  body  should 
be  regarded  as  consisting  of  water,  ash,  fat,  and  carbohydrates. 
There  is  undoubtedly  a  certain  degree  of  justification  for  this  con- 
tention, and  the  significance  of  small  gains  of  carbon,  or  of  gains 
observed  during  short  periods,  is  by  no  means  unambiguous.  But 
when  such  a  gain  is  observed  to  continue  day  after  day  for  weeks 
on  an  unchanged  ration,  as  in  some  of  the  experiments  cited  on 
subsequent  pages,  the  objection  loses  all  force. 

Computation  of  Total  Metabolism. — The  same  principle 
may  be  applied  to  the  computation  of  the  total  amount  of  protein 
and  fat  metabolized.  From  the  urinary  nitrogen  (plus  that  of  the 
feces  if  the  latter  be  regarded  as  a  metabolic  product)  by  multipli- 
cation by  the  conventional  factor  we  obtain,  as  already  explained, 
the  total  proteid  metabolism.  Subtracting  the  amount  of  carbon 
corresponding  to  this  quantity  of  protein  from  the  total  carbon 
excretion  leaves  a  remainder  which  must  have  been  derived  from 
non-nitrogenous  material.  If  carbohydrates  are  absent  from  the 
food,  this  material,  in  an  experiment  of  any  length,  must  be 
substantially  fat,  and  the  amount  of  the  latter  can  be  computed 
from  the  carbon  by  the  use  of  the  factor  1.3.  In  the  presence  of 
any  considerable  amount  of  carbohydrates,  however,  the  results 
are  ambiguous  unless  we  know  also  the  quantity  of  oxygen  con- 
sumed. 

Other  Determinations. — The  great  majority  of  investigations 
upon  the  metabolism  of  matter  have  been  confined  to  determi- 
nations of  the  nitrogen  and  carbon  balance.  Occasionally,  how- 
ever, other  determinations  have  been  made. 

Hydrogen  Balance. — Determinations  of  water  and  of  hydrogen 
in  organic  combination  in  food  and  excreta  enable  us,  after  making 


METHODS  OF  INVESTIGATION.  79 

allowances  for  the  hydrogen  gained  or  lost  in  protein  and  fat,  to 
compute  the  gain  or  loss  of  water  by  the  body. 

With  the  earlier  forms  of  respiration  apparatus,  great  diffi- 
culty was  experienced  in  obtaining  satisfactory  results  for  the 
water,*  and  Stohmann  f  has  traced  the  difficulty  to  an  invisible 
condensation  of  water  on  the  walls  of  the  chamber  and  connections. 
More  recently  Rubner  %  has  been  able  to  make  satisfactory  deter- 
minations of  water  with  a  Pettenkofer  apparatus  by  avoiding  as 
much  as  possible  differences  of  temperature  between  different  parts 
of  the  apparatus  and  by  taking  the  sample  of  the  outcoming  air  for 
analysis  as  close  to  the  respiration  chamber  as  possible.  Atwater 
&  Rosa  have  shown  that  their  form  of  Pettenkofer  apparatus 
(p.  72)  permits  of  very  accurate  determinations  of  water. 

Oxygen  Balance. — Owing  to  the  technical  difficulties  already 
indicated  in  considering  the  different  types  of  respiration  apparatus, 
direct  determinations  of  the  oxygen  balance  have  rarely  been  made. 
This  is  the  more  to  be  regretted  since  such  a  determination  would 
erve  to  check  those  of  nitrogen,  carbon,  and  hydrogen,  and  would 
be  a  test  of  the  accuracy  of  our  deductions  from  those  determina- 
tions as  to  the  nature  of  the  material  gained  or  lost  by  the  body. 

Ash  Ingredients. — The  gain  or  loss  of  ash  ingredients  can  of 
course  be  readily  determined,  but  the  subject  as  yet  has  hardly 
received  the  attention  which  it  deserves. 

Sulphur  and  Phosphorus. — Sulphur  forms  an  essential  con- 
stituent of  the  proteids,  while  phosphorus  enters  into  the  composi- 
tion of  the  nucleins  and  also  of  lecithin.  The  determination  of  the 
income  and  outgo  of  these  two  elements  is  often  of  value  in  rela- 
tion to  special  physiological  questions,  but  from  the  somewhat 
general  standpoint  of  this  work  may  be  considered  as  of  rather 
minor  importance. 

*Zeit.  f.  Biol,  11,  126. 
fLandw.  Vers.  Stat.,  19,  81. 
JArch.  f.  Hygiene,  11,  160. 


CHAPTER  IV. 
THE  FASTING  METABOLISM. 

The  matter  which  the  animal  organism  derives  from  its  food  is 
applied  substantially  in  three  general  directions :  first,  to  the  main- 
tenance of  those  vital  activities,  such  as  circulation,  respiration, 
secretion,  the  metabolic  activity  of  the  various  tissues,  etc.,  and 
probably  to  some  extent  the  direct  production  of  heat,  which  in 
their  entirety  make  up  the  physical  life  of  the  organism;  second, 
to  the  support  of  those  functions  by  which  the  crude  materials 
ingested  are  prepared  to  nourish  the  body,  that  is,  to  the  work  of 
digestion  and  assimilation;  third,  to  the  production  of  external 
mechanical  work  or  to  the  storage  of  surplus  material  in  the  form 
of  growth  of  tissue. 

Of  these  three  general  functions  of  the  food,  the  one  first  named 
is  obviously  of  fundamental  significance,  and  a  determination  of 
the  nature  and  amount  of  its  demands  constitutes  the  natural  first 
step  in  a  study  of  the  laws  of  nutrition.  For  this  purpose  we  can 
eliminate  the  influence  of  the  other  two  factors  by  keeping  the  ani- 
mal as  nearly  as  possible  in  a  state  of  absolute  rest  and  by  with- 
holding food.  Under  these  circumstances  the  expenditure  of  matter 
from  the  tissues  of  the  body  may  be  taken  as  representing  the 
miminum  demands  of  the  vital  functions.  It  will  therefore  be  both 
logical  and  convenient  to  consider  first,  in  the  present  chapter,  the 
fasting  metabolism  of  the  quiescent  animal,  while  in  succeeding  chap- 
ters we  take  up  the  influence  respectively  of  the  food-supply  and  of 
external  work  upon  metabolism.  The  protein  of  the  food  has  such 
peculiar  and  distinct  functions  in  the  animal  economy  that  it  will 
be  a  matter  of  practical  convenience  to  follow  the  historical  order 
of  investigation  and  consider  *first  the  proteid  metabolism  by  itself 

80 


THE  FASTING   METABOLISM.  81 

and  subsequently  the  total  metabolism  as  shown  by  the  combined 
nitrogen  and  carbon  balance. 

§  i.  The   Proteid  Metabolism. 

Tends  to  Become  Constant. — When  food  is  withheld  from  a 
well-nourished  animal,  particularly  a  carnivorous  animal,  the  proteid 
metabolism  usually  diminishes,  at  first  rapidly  and  more  slowly  later, 
until  within  a  few  days  it  reaches  a  minimum  value  which  may  then 
remain  nearly  unchanged  for  a  considerable  time.  This  was  first 
shown  by  the  investigations  of  Carl  Voit,  in  conjunction  with 
Bischoff  and  later  with  v.  Pettenkofer,  and  has  been  fully  confirmed 
by  later  results. 

The  following  table  shows  the  results  obtained  by  Voit*  in 
several  experiments  upon  a  dog  weighing  about  35  kgs.,  the  pro- 
teid metabolism  being  expressed  in  grams  of  urea  per  day.  As 
noted  in  Chapter  III,  such  results  are  not  absolutely  accurate  and  do 
not  represent  the  total  proteid  metabolism,  but  the  fact  that  they 
are  comparable  is  sufficient  for  our  present  purpose. 


Previous  Food  per  Day. 

2500  Grms. 
Meat. 

1800  Grms. 

Meat; 

250  Grms. 

Fat. 

1500  Grms. 
Meat. 

1500  Grms. 
Meat. 

Nothing. 

Urea  per  day: 
Last  day  of  feeding .... 

Grms. 
180.8 

Grms. 
130.0 

Grms. 
110.8 

Grms. 

110.8 

Grms. 
24.7 

1st 

•  fasting 

60.1 

37.5 

29.7 

26.5 

19.6 

2d       ' 

'         "     

24.9 

23.3 

18.2 

18.6 

15.6 

3d      ' 

'         "     

19.1 

16.7 

17.5 

15.7 

14.9 

4th     « 

•         "     

17.3 

14.8 

14.9 

14.9 

13.2 

5th      ' 

'         "     

12.3 

12.6 

14.2 

14.8 

12.7 

6th      ' 

'         «•     

13.3 

12.8 

13.0 

12.8 

13.0 

7th     ' 

'         "     

12.5 

12.0 

12.1 

12.9 

8th     ' 

,         tt     

10.1 

12.9 

12.1 
11.9 
11.4 

9th      ' 

10th    ' 

«        i< 

Two  Factors  of  Proteid  Metabolism. — In  these,  as  in  many 
similar  experiments,  the  proteid  metabolism  was  quite  unequal  on 
the  last  day  of  the  feeding  and  on  the  first  fasting  day,  but  in  a 


*Zeit.  f.  Biol.,  2,  311. 


82  PRINCIPLES   OF  ANIMAL   NUTRITION. 

comparatively  short  time  it  sank  to  a  minimum  which  was  practi- 
cally the  same  in  all  the  experiments  upon  this  particular  animal, 
viz.,  the  equivalent  of  about  12  grams  of  urea  per  day.  This  mini- 
mum we  may  fairly  regard  as  representing  the  necessary  and  inev- 
itable destruction  of  proteids  involved  in  the  vital  processes  of 
the  organism,  and  therefore  may  consider  as  taking  place  also 
when  the  animal  was  fed.  If,  now,  we  subtract  from  the  total 
urea  excreted  the  12  grams  corresponding  to  the  minimum  de- 
mand of  the  body,  there  is  revealed  the  second  and  variable  factor 
of  the  proteid  metabolism,  which  is  large  in  the  well-fed  animal  but 
rapidly  disappears  during  fasting,  as  the  following  table  shows: 


Previous  Food  per  Day. 

2500  Grms. 
Meat. 

1800  Grms. 

Meat  and 

250  Grms. 

Fat. 

1500  Grms. 
Meat. 

1500  Grms. 
Meat. 

Nothing. 

Urea  per  day: 

Last  day  of  feeding 

1st      "      "    fasting 

2d       "     "        "       

3d       "     «        "       

4th     "     "        "       

5th     "     "        "       

6th     "     "       "       

7th     "     "       "       

8th     "     "       "       

9th     "     "       "       

Grms. 

168.8 

48.1 

12.9 

7.1 

5.3 

0.3 

1.3 

0.5 

-1.9 

Grms. 

118.0 

25.5 

11.3 

4.7 

2.8 

0.6 

0.8 

0.0 

Grms. 
98.8 
17.7 
6.2 
5.5 
2.9 
2.2 
1.0 
0.1 
0.9 

Grms. 

98.8 

14.5 

6.6 

3.7 

2.9 

2.8 

0.8 

0.9 

0.1 

-0.1 

-0.6 

Grms. 
12.7 
7.6 
3.6 
2.9 
1.2 
0.7 
1.0 

10th   "     "        " 

Organized  and  Circulatory  Protein. — It  is  evident  from 
the  above  results,  and  will  appear  still  more  clearly  when  we  come 
to  consider  the  influence  of  the  food-supply  upon  proteid  meta- 
bolism, that  in  addition  to  the  great  mass  of  proteid  tissue  in  the 
body,  whose  metabolism  results  in  the  excretion  of  a  relatively 
small  and  constant  amount  of  nitrogenous  products,  the  well- 
nourished  organism  may  also  contain  variable  amounts  of  nitrogen- 
ous matter  which  is  subject  to  rapid  metabolism  and  which  speed- 
ily disappears  during  fasting.  Voit  employed  the  term  circula- 
tory protein  (Zirkulationseiweiss)  to  designate  this  variable  store 
of  rapidly  metabolized  nitrogenous  matter,  which  he  regards  as 
being  substantially  the  dissolved  protein  which  penetrates  from 


THE  FASTING  METABOLISM.  83 

the  blood  and  lymph  into  the  cells  of  the  tissues,  while  he  termed 
the  protein  of  the  organized  tissues,  which  is  relatively  stable  and 
but  slowly  metabolized,  organized  protein  (Organeiweiss) .  The 
amount  of  the  circulatory  protein  is  small  in  all  cases  as  compared 
with  that  of  the  organized  protein,  its  absolute  amount  being  de- 
pendent, as  the  above  tables  indicate  and  as  will  appear  more 
clearly  in  the  next  chapter,  upon  the  supply  of  proteids  in  the 
food.  Owing  to  its  rapid  metabolism,  however,  it  furnishes  by 
far  the  larger  part  of  the  nitrogenous  waste  products  in  the  liberally 
fed  animal. 

That  the  anatomical  distinctions  implied  in  the  terms  used  by 
Voit  correspond  to  the  actual  facts  of  the  case  has  been  disputed 
and  may  be  open  to  question,  but  for  our  present  purpose  this  does 
not  particularly  concern  us.  The  fact  of  the  existence  of  the  two 
factors  of  pr-oteid  metabolism,  viz.,  a  variable  one,  depending  upon 
the  previous  food-supply  and  a  relatively  constant  one  independ- 
ent of  the  latter  is  fully  established,  by  whatever  names  we  may 
choose  to  call  them. 

A  Minimum  of  Protein  Indispensable. — While  the  proteid 
metabolism  of  the  fasting  animal  is  speedily  reduced  to  relatively 
small  proportions,  it  is  never  entirely  suspended  as  long  as  the 
animal  lives.  Moreover,  to  anticipate  a  portion  of  the  follow- 
ing chapter,  even  the  most  liberal  supply  of  non-nitrogenous 
nutrients  is  powerless  to  suspend  or  very  greatly  reduce  the  pro- 
teid metabolism  of  a  fasting  animal.  A  certain  amount  of  proteid 
metabolism  is  indissolubly  associated  with  the  continuance  of  life, 
and  neither  the  fat  of  the  body  nor  the  non-nitrogenous  ingredients 
supplied  in  the  food  can  perform  these  special  functions  of  protein 
in  the  body. 

§2.  Total  Metabolism. 

Constant  Loss  of  Tissue. — Common  observation,  no  less  than 
scientific  investigation,  teaches  that  a  fasting  animal  suffers  a  con- 
tinual loss  of  tissue.  Such  an  animal  derives  the  energy  required 
for  its  vital  activities  from  the  metabolism  of  its  store  of  proteids 
and  of  fat.  As  regards  the  former,  we  have  just  seen  that  in  a 
short  time,  or  as  soon  as  the  influence  of  the  previous  supply  of 


84 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


proteids  in  the  food  is  exhausted,  the  proteid  metabolism  reaches 
a  minimum  and  thereafter  remains  nearly  constant  for  a  consider- 
able time,  and  subsequent  investigations  have  shown  that  this 
constancy  is  still  more  marked  when  the  proteid  metabolism  is 
computed  per  unit  of  live  weight. 

What  has  thus  been  found  to  be  true  of  the  proteid  metabolism 
has  also  been  shown  to  hold  good  of  the  total  metabolism  of  pro- 
teids plus  body  fat.  As  soon  as  the  influence  of  the  previous  food 
has  disappeared,  the  rate  of  metabolism  of  both  proteids  and  fat 
shows  but  slight  variations  throughout  a  considerable  time.  Of 
the  early  experiments  of  Pettenkofer  and  Voit,  the  following  *  may 
be  cited  as  illustrating  approximately  this  constancy: 


Series  a,  1862. 

Series  6,  1861. 

March  10, 
6th  Day. 

March  14, 
10th  Day. 

April  5, 
2d  Day. 

April  8, 
5th  Day. 

April  11, 
8th  Day. 

Kgs. 
31.21 

Grms. 
104.1 
5.95 

37.18 
107. 

1.19 
3.43 

Kgs. 
30.05 

Grms. 
82.4 
5.23 

32.69 
83. 

1.09 
2.76 

Kgs. 
32.87 

Grms. 

108.7 

11.6 

72.51 

86. 

2.21 
2.62 

Kgs. 

31.67 

Grms. 

100.0 

5.7 

35.63 
103. 

1.13 
3.25 

Kgs. 
30.54 

Carbon  of  excreta 

Nitrogen  of  excreta 

Total  loss: 

Grms. 

93.2 

4.7 

29.38 

Fat 

99.2 

Loss  per  Kg.  live  weight: 

0.96 

Fat 

3.25 

Finkeler  f  determined  the  respiratory  exchange  of  fasting 
guinea-pigs  in  two-hour  periods.  Upon  the  highly  probable  assump- 
tion that  their  proteid  metabolism  was  relatively  small  and  con- 
stant, the  results  of  such  experiments  would  furnish  a  measure  of 
the  relative  intensity  of  the  total  metabolism.  Finkeler 's  average 
results  are  contained  in  the  table  on  the  opposite  page. 

But  a  slight  decrease  in  the  amount  of  oxygen  consumed  is 
observed  in  the  different  stages  of  the  fasting,  while  there  is  a 
marked  decrease  in  the  amount  of  carbon  dioxide  produced.  The 
relation  between  these  two  quantities,  as  expressed  by  the  respira- 
tory quotient,t  shows  us  that  at  the  beginning  of  the  fasting  the 
metabolism  was  largely  at  the  expense  of  the  carbohydrates  of  the 
*  Zeit.  f.  Biol.,  5,  369.     t  Arch.  ges.  Physiol.,  23,  175.      X  Compare  p.  74. 


THE  FASTING   METABOLISM. 


35 


Per  Hour  and  Kg.  Live  Weight. 

Length  of  Fasting, 
Minutes. 

Oxygen  Consumed, 
c.c. 

Carbon  Dioxide 

Excreted, 

c.c. 

Respiratory 
Quotient. 

0 
1468 
2950 

1202.19 
1154.53 
1146.76 

1111.80 
923.75 
811.12 

0.93 
0.80 
0.71 

0 
1575 
3543 
5940 

1250.28 
1226.18 
1241.78 
1192.50 

334 
1712 
3233 

1959.45 
1850.02 
1809.85 

1494.68 
1318.19 
1289.63 

0.76 
0.71 
0.71 

body,  while  as  the  experiment  progressed  the  store  of  carbohydrates 
(glycogen)  in  the  body  was  gradually  exhausted  and  the  meta- 
bolism finally  became  a  fat  metabolism.  Since,  now,  as  will  be 
shown  in  Chapter  VIII,  the  consumption  of  equal  amounts  of  oxygen 
results  in  the  liberation  of  approximately  equal  amounts  of  energy 
whether  that  oxygen  is  employed  to  oxidize  carbohydrates  or  fats, 
Finkeler  concludes  that  the  total  metabolism,  as  measured  in  terms 
of  energy,  was  nearly  constant. 

Lehmann  and  Zuntz  *  have  observed  a  similar  constancy  of 
the  respiratory  exchange  per  unit  of  weight  in  the  case  of  two 
men  fasting  for  eleven  and  six  days  respectively,  while  Munk  f 
found  their  urinary  nitrogen  to  be  also  approximately  constant. 
Magnus-Levy  %  has  likewise  observed  a  similar  constancy  in  the 
respiratory  exchange  of  the  dog  and  of  man  during  fasting,  as  have 
also  Johansson,  Landgren,  Sonden,  &  Tigerstedt  §  for  man. 

Rubner,||  as  a  preliminary  to  his  investigations  upon  the  re- 
placement values  of  the  nutrients,  discusses  this  question  at  some 
length  and  gives  the  results  of  experiments  upon  dogs,  rabbits, 
guinea-pigs  and  fowls,  in  which  the  excretion  of  nitrogen  and  car- 
bon per  unit  weight  shows  a  marked  degree  of  constancy  through 
considerable  periods. 

*Virchow's  Archiv,  131,  Supp.  J  Arch,  ges.  Physiol.,  55,  1. 

ilbid.  §Skand.  Archiv.  f.  Physiol.,  7,  29. 

II  Zeit.  f.  Biol.,  17,  214;  19,  313;  Biologische  Gesetze,  p.  15. 


86  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Metabolism  Proportional  to  Active  Tissue. — In  a  critical 
discussion  of  these  and  other  results  on  fasting  animals,  to  which 
we  shall  have  occasion  to  refer  again  in  Part  II,  E.  Voit  *  shows 
that  a  still  more  constant  relation  is  obtained  when  either  the  pro- 
teid  or  the  total  metabolism  is  compared  with  the  total  mass  of 
proteid  tissue  estimated  to  be  contained  in  the  body  on  the  several 
days  of  the  experiment.  The  total  protein  of  the  body,  however, 
may  be  regarded  as  at  least  an  approximate  measure  of  the  active 
cell  mass,  as  distinguished  from  the  relatively  inactive  cells  of 
adipose  tissue.  It  is  the  vital  activities  of  the  former,  in  the  fast- 
ing animal,  that  mainly  determine  the  amount  of  the  total  meta- 
bolism, the  energy  liberated  being  supplied  in  part  by  the  relatively 
small  amount  of  proteid  metabolism  which  goes  on  in  the  cells  of 
the  fasting  animal,  but  largely  by  the  metabolism  of  fat  supplied 
to  the  active  cells  from  the  adipose  tissue. 

Ratio  of  Proteid  to  Total  Metabolism. — In  the  preceding 
paragraph  it  was  implied  that  the  proteid  metabolism  constitutes 
but  a  small  portion  of  the  total  metabolism  of  the  fasting  animal, 
the  remainder  of  the  necessary  energy  being  supplied,  after  the 
small  store  of  glycogen  in  the  body  is  exhausted,  by  the  metabo- 
lism of  body  fat.  Rubner  f  appears  to  have  been  the  first  to  call 
specific  attention  to  this  aspect  of  the  question.  In  his  investiga- 
tions upon  the  relation  of  size  of  animal  to  total  metabolism  he 
adduces  experimental  results  to  prove  that  this  ratio  is  not  mate- 
rially different  in  large  and  in  small  animals.  The  question  has, 
however,  been  more  recently  discussed  by  E.  Voit  J  from  a  general 
point  of  view,  the  results  of  numerous  investigators  being  summa- 
rized. In  discussing  these  results,  Voit  has  computed  from  the 
nitrogen  and  carbon  balance,  when  these  data  were  available,  in 
substantially  the  manner  described  in  Chapter  VIII,  the  amount  of 
energy  liberated  by  the  metabolism  of  the  protein  and  fat.  lost  by 
the  body.  In  those  instances  in  which  only  the  nitrogen  balance 
was  determined,  he  estimates  the  amount  of  energy  liberated  in 
the  body  from  the  computed  surface  on  the  basis  of  average  results 
with  similar  animals.  (Compare  Chapter  XI,  §  2.)  Taking  this 
amount,  expressed  in  calories,  as  the  measure  of  the  total  meta- 
bolism, and  including  only  experiments  in  which  the  animals 
*  Zeit.  f.  Biol.,  41,  113.     t  Ibid;  19,  557.     \  Ibid.,  41,  167. 


THE  FASTING  METABOLISM. 


«7 


are  believed  to  have  been  in  good  bodily  condition  (well  nourished) 
at  the  beginning  of  the  trials,  he  obtains  the  following  average 
results : 


Live  Weight, 
Kgs. 

Nitrogen  Excretion  per  Day. 

Proteid 

Total, 
Grms. 

Per  Kg. 

Live  Weight, 

Grms. 

Metabolism 
in  %  of  Total 
Metabolism. 

115.0 

63.7 

(28.6 

\  18.7 

1    7.2 

2.7 

0.6 

3.3 

2.1 

6.8 
12.6 
5.1 
3.8 
2.2 
1.2 
0.4 
0.8 
0.7 

0.06 
0.20 
0.18 
0.20 
0.30 
0.46 
0.65 
0.23 
0.34 

7.3 

Man 

15.6 

Dog 

13.2 
10.7 

Rabbit 

13.5 
16.5 

Guinea  pig 

10.8 
7.4 

Hen 

10.0 

As  will  appear  later,  the  total  metabolism  of  a  small  animal  is 
greater  per  unit  of  weight  than  that  of  a  large  animal.  The  above 
figures  show  that  the  same  thing  is  true  of  the  proteid  metabolism. 
When,  however,  the  proteid  metabolism  is  computed  as  a  percent- 
age of  the  total  metabolism,  as  in  the  last  column  of  the  table,  this 
dependence  upon  the  live  weight  disappears.  While  the  figures 
still  show  considerable  variations,  these  are  much  reduced  and 
show  no  connection  with  the  live  weight.  In  other  words,  the  proteid 
metabolism  tends  to  be  a  somewhat  uniform  percentage  of  the 
total  metabolism,  ranging  in  these  experiments,  aside  from  two 
apparently  exceptional  results,  between  10  and  16  per  cent. 

The  individual  experiments  cited  by  Voit  show  a  similar  general 
uniformity,  both  in  the  same  animal  on  successive  days  of  fasting 
and  in  case  of  different  animals.  Thus  twenty-seven  experiments 
on  the  dog  gave  the  following : 


Range  of  Proteid  Metabolism  in  Per  Cent. 

Number  of  Cases. 

of  Total  Metabolism. 

Absolute. 

Per  Cent. 

4 

15 

5 

3 

27 

14.8 

10-14  

55.6 

14-17  

18.5 

More  than  17 

11.1 

100.0 

88  PRINCIPLES  OF  ANIMAL   NUTRITION. 

The  great  majority  of  cases  gave  values  lying  between  10  and 
17  per  cent. 

Effect  of  Body  Fat. — Both  from  the  summary  on  p.  87  and 
from  the  individual  results  cited  by  Voit,  it  is  evident  that  while 
the  proportion  of  energy  supplied  by  the  metabolism  of  pro- 
teids  in  the  fasting  animal  is  normally  small  and  varies  only  within 
rather  narrow  limits,  it  is  still  subject  to  relatively  considerable 
variations.  The  most  important  cause  of  these  variations  in  the 
fasting  animal  under  uniform  external  conditions  appears  to  be 
the  ratio  of  fat  to  protein  in  the  body. 

C.  Voit  *  appears  to  have  first  noted  that  when  fasting  is  pro- 
longed sufficiently  to  nearly  exhaust  the  reserve  of  visible  fat  in 
the  body,  the  proteid  metabolism,  after  remaining  nearly  constant 
or  decreasing  slightly  for  some  days,  as  in  the  examples  just  given, 
begins  to  increase  somewhat  rapidly.  This  increase  Voit  attrib- 
uted to  the  exhaustion  of  the  fat,  the  oxidation  of  which  had  hith- 
erto partially  protected  the  organized  proteids  of  the  body.  Sub- 
sequent investigations,  particularly  Rubner's,f  have  in  general 
confirmed  Voit's  observation,  while  giving  it  a  somewhat  more 
general  form. 

E.  Voit  X  has  recently  reviewed  the  available  experiments  upon 
fasting  metabolism  in  their  bearing  on  this  question.  From  the 
experimental  data  he  computes  or  estimates,  first  the  ratio  of  pro- 
teid to  total  metabolism  (expressed  in  terms  of  energy),  and  second 
the  ratio  of  proteids  to  fat  in  the  body  on  the  several  days  of  each 
experiment.  A  comparison  of  these  ratios  shows  a  very  marked 
correspondence,  a  high  ratio  of  proteids  to  fat  in  the  body  coin- 
ciding with  a  large  proteid  metabolism  compared  with  that  of  fat, 
and  vice  versa.  The  graphic  representations  of  the  relations  as 
given  by  Voit  are  especially  convincing.  Moreover,  the  results 
show  that  the  extent  of  the  proteid  metabolism  does  not  depend 
directly  upon  the  duration  of  the  fasting.  With  different  animals, 
or  with  the  same  animal  under  different  conditions,  a  certain  ratio 
of  proteid  to  total  metabolism  is  attained  whenever  the  correspond- 
ing ratio  of  proteid  tissue  to  fat  in  the  body  is  reached,  whether  this 
be  early  or  late  in  the  experiment. 

The  growing  ratio  of  proteid  to  total  metabolism  in  the  fasting 
*  Zeit.  f.  Biol.,  2,  326.      f  Loc.  cit.     See  p.  86.      X  Zeit.  f.  Biol.,  41,  502. 


THE  FASTING   METABOLISM.  89 

animal  is  explained  by  Voit  to  be  due  to  an  increasing  difficulty  in 
transferring  tne  reserve  fat  from  the  adipose  tissues,  thus  resulting 
in  a  diminution  of  the  amount  of  fat  (or  its  cleavage  products?) 
circulating  in  the  organism.  If  the  body  is  well  supplied  with  fat 
at  the  outset  this  phenomenon  does  not  at  first  appear,  and  the 
ratio  of  proteid  to  total  metabolism  remains  nearly  constant  for  a 
time. 

With  continued  fasting  the  store  of  body  fat  is,  as  has  just  been 
shown,  drawn  upon  much  more  rapidly  than  that  of  protein,  while 
at  the  same  time  the  total  amount  of  the  former  present  at  the 
beginning  of  fasting  is  often  less  than  that  of  the  latter.  As  a 
necessary  result,  the  ratio  of  fat  to  protein  in  the  body  decreases. 
When  this  decrease  passes  a  certain  point,  the  fat  of  the  adipose 
tissue  is  drawn  upon  with  more  and  more  difficulty  for  material  to 
supply  the  demand  for  energy,  and  as  a  result  additional  protein  is 
metabolized  to  make  good  the  deficiency  of  available  fat.  From 
this  time  on,  the  ratio  of  proteid  to  total  metabolism  shows  a  con- 
tinually accelerated  increase.  The  time  when  the  increase  in  the 
proteid  metabolism  becomes  marked  depends  upon  the  original 
condition  of  the  body.  If  the  animal  is  well  nourished,  and  espe- 
cially if  it  contains  large  reserves  of  fat,  the  increase  may  be  long 
deferred  or  even  fail  to  appear  at  all.  If,  on  the  other  hand,  it  is 
poorly  nourished  and  contains  little  fat,  an  increase  of  the  proteid 
metabolism  may  take  place  almost  from  the  outset.  The  following 
three  examples,  cited  by  E.  Voit  from  Rubner's  experiments,  may 
serve  to  illustrate  these  three  types  of  fasting  metabolism: 


Guinea  Pig. 

Dog. 

Rabbit. 

Day  of 
Fasting. 

Proteid 
Metabolism 
in  %  of  Total 
Metabolism. 

Day  of 
Fasting. 

Proteid 
Metabolism 
in  %  of  Total 
Metabolism. 

Day  of 
Fasting. 

Proteid 
Metabolism 
in  %  of  Total 
Metabolism. 

2 

3 

4 

5 

6 

7 

8 

9 

10.4 
11.1 
11.0 
11.9 
11.8 
6.9 
11.2 
10.9 

2-4 
10-11 
12 
13 
14 

16.3 
13.1 
15.5 
17.4 
20.0 

3 

5-7 
9-12 

13-15 

16 

17-18 

16.5 
23.6 
26.5 
29.8 
50.1 
96.4 

90  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Schulze  *  claims  that  this  increase  in  the  proteid  metabolism  of 
the  fasting  animal  is  not,  in  all  cases  at  least,  due  to  lack  of  fat  or 
other  non-nitrogenous  material  to  protect  the  protein  from  destruc- 
tion. He  advances  the  hypothesis  that  the  loss  of  protein  incident 
to  the  fasting  so  injures  the  cells  that  finally  many  of  them  die 
and  the  protein  of  their  protoplasm  becomes  part  of  the  circula- 
tory protein  of  the  body  and  is  rapidly  decomposed,  thus  giving 
rise  to  an  increased  excretion  of  nitrogen. 

While  it  is  not  impossible  that  this  ingenious  hypothesis  has 
some  basis  of  fact,  Kaufmann,f  in  a  quite  full  review  of  the  litera- 
ture of  the  subject,  together  with  original  experiments,  shows 
that  it  can  by  no  means  supplant  Voit's  explanation.  He  points 
out  in  particular  that  the  time  when  the  increase  in  the  proteid 
metabolism  begins  seems  to  bear  no  relation  to  the  loss  of  protein 
which  the  body  has  sustained,  while,  on  the  other  hand,  it  coin- 
cides quite  closely  with  the  time  when  the  supply  of  visible  fat  is 
nearly  exhausted.  £ 

Summary. — In  the  light  of  the  facts  set  forth  in  the  foregoing 
paragraphs  we  may  sketch  the  general  outlines  of  the  fasting  meta- 
bolism somewhat  as  follows: 

In  the  early  stages  of  fasting,  particularly  if  the  previous  food 
has  contained  an  abundance  of  proteids,  the  proteid  metabolism 
may  be  considerable.  As  the  effect  of  the  previous  food  disappears, 
however,  and  the  store  of  "  circulatory  protein  "  in  the  body  is  ex- 
hausted, the  proteid  metabolism  speedily  falls  to  the  minimum 
amount  required  for  the  vital  activities  of  the  protoplasm,  and  the 
remaining  demands  of  the  body  for  energy  are  supplied  by  the 
metabolism  of  the  stored-up  fat.  If  the  latter  is  fairly  abundant, 
this  stage  may  last  several  days,  the  total  metabolism  remaining 
nearly  constant  and  the  proteids  supplying  a  nearly  constant  pro- 
portion of  the  necessary  energy  (according  to  E.  Voit  about  15-16 
per  cent.).  Sooner  or  later,  however  (unless  in  a  very  fat  animal), 
the  supply  of  fat  from  the  adipose  tissue  begins  to  flag.  The  de- 
mand for  energy,  however,  remains  unabated,  and  as  the  fat-supply 
falls  off,  more  and  more  protein  is  metabolized  in  its  place,  until  at 

*Arch.  ges.  Physiol.,  76,  379.  fZeit.  f.  Biol.,  41,  75. 

\  Compare  also  E.  Voit's  critique  of  Schulze's  investigations.  (Zeit.  f. 
Biol.,  41,  550.) 


THE  FASTING   METABOLISM.  91 

last  the  metabolism  may  even  become  almost  entirely  proteid  in 
its  character.  We  have  in  these  facts  the  first  of  the  numerous 
illustrations  which  we  shall  meet  in  the  course  of  this  discussion 
of  the  plasticity  of  the  organism  in  adapting  itself  to  differences 
in  the  food-supply,  and  of  the  controlling  influence  exerted  upon 
the  course  of  its  metabolism  by  the  demand  for  energy. 

The  Intermediary  Metabolism. — The  prime  object  of  the 
metabolism  of  the  quiescent  fasting  animal  is,  as  already  pointed 
out,  to  supply  energy  for  the  performance  of  the  vital  functions. 

Mention  has  already  been  made  in  Chapter  II  of  the  hypothesis 
that  the  immediate  source  of  energy  to  the  cells  of  both  muscles 
and  glands  is  the  metabolism  of  carbohydrate  material.  This 
hypothesis  in  effect  regards  the  metabolism  of  the  fasting  animal  as 
divisible  into  three  processes :  first,  the  splitting  up  of  the  proteids, 
yielding  urea  and  fat;  second,  the  partial  oxidation  of  fat,  whether 
derived  from  the  proteids  or  from  the  adipose  tissue,  yielding  dex- 
trose; third,  the  oxidation  of  the  resulting  dextrose  in  the  tissues. 

So  far  as  the  kind  and  amount  of  excretory  products  are  con- 
cerned, it  of  course  makes  no  difference  whether  the  metabolism 
takes  place  in  accordance  with  this  hypothesis  or  whether  the 
proteids  and  fat  are  oxidized  directly  in  the  tissues.  In  either 
case  the  fasting  animal  lives  upon  its  store  of  proteids  and  fat,  and 
the  resulting  excretory  products,  as  well  as  the  amount  of  heat 
produced,  are  qualitatively  and  quantitatively  the  same,  so  that 
the  coincidence  observed  by  Kaufmann  *  between  the  observed 
results  and  those  computed  from  his  equations  is  without  special 
significance  in  this  case. 

There  is,  nevertheless,  an  important  and  essential  difference  in 
the  two  views.  If  we  regard  the  proteids  and  fat  as  yielding  up 
their  energy  directly  for  the  vital  activities,  then  all  the  energy 
thus  liberated  is  available  for  this  purpose.  If,  on  the  contrary, 
we  suppose  these  substances  to  be  first  partially  metabolized  in  the 
liver  or  elsewhere  in  the  organism,  then  only  that  portion  of  their 
potential  energy  which  is  contained  in  the  resulting  dextrose  is 
available  directly  for  the  general  purposes  of  the  body.  The  re- 
mainder of  their  energy  is  liberated  as  heat  during  the  preliminary 
*Archives  de  Physiologie,  1896,  pp.  329  and  352. 


92  PRINCIPLES   OF  ANIMAL    NUTRITION. 

metabolism,  and  while  contributing  its  quota  towards  maintaining 
the  normal  temperature  of  the  body  is  not  directly  available  for 
other  purposes.  In  other  words,  the  question  is  not  one  as  to  the 
total  energy  liberated,  but  as  to  its  form  and  distribution.  As 
regards  the  fasting  animal  itself,  the  question  is  of  minor  impor- 
tance; but,  as  will  appear  in  subsequent  chapters,  it  materially 
affects  our  views  as  to  the  relative  values  of  the  several  nutrients 
of  the  food. 


CHAPTER  V. 
THE  RELATIONS  OF  METABOLISM  TO  FOOD-SUPPLY. 

The  metabolism  of  the  fasting  animal  was  regarded  in  the  pre- 
ceding chapter  as  representing  the  essential  demands  of  the  vital 
functions  for  a  supply  of  matter  as  a  vehicle  of  potential  energy. 
Under  these  conditions,  as  we  have  seen,  the  total  metabolism  bears 
a  close  relation  to  the  mass  of  active  tissue,  while  the  qualitative 
character  of  the  metabolism,  that  is  the  ratio  of  proteid  to  non- 
proteid  matter  consumed,  appears  to  be  likewise  constant  for  any 
given  condition  of  the  body,  depending  upon  the  relative  supply 
of  proteids  and  non-nitrogenous  matters  to  the  active  cells.  When 
food  is  given  to  such  an  animal  the  conditions  are  modified  in  essen- 
tially three  ways: 

First,  to  the  metabolism  incident  to  the  fasting  state  is  added 
that  required  to  supply  the  energy  consumed  in  the  digestion  and 
assimilation  of  the  food. 

Second,  the  food-supply  may  alter  the  proportions  in  which  the 
various  nutrients  are  supplied  to  the  active  cells,  and  thus  affect 
the  metabolism  qualitatively,  giving  rise  to  a  relatively  greater 
or  less  metabolism  of  proteids,  fats,  carbohydrates,  etc. 

Third,  the  food-supply  may  be  in  excess  of  the  requirements  of 
the  body  and  lead  to  a  storage  of  matter  of  one  sort  or  another. 

The  quantitative  relations  of  the  food-supply  to  the  total 
metabolism  and  to  the  storage  of  matter  and  energy  in  the  body 
may  be  most  satisfactorily  considered  upon  the  basis  of  the  amounts 
of  energy  involved.  Accordingly  we  may  content  ourselves  here 
with  a  simple  mention  of  this  side  of  the  question,  deferring  a  dis- 
cussion of  it  to  Part  II  and  confining  the  present  chapter  largely 
to  a  study  of  the  qualitative  changes  in  the  metabolism  brought 
about  by  variations  in  the  food-supply.  As  in  the  previous  chapter, 
it  will  be  convenient  to  consider  the  relations  of  the  proteids  of  the 

93 


94  PRINCIPLES   OF  ANIMAL    NUTRITION. 

food  and  of  the  body  separately  from  those  of  the  non-nitrogenous 
nutrients. 

§  i.  The  Proteid  Supply. 
The  effects  of  the  proteid  supply  upon  metabolism  may  be  most 
readily  and  clearly  traced  in  experiments  in  which  the  food  consists 
solely,  or  nearly  so,  of  proteids,  deferring  to  the  next  section  a 
consideration  of  the  modifications  introduced  by  the  presence  of 
non-nitrogenous  nutrients  in  the  food. 

Effects  on  Proteid  Metabolism. 

Our  knowledge  of  the  relations  between  proteid  supply  and 
proteid  metabolism  in  the  animal  body  is  based  upon  the  funda- 
mental investigations  of  Bischoff  &  Voit,*  Carl  Voit,f  and  Petten- 
kofer  &  Voit,f  at  Munich.  The  results  of  these  researches  have 
been  so  fully  confirmed  by  subsequent  investigators  and  have 
become  so  much  the  common  property  of  science  that  it  is  unneces- 
sary to  do  more  than  summarize  them  here,  with  the  addition  of 
such  examples  as  may  seem  best  adapted  to  illustrate  them. 

Amount  Required  to  Reach  Nitrogen  Equilibrium. — As 
we  have  seen,  the  proteid  metabolism  of  a  fasting  animal  speedily 
reaches  a  minimum  which  we  may  probably  regard  as  representing, 
at  least  approximately,  the  amount  of  proteids  necessarily  broken 
down  and  oxidized  in  the  vital  activities  of  the  tissues  of  the  body. 
If  we  supply  proteid  food  to  such  an  animal,  we  might  naturally 
be  inclined  to  expect  that  the  first  use  to  which  the  proteids  of 
the  food  would  be  put  would  be  to  stop  the  loss  of  proteid  tissue, 
and  that  if  as  much  proteid  was  supplied  in  the  food  as  was  being 
metabolized  in  the  body,  nitrogen  equilibrium  would  be  reached. 

Experiment  shows,  however,  that  this  is  very  far  from  being 
the  case.  Even  the  least  amount  of  proteids  causes  a  prompt 
increase  in  the  urinary  nitrogen,  and  each  successive  addition  of 
proteids  results  in  a  further  increase,  so  that  it  is  not  until  the  food 
proteids  largely  exceed  the  amount  metabolized  during  fasting 
that    nitrogen    equilibrium    is    reached.     Thus  Bischoff  &  Voit,  J 

*  Gesetze  der  Ernilhrung  des  Fleischfressers,  1860. 

f  Published  chiefly  in  the  Annalen  der  Chemie  und  Pharmacie  and  the 
Zeitschrift  fur  Biologie.  See  also  Voit,  "Physiologie  des  Stoffwechsels,"  in 
Herman's  Handbuch  der  Physiologie. 

%  Zeit.  f.  Biol.,  3,  29  and  33. 


THE  RELATIONS   OF  METABOLISM   TO  FOOD-SUPPLY. 


95 


in  a  series  of  experiments  upon  a  dog  fed  exclusively  on  lean  meat, 
obtained  the  results  shown  in  the  following  table,  the  proteid 
metabolism  being  expressed  in  terms  of  flesh  with  its  normal  water 
content  (N  X  29.4)  instead  of  dry  proteids: 


"  Flesh  " 
Metabolized. 


Gain  or  Loss 
of  Flesh. 


1858. 
Aug.  25 

"  26 

"  27  and  28 . 

"  29-Sept.  1. 
Sept.  2  and  3.  . . 

"   4  "  5.  .. 

"  6  "  7.  .. 


0 

0 

300 

600 

900 

1200 

1500 


223 
190 
379 
665 
941 
1180 
1446 


-223 
-190 

-  79 

-  65 

-  41 
+  20 
+   54 


Nov.  16 

1800 

1500 

1200 

900 

600 

300 

176 

0 

1764 
1510 
1234 
945 
682 
453 
368 
226 

+  36 

"     17  and  18 

-    10 

"     18    "    19 

-   34 

"     20    "    21 

—  45 

"     22    "    23 

-  82 

"     24    "    25.  .  . 

-153 
-192 

"     26    "    27 

"     28-Dec.  1 

—  226 

A  much  later  series  by  E.  Voit  &  Korkunoff,*  in  which  the 
results  were  determined  in  terms  of  nitrogen,  may  be  cited  to  illus- 
trate the  same  point.  The  food  was  lean  meat  from  which  the 
extractives  had  been  removed  by  treatment  with  cold  water.  It 
contained  1.25  to  1.96  per  cent,  of  fat. 


Food. 

Nitrogen  in 

Food, 
Grms. 

Feces  and  Urine , 
Grms. 

Gain  or  Loss, 
Grms. 

0 

4.10 

5.74 

6.77 

7.59 

8.20 

10.24 

11.99 

15.58 

13.68 

3.996 

5.558 

6.495 

7.217 

7.804 

8.726 

10.579 

12.052 

14.314 

13.622 

3  996 

100  grms.  extracted  meat 

140      "            "            "     

-1.458 
-0  755 

165      "            "            "     

—  0  447 

185     "             "           "     

-0  214 

200      "             "            "     . 

-0  526 

230      "             "           "     . 

-0.339 

360      "            "            "... 

-0  062 

410     "            "            "     

+  1  266 

460      "            "            "     

+  0  058 

*  Zeit.  f.  Biol.,  32,  67. 


96  PRINCIPLES   OF  ANIMAL   NUTRITION. 

The  proteid  supply  gradually  overtakes  the  proteid  metabolism, 
but  when  only  proteids  are  fed  the  supply  must  largely  exceed  the 
fasting  metabolism  in  order  to  attain  nitrogen  equilibrium.  E. 
Voit  has  endeavored  to  obtain  a  numerical  expression  for  this 
relation  by  taking  as  the  basis  of  comparison  the  fasting  meta- 
bolism. He  estimates  (loc.  cit.,  p.  101)  that  of  the  total  nitro- 
gen excretion  of  a  fasting  animal  81.55  per  cent,  is  derived  from 
true  proteids  and  18.45  per  cent,  from  the  extractives  of  the 
muscles.  Since  the  food  in  his  experiments  consisted  substan- 
tially of  true  proteids,  he  compares  its  nitrogen  with  81.55  per 
cent,  of  the  nitrogen  of  the  excreta  and  thus  finds  that  the  mini- 
mum supply  of  proteid  nitrogen  required  to  reach  nitrogen  equi- 
librium was  between  3.67  and  4.18  times  that  metabolized  during 
fasting,  the  true  value  being  estimated  at  3.68.  Five  other  less 
exact  experiments  gave  confirmatory  results  and  similar  confirma- 
tion is  found  in  the  experimental  results  of  C.  Voit. 

Effect  of  Excess  of  Proteids. — If  the  supply  of  proteids  to 
a  mature  animal  be  still  further  increased  after  nitrogen  equilib- 
rium is  reached,  the  excess  of  proteids  is  promptly  metabolized, 
its  nitrogen  reappearing  in  the  excreta.  In  other  words,  the  ex- 
cretory nitrogen  keeps  pace  with  the  supply  of  nitrogen  in  the  food. 
The  experiments  by  Bischoff  &  Voit  just  cited  serve  to  illustrate 
this  fact  also.  Approximate  nitrogen  equilibrium  was  reached  on 
1200-1500  grams  of  meat,  but  in  other  trials  even  double  this 
supply  caused  but  a  slight  apparent  gain  of  nitrogen,  and  it  is 
probable  that  if  the  total  urinary  nitrogen  had  been  determined 
instead  of  the  urea,  and  account  taken  of  the  nitrogen  of  the 
feces,  even  this  small  difference  would  have  disappeared. 

It  is  needless  to  multiply  examples  of  this  perfectly  well-estab- 
lished fact.  The  animal  body  puts  itself  very  promptly  into  equi- 
librium with  its  nitrogen  supply  and  no  considerable  or  long-Con- 
tinued gain  of  proteid  tissue  can  be  produced  in  the  mature  animal 
by  even  the  most  liberal  supply  of  proteid  food. 

Transitory  Storage  of  Proteids. — But  while  no  continued 
gain  of  protein  by  the  body  can  be  brought  about  by  additions  to 
the  proteid  food,  nevertheless,  during  the  first  few  days  following 
such  an  increase  in  the  proteid  supply  a  transitory  storage  of  nitro- 
gen takes  place.     Conversely,  too,  a  decrease  in  the  proteid  supply 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY. 


9  7 


causes  at  first  a  loss  of  nitrogen  from  the  body,  which,  however, 
unless  the  new  supply  of  proteids  falls  below  a  certain  minimum  is 
as  transitory  as  the  gain  in  the  other  case.  In  other  words,  while 
the  nitrogen  excretion  of  the  mature  animal  is  in  the  long  run 
equal  to  the  supply  in  the  food,  when  the  amount  of  the  latter 
is  changed  the  full  effect  on  the  excretion  is  not  realized  at  once. 
This  fact  is  well  illustrated  by  the  following  selection  from 
C.  Yoit's  investigations  upon  the  dog,*  the  results  being  expressed 
in  terms  of  "  flesh  " : 


New 

Ration. 
Grms. 
Meat. 

"  Flesh  "  Metabolized  per  Day. 

Previous 

On 

Previous 

Ration. 

Grms. 

On  New  Ration. 

Grms. 
Meat. 

1st  Day. 
Grms. 

2d  Day. 
Grms. 

3d  Day. 
Grms. 

4th  Day. 
Grms. 

5th  Day. 
Grms. 

6th  Day. 
Grms. 

7th  Day. 
Grms. 

1800 

2500 

1800 

2153 

2480 

2532 

500 

1500 

547 

1222 

1310 

1390 

1410 

1440 

1450 

1500 

0 

1500 

176 

1267 

1393 

1404 

2500 

2000 

2500 

2229 

1970 

1500 

1000 

1500 

1153 

1086 

1088 

1080 

1027 

1000 

5000 

1000 

706 

610 

623 

560 

An  example  of  the  same  fact  is  found  in  the  experiments  cited 
on  p.  81,  in  which  &\.  proteid  food  was  withdrawn,  the  nitrogen 
excretion  falling  rapidly,  but  reaching  its  minimum  only  after  three 
or  four  days. 

Voit  explained  the  facts  just  adduced  as  the  consequence  of 
the  difference  between  organized  and  circulatory  proteids  already 
noted  on  p.  82.  According  to  this  hypothesis,  the  amount  of 
the  proteid  metabolism  is  chiefly  determined  by  the  store  of  circu- 
latory proteids  in  the  body.  The  ingestion  of  additional  proteids 
increases  the  amount  of  these  circulatory  proteids  in  the  body,  and 
as  a  consequence  the  proteid  metabolism  increases  until  the  nitrogen 
excretion  overtakes  the  supply.  Similarly,  a  decrease  in  the  pro- 
teid food  has  the  converse  effect. 

Proteid  Metabolism  and  Nitrogen  Excretion. — Up  to  this 
point,  following  common  usage,  the  terms  nitrogen  excretion  and 
proteid  metabolism  have  been  employed  as  practically  synony- 
mous. In  one  sense  this  usage  is  correct,  but  it  is  liable  to  give 
*  E.  v.  Wolff,  Ernahrung  Landw.  Nutzthiere,  p  271. 


98  PRINCIPLES   OF  ANIMAL   NUTRITION. 

rise  to  a  misconception.  It  is  perfectly  true  that  the  presence  of 
one  gram,  e.g.,  of  nitrogen  in  the  urine,  implies  that  about  six  grams 
of  protein  have  yielded  up  their  nitrogen  in  the  form  of  urea  or 
other  metabolic  products  and  therefore  have  ceased  to  exist  as  pro- 
tein. It  by  no  means  follows  from  this,  however,  that  this  protein 
has  been  completely  oxidized  to  carbon  dioxide  and  water.  We 
have  already  seen  (Chapter  II,  p.  48)  that  the  abstraction  of  the 
elements  of  urea  from  protein  leaves  a  non-nitrogenous  residue 
equal  to  nearly  tworthirds  of  the  protein,  and  that  there  is  reason 
to  believe  that  this  residue  may,  according  to  circumstances,  be 
oxidized  to  supply  energy  or  give  rise  to  a  production  of  glycogen 
or  of  fat.  In  other  words,  the  separation  of  its  nitrogen  from  pro- 
tein and  the  complete  oxidation  of  its  carbon  and  hydrogen  are 
two  distinct  things.  When,  therefore,  we  assert,  on  the  basis  of 
the  evidence  noted  above,  that  the  proteid  metabolism  of  the  mature 
animal  is  determined  by  the  supply  of  proteids  in  the  food,  what 
we  really  mean  is  that  the  cleavage  of  proteids  and  the  excretion 
of  their  nitrogen  is  so  determined. 

Rate  of  Nitrogen  Excretion. — A  consideration  of  the  course 
of  the  nitrogen  excretion  after  a  meal  of  proteids  is  calculated  to 
throw  light  upon  the  relations  of  nitrogen  cleavage  to  the  total 
metabolism  of  the  proteids.  The  early  investigations  of  Becher, 
Voit,  Panum,  Forster,  and  Falck  showed  that  when  proteids  are 
given  to  a  fasting  animal  the  rate  of  nitrogen  excretion  shows  a 
rapid  increase,  reaching  a  maximum  within  a  few  hours. 

Feder  *  observed  the  maximum  rate  of  nitrogen  excretion  by 
dogs  in  different  experiments  between  the  fifth  and  eighth  hour 
after  a  meal  of  meat.  From  this  point  the  rate  of  excretion  de- 
creased less  rapidly  than  it  had  increased  and  continued  to  decrease 
until  about  thirty-six  hours  after  the  meal. 

Graff enberger,f  experimenting  upon  himself,  obtained  similar 
results  after  the  consumption  of  fibrin,  gelatin,  and  asparagin,  while 
the  results  with  a  commercial  " meat  peptone"  were  markedly 
different;  and  Rosemann,J  in  studies  upon  the  rate  of  nitrogen 
excretion  by  man,  traces  clearly  a  similar  influence  of  the  ingestion 
of  nitrogenous  food,  while  Krummacher's  §  results   on  dogs  fully 

*  Zeit.  f.  Biol.,  17,  531 ;  Thier  Chem.  Ber.,  12,  402.    t  Zeit.  f.  Biol.,  28,  318. 
%  Arch.  ges.  Physiol.,  65,  343.  §  Zeit.  f.  Biol.,  36, 481, 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY. 


99 


confirm  those  of  Feder.  Sherman  and  Hawk  *  have  likewise  found 
the  curve  of  nitrogen  excretion  by  man  after  the  ingestion  of  lean 
meat  to  show  the  same  general  form  observed  by  Feder  and  by  Graf- 
fenberger. 

Nitrogen  Cleavage  Independent  of  Total  Metabolism. — 
Kaufmann,f  by  the  method  outlined  in  Chapter  VIII,  has  made 
a  series  of  determinations  of  the  nitrogen  excretion,  respiratory 
exchange,  and  heat  production  of  dogs  during  the  time  when  nitro- 
gen cleavage  is  most  active,  i.e.,  from  the  second  to  the  seventh  hour 
after  a  full  meal  of  meat.  From  his  theoretical  equations  for  the 
complete  metabolism  of  proteids  (pp.  51  &  75)  he  computes  the 
respiratory  exchange  and  heat  production  corresponding  to  the 
observed  excretion  of  urinary  nitrogen  and  compares  them  with 
the  actual  results  per  hour  as  follows: 


Proteid 

Computed. 

Observed. 

Meta- 
bolism.J 

C0a 

o, 

Heat  Pro- 

CO, 

o, 

Heat  Pro- 

' 

Excreted. 

Consumed. 

duction. 

jExcreted. 

Consumed. 

duction. 

Liters. 

Liters. 

Cals. 

Liters. 

Liters. 

Cals. 

No.  1... 

9.329 

8.132 

9.745 

45.0 

5.953 

6.767 

30.6 

"     2... 

9.926 

8.565 

10.373 

48.0 

7.064 

7.972 

34.6 

"     3... 

9.350 

8.153 

9.771 

45.4 

7.161 

8.236 

34.0 

"     4... 

9.540 

8.231 

9.864 

45.8 

7.398 

8.673 

34.0 

"     5... 

6.632 

5.783 

6.930 

32.0 

5.228 

6.596 

27.7 

"     6... 

9.491 

8.276 

9.918 

46.1 

6.393 

7.813 

29.7 

"     7... 

8.685 

7.573 

9.075 

42.2 

6.325 

7.730 

29.0 

"     8... 

9.958 

8.683 

10.406 

48.4 

6.702 

7.903 

33.6 

"     9... 

8.928 

7.785 

9.235 

43.0 

6.062 

7.916 

35.3 

"   10... 

10.553 

9.202 

11.027 

51.0 

7.125 

8.589 

32.7 

But  a  glance  is  needed  to  show  that  the  total  metabolism, 
whether  measured  by  the  gaseous  exchange  or  by  the  heat  produc- 
tion, is  much  less  than  that  computed,  which  is  equivalent  to  saying 
that  the  non-nitrogenous  residue  of  the  proteids  was  not  completely 
oxidized.  Gruber,§  whose  experimental  results  upon  the  rate  of 
nitrogen  excretion  fully  confirm  those  above  cited,  has  shown  very 
clearly  the   bearing   of  these   facts.      He   points  out  that  if  we 

*Amer.  Jour.  Physiol.,  4,  25. 
t  Archives  de  Physiologie,  1896,  pp.  346  and  768. 

%  Kaufmann's  factor  for  proteids,  derived  from  the  formula  CnHn!iNi8Oa,S, 
is  6.39. 

§  Zeit.  f.  Biol.,  42,  407. 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


regard  the  nitrogen  excretion  as  denoting  the  complete  metabo- 
lism to  carbon  dioxide,  water,  urea,  etc.,  of  a  corresponding 
amount  of  proteids,  we  get  figures  for  the  total  evolution  of 
energy  (heat)  in  the  organism  which  are  entirely  incompatible 
with  those  derived  from  other  considerations.  For  example,  a 
daily  diet  of  1500  grams  of  lean  meat  given  to  a  dog  not  only  suf- 
ficed to  supply  the  demands  for  energy  but  produced  a  storage  of  fat 
in  the  body.  The  total  daily  production  of  heat,  computed  from 
the  results  of  respiration  experiments  (see  Chapter  VIII),  was 
1060.2  Cals.,  equivalent  to  88.3  Cals.  in  two  hours,  which  must  have 
been  derived  essentially  from  the  metabolism  of  proteids.  If,  how- 
ever, we  compute  the  evolution  of  energy  from  the  results  of  the 
nitrogen  excretion  as  determined  in  two-hour  periods,  we  get  strik- 
ingly variable  results. 


Hour. 

Urinary  Nitrogen, 
Grams. 

Equivalent  Energy,* 
Cals. 

9   11 

3.11 
5.71 
6.62 
6.98 
6.35 
6.04 
5.08 
2.65 
1.24 

80  6 

11-1      .                                         

148  2 

1-3                                              

171  6 

3-5                                      .    .. 

181  2 

5-7 

165  1 

7-9 

156  0 

9-11 

132  6 

68  9 

7-9 

32  5 

The  heat  production  as  thus  computed  varies  from  over  twice 
the  average  two-hour  rate  to  an  amount  equal  to  scarcely  more  than 
one  half  of  the  average  fasting  metabolism  of  the  same  animal 
(62  Cals.  per  two  hours).  Such  fluctuations  are  entirely  inconsistent 
with  all  data  as  to  the  heat  production  of  the  body,  which,  as  we 
shall  see  later,  appears  to  go  on  with  a  remarkable  degree  of  uni- 
formity under  uniform  conditions.  The  only  reasonable  conclu- 
sion, then,  appears  to  be  that  the  nitrogen  cleavage  and  the  total 
oxidation  of  the  proteids  are  distinct  and  at  least  largely  inde- 
pendent processes. 

Gruber's  explanation  of  these  facts  is  substantially  as  follows: 
It  is  well  established  that  a  relatively  constant  composition  of  the 
blood  and  of  the  fluids  of  the  body  generally  is  an  essential  condi- 
*  One  gram  N  equivalent  to  26  Cals.     See  Chapter  VIII. 


THE  RELATIONS   OF  METABOLISM   TO  FOOD-SUPPLY.       101 

tion  of  normal  physiological  activity.  It  has  been  repeatedly 
demonstrated,  however,  that  when  the  period  of  growth  is  past,  the 
animal  body  has  not  the  ability  to  produce  any  material  amount  of 
proteid  tissue.  A  large  supply  of  proteid  food,  then,  necessarily 
tends  to  alter  the  composition  of  the  blood  and  other  fluids  of  the 
body,  and  the  nitrogen  cleavage  is  evidently  an  effort  on  the  part 
of  the  organism  to  counteract  this  effect  by  splitting  off  from  the 
proteids  a  nitrogenous  group  which  can  be  rapidly  excreted,  leav- 
ing a  non-nitrogenous  residue  which,  so  far  as  it  is  not  immediately 
needed  to  supply  energy,  is  capable  of  storage  in  the  relatively  inert 
and  insoluble  forms  of  glycogen  and  of  fat. 

According  to  Rosemann,*  the  rapid  increase  in  the  nitrogen  ex- 
cretion after  a  meal  arises  from  two  concurrent  causes:  first,  a 
direct  stimulus  to  the  proteid  metabolism,  due  to  the  rapid  increase 
of  proteids  and  their  digestion  products  in  the  blood,  which  is 
somewhat  transitory  in  character;  and,  second,  the  effect  of  a 
larger  relative  supply  of  proteids  in  causing,  according  to  well- 
known  physico-chemical  laws,  a  relatively  larger  number  of  mole- 
cules of  these  substances  to  enter  into  reactions  with  the  cell  proto- 
plasm. The  accompanying  graphic  representation  by  Gruber  f  of 
the  course  of  the  nitrogen  excretion  of  a  dog  on  the  second  day  of 


A 

[ 

\ 

.  f 

t 

\ 

I 

\ 

•\ 

\ 

I 

i 

\ 

10  IS  U16  18  20  22  24 


10  12  14.16  18  20  22  24  2  4  6  8  10  12  14.16  18  20  22  24 


10  12  1AJ6  l,  2u  -J2  .i 


RATE  OF  NITROGEN   EXCRETION   PER  TWO  HOURS. 


feeding  with  1000  grams  of  lean  meat  and  on  the  three  following 

fasting  days  shows  plainly  the  sudden  stimulation  of  the  excretion 

*  hoc.  eit.  t  Loc.  cit.,  p.  421. 


102  PRINCIPLES   OF  ANIMAL   NUTRITION. 

at  first  and  the  fall,  rapid  at  first,  and  then  very  gradual,  until  the 
minimum  of  the  fasting  excretion  is  reached  about  the  third  day. 

On  the  other  hand,  Rjasantzeff  *  and  Shepski  \  ascribe  the  in- 
crease in  the  nitrogen  cleavage  after  a  meal  to  the  increase  in  the 
digestive  work  rather  than  to  the  proteids  as  such.  They  find  it 
possible,  by  stimulating  the  activity  of  the  digestive  organs  without 
introducing  food,  to  considerably  increase  the  nitrogen  excretion 
in  the  urine,  while,  on  the  other  hand,  the  introduction  of  proteid 
food  through  a  gastric  fistula  produced  little  or  no  effect.  They 
also  find  the  increase  with  the  same  amount  of  food  nitrogen  to  be 
proportional  to  the  (estimated?)  amount  of  digestive  work,  but  seem 
to  offer  no  explanation  of  the  equality  of  nitrogen  cleavage  and 
nitrogen  supply. 

Cause  of  Transitory  Storage. — As  already  noted  (p.  96), 
any  change  in  the  rate  of  proteid  supply  in  the  food,  while  resulting 
ultimately  in  a  corresponding  change  in  the  rate  of  nitrogen  excre- 
tion, gives  rise  to  a  transitory  gain  or  loss  of  nitrogen  by  the  body, 
which  was  interpreted  by  Voit  as  consisting  in  a  corresponding 
change  in  the  stock  of  "circulatory  protein"  in  the  body.  The 
facts  which  we  have  just  been  considering  permit  us  to  trace  some- 
what more  fully  the  details  of  the  phenomenon.  Gruber  points 
out  that  while  the  larger  part  of  the  nitrogen  cleavage  consequent 
upon  a  single  meal  of  proteids  takes  place  within  a  few  hours,  the 
remainder  is  prolonged  over  two  or  three  days,  as  in  the  case  illus- 
trated above,  while  he  likewise  shows  experimentally  that  this 
effect  is  not  due  to  a  retention  of  the  nitrogenous  metabolic  prod- 
ucts, but  represents  the  actual  course  of  nitrogen  cleavage. 

Such  being  the  case,  the  transitory  gain  or  loss  incident  to  a 
change  in  the  rate  of  proteid  supply  is  most  simply  explained  as 
the  result  of  a  superposition  of  the  daily  curves.  Let  it  be  assumed, 
for  example,  that  80  per  cent,  of  the  nitrogen  cleavage  incident  to 
a  single  meal  of  proteids  takes  place  on  the  first  day,  13  per  cent, 
on  the  second,  5  percent,  on  the  third,  and  2  per  cent,  on  the  fourth. 
Then  if  we  give  to  a  fasting  animal  an  amount  of  proteids  contain- 
ing 100  grams  of  nitrogen  for  five  successive  days  and  then  with- 
draw the  food,  the  food  nitrogen  will  be  excreted  as  follows  on  the 
several  days: 

*  Jahr.  Thier  Chem.,  26,  349.  f  Ibid.,  30,  711. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       103 


Feeding. 

Fasting. 

1st 
Day. 

2d 
Day. 

3d 
Day. 

4th 
Day. 

5th 
Day. 

1st 
Day. 

2d 
Day. 

3d 
Day. 

From  food  of  1st  day  of  feeding .... 
"         «     «2d      tt     «       « 

"         "     "3d      "     "       " 

"     "4th    "     "       "       .... 
it     «5th    tt     tt       tt 

Total 

80 
80 

13 

80 

93 

5 
13 

80 

98 

2 
5 

13 

80 

100 

2 

5 

13 

80 

100 

2 
5 
13 

20 

2 

5 

7 

2 
2 

On  the  above  assumptions,  there  remained  in  the  body  at  the 
end  of  the  first  day  20  grams  of  nitrogen  in  the  form  of  unmeta- 
bolized  proteids.  At  the  end  of  the  second  day  this  had  increased 
to  27  grams,  and  at  the  end  of  the  third  day  had  reached  the  maxi- 
mum of  29  grams.  At  the  end  of  the  first  day's  fasting  it  had  fallen 
to  9  grams,  at  the  end  of  the  second  day  to  2  grams,  and  at  the  end 
of  the  third  day  to  zero.  In  other  words,  the  transitory  storage 
of  proteids  observed  by  Voit  and  others  is  explained  by  Gruber  as 
due  to  the  fact  that  the  nitrogen  cleavage  extends  over  more  than 
a  single  day. 

In  reality,  of  course,  the  excretion  does  not  take  place  with  any 
such  mathematical  exactness  as  in  this  schematic  example,  and 
after  long  fasting  in  particular  a  certain  rebuilding  of  proteid  tissue 
may  occur,  but  the  assumed  figures  may  serve  to  give  a  general 
notion  of  the  relations  of  food-supply  and  excretion. 

In  brief,  then,  we  may  suppose  that  when  proteid  food  is  given 
to  a  fasting  animal  the  stimulating  effect  upon  the  nitrogen  cleavage 
anticipates  the  use  of  the  proteids  for  constructive  purposes  and 
that  a  large  proportion  of  them  is  thus  destroyed  as  proteids  before 
it  can  be  used  to  make  good  the  loss  of  proteids  by  the  organized 
tissues.  In  other  words,  the  proteids  actually  available  for  the 
tissues  are  much  less  than  the  amount  supplied  in  the  food.  In 
this  view  of  the  matter  we  can  readily  see  why  the  proteid  supply 
overtakes  the  nitrogen  excretion  so  slowly  and  why  two  or  three 
times  the  amount  metabolized  in  fasting  is  necessary  to  make  good 
the  loss  from  the  body  and  ensure  nitrogen  equilibrium. 


io4 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Effects  on  Total  Metabolism. 

In  the  preceding  paragraphs  the  effects  of  an  exclusive  proteid 
diet  upon  the  proteid  metabolism  have  been  discussed.  There 
remain  to  be  considered  its  effects  upon  the  metabolism  of  fat. 

Proteids  Substituted  for  Body  Fat. — When  proteids  are 
given  to  a  fasting  animal  the  proteid  metabolism  is  increased,  as  we 
have  seen,  but  at  the  same  time  the  loss  of  body  fat  is  diminished. 

Pettenkofer  &  Voit  *  fed  a  dog  with  varying  amounts  of  lean 
meat,  which  may  be  regarded  as  consisting  chiefly  of  proteids 
together  with  small  amounts  of  fat,  with  the  following  average 
results  in  terms  of  nitrogen: 


Meat  Fed, 
Grms. 

Nitrogen 
of  Food, 
Grms. 

Nitrogen 

Metabolized, 

Grms. 

Gain  or  Loss 

of  Nitrogen, 

Grms. 

Gain  or  Loss 
of  Fat, 
Grms. 

ot 

500 
1000 
1500| 

0 
17.0 
34.0 
51.0 

5.6 
20.4 
36.7 
51.0 

-5.6 

-3.4 

-2.7 

0 

-95 
-47 
-19 

+  4 

Rubner  §  has  obtained  a  similar  result  by  the  use  of  the  proteid 
mixture  resulting  from  the  extraction  of  lean  meat  with  water,  and 
which  still  contained  some  fat.  As  compared  with  the  fasting  state, 
the  consumption  of  740  grams  of  the  moist  material  (containing 
72.2  per  cent,  of  water)  produced  the  following  effect: 


Nitrogen  of  Food, 
Grms. 

Nitrogen 

Metabolized, 

Grms. 

Fat 

Metabolized, 

Grms. 

0 
35.22 

5.25 
2G.37 

+  21.12 

84  39 

Fed  

28  37 

Difference 

-56.02 

The  increased  nitrogen  cleavage  resulting  from  an  increase  in 
the  proteid  supply  liberates  a  certain  amount  of  energy  for  the  vital 
activities  of  the  body,  while  the  non-nitrogenous  residue  of  the  cleav- 

*  Zeit  f .  Biol.,  7,  489. 

t  Average  of  first  two  experiments,  p.  84,  Chapter  IV. 

%  Series  I  only.     The  others  showed  a  greater  gain  of  fat  and  of  nitrogen. 

§  Zeit.  f.  Biol.,  22,  51. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       105 

age  becomes  available  also  as  a  source  of  energy  to  the  organism, 
and  the  metabolism  of  fat  is  correspondingly  diminished.  In 
effect,  then,  the  proteids  are  simply  substituted  for  more  or  less  of 
the  body  fat  as  a  source  of  energy,  and  Rubner,  in  a  series  of  experi- 
ments which  will  be  considered  in  Part  II,  has  shown  that  the  sub- 
stitution takes  place,  under  the  condition  of  these  experiments,  ap- 
proximately in  proportion  to  the  amount  of  available  potential 
energy  contained  in  the  proteids  and  fats  respectively.  That  isr 
if  the  extra  proteids  metabolized  can  supply  a  certain  amount, 
100  Cals.,  e.g.,  of  energy  to  the  organism,  the  fat  metabolism  is 
diminished  by  a  corresponding  amount,  so  that  the  total  expend- 
iture of  energy  by  the  body  remains  unchanged,  being  simply 
drawn  from  different   sources  in  the   two  cases. 

Amount  Required  to  Produce  Carbon  Equilibrium. — In 
the  experiments  by  Pettenkofer  &  Voit  cited  above,  the  quantity 
of  food  proteids  which  resulted  on  the  average  in  nitrogen  equili- 
brium produced  substantially  an  equilibrium  also  between  the 
supply  and  excretion  of  carbon.  The  earlier  experiments  of  Bidder 
&  Schmidt  *  gave  similar  results.  Later  experiments,  however, 
have  given  divergent  results,  nitrogen  equilibrium  appearing  to 
be  reached  with  an  amount  of  proteids  which  is  far  from  supplying 
sufficient  energy  for  the  organism,  so  that  while  the  stock  of  pro- 
teids in  the  body  is  maintained,  its  store  of  fat  is  still  drawn  upon. 

We  have  seen  that  the  proteid  metabolism  in  the  normal  fast- 
ing animal  amounts  to  10-14  per  cent,  of  the  total  metabolism, 
while  according  to  E.  Voit  (p.  96)  the  food  proteids  required  for 
nitrogen  equilibrium  are,  roughly,  2\  to  3  times  the  fasting  proteid 
metabolism.  It  follows,  then,  that  an  amount  of  proteids  con- 
taining from  25  to  42  per  cent,  of  the  total  available  energy  expended 
by  the  fasting  organism  will  maintain  its  store  of  proteids,  and  this 
being  so,  the  remaining  58-75  per  cent,  must  necessarily  be  sup- 
plied by  the  metabolism  of  body  fat.  Thus  with  the  dog  on  which 
E.  Voit's  main  experiment  was  made,  nitrogen  equilibrium  was 
approximately  reached  with  12.05  grams  of  nitrogen  in  the  food,f 
equivalent  to  75.31   grams  of  protein  (NX6.25)   and  containing 

♦Compare  Atwater  &  Langworthy;  Digest  of  Metabolism  Experiments; 
U.  S.  Dept.  of  Agr.,  Office  of  Experiment  Stations,  Bui.  45,  388. 
■\Loc.  cit.,  p.  69. 


io6 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


approximately,  according  to  Rubner  (see  Chapter  X),  321  Cals.  of 
available  energy.  The  actual  expenditure  of  energy  by  the  animal 
was  not  determined,  but  is  estimated  by  the  author  on  the  basis 
of  Rubner's  investigations  at  about  1280  Cals. 

Several  experiments  by  Rubner*  lead  to  the  same  conclusion. 
In  these  experiments  the  carbon  and  nitrogen  of  the  excreta  were 
determined  and  the  nitrogen  of  the  food  estimated  from  average 
figures.  The  proteid  metabolism  having  been  computed  from  the 
total  excretory  nitrogen,  the  corresponding  amount  of  carbon  is 
computed  from  the  average  composition  of  the  proteids  and  any  ex- 
cess in  the  excreta  is  assumed  to  be  derived  from  the  metabolism 
of  fat.  (Compare  p.  78.)  The  following  are  the  results  in  brief, 
including  the  one  cited  above  (p.  104) : 


Food. 


Nitrogen 

of  Food, 

Grms. 


Nitrogen 

of 
Excreta, 
Grms. 


Fat 
metab- 
olized, 
Grms. 


Nothing 

415  grms.  lean  meat 


14.11 


Nothing 

740  grms.  lean  meat  . . .  I  25 .  16 


Nothing 

740  grms.  extracted  lean 
meat    


Nothing 

390  grms.  lean  meat 


Nothing 

350  grms.  lean  meat 


Nothing 

580  grms.  lean  meat 


35.22 
i3!26 
ii!90 
19^72 


4.38 
13.72 

2.80 
20.63 

5.25 

26.37 

1.08 
8.53 

1.08 
10.10 

3.50 

18.47 


49.33 
25.44 


79.94 
30.73 


84.36 
28.37 


22.88 
11.42 


22.88 
11.79 


37.24 
21.45 


Average  of  several  days. 
1st  two  days  of  feeding. 

1st  to  4th  day  of  feeding 
1st  day  of  feeding. 
3d  to  6th  day  of  feeding. 
1st  to  7th  day  of  feeding. 


While  some  of  the  experiments  were  hardly  continued  long 
enough  to  absolutely  establish  the  sufficiency  of  the  proteid  supply, 
nevertheless  we  see  in  all  cases  a  material  loss  of  fat  on  rations  which 
apparently  are  sufficient  to  prevent  a  loss  of  nitrogen  from  the 
body. 

It  should  perhaps  be  noted  that  in  Pettenkofer  &  Voit's  ex- 
periments 1000  grams  of  meat  nearly  prevented  a  loss  of  nitrogen 
*Zeit,  f.  Biol.,  22,  43-4S;  30,  122-134. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY.       107 

from  the  body.  It  appears  possible,  then,  that  nitrogen  equilib- 
rium might  have  been  reached  with  a  less  amount  than  1500  grams, 
and  that  with  this  less  amount  there  might  still  have  been  a  loss  of 
fat  from  the  body.  Whether  this  possibility  is  sufficient  to  explain 
the  apparent  discrepancy  between  these  and  later  results  must, 
however,  remain  a  matter  of  conjecture. 

Utilization  of  Excess  of  Proteids. — We  have  seen  that  no 
very  considerable  or  long-continued  storage  of  protein  takes  place 
in  the  body  of  the  mature  animal.  However  large  the  supply  of 
food  proteids,  the  body  very  soon  reaches  the  condition  of  nitrogen 
equilibrium,  the  outgo  of  this  element  in  the  excreta  equaling  the 
supply  in  the  food.  This  fact,  as  has  been  pointed  out,  does  not 
necessarily  prove  that  the  elements  of  the  food  proteids  are  com- 
pletely oxidized  in  the  organism.  As  was  shown  in  Chapter  II, 
the  abstraction  from  proteid  matter  of  the  elements  of  urea  (or, 
more  strictly  speaking,  of  the  elements  found  in  the  urine)  leaves 
a  very  considerable  non-nitrogenous  residue  available  for  the  pur- 
poses of  the  organism.  It  was  there  stated  that  this  residue  could 
serve  as  a  source  of  energy,  and  likewise  that  there  was  good  reason 
to  believe  that  sugar  was  formed  from  it,  while  finally  the  question 
of  its  ability  to  serve  as  a  source  of  fat  was  reserved  for  discussion 
in  the  present  connection. 

Formation  of  Fat  from  Proteids. 

Mention  has  already  been  made  in  Chapter  II  (p.  29)  of  the  fact, 
first  asserted  by  Liebig,*  that  the  animal  body  manufactures  fat 
from  other  ingredients  of  its  food.  As  a  result  of  the  investiga- 
tions incited  by  the  publication  of  his  views  regarding  the  origin 
of  animal  fat,  Liebig's  classification  of  the  nutrients  into  "  plastic  " 
and  "respiratory"  was  generally  accepted.  The  proteids  were 
regarded  as  the  material  for  the  growth  and  repair  of  the  muscles 
and  the  force  exerted  by  the  latter  was  considered  to  arise  from 
their  oxidation,  while  the  non-nitrogenous  ingredients  of  the  food, 
especially  the  carbohydrates,  were  the  source  of  the  animal  heat, 
and  when  present  in  excess  gave  rise  to  a  production  of  fat. 

As  time  went  on,  however,  observations  began  to  accumulate 

*  Compare  p.  163. 


108  PRINCIPLES    OF  ANIMAL   NUTRITION. 

tending  to  show  that  the  proteids  were  not  without  influence  on 
fat-production. 

As  early  as  1745  R.  Thomson,*  in  experiments  on  tnilch  cows, 
noted  an  apparent  connection  between  the  supply  of  proteids  in 
the  food  and  the  production  of  butter. 

Hoppe  f  in  1856  interpreted  the  results  of  an  experiment  in 
which  a  dog  was  fed  lean  meat  with  and  without  the  addition  of 
sugar  as  showing  a  formation  of  fat  from  proteids.  The  same 
author  %  in  1859  claimed  to  have  shown  a  slight  formation  of  fat 
from  casein  in  milk  exposed  to  the  air,  and  this  was  confirmed  later 
by  Szubotin.§  The  latter  author,  and  also  Kemmerich,||  and  later 
Voit,l  experimented  upon  the  production  of  milk-fat  by  dogs. 
Their  results,  while  indicating  the  possibility  of  a  formation  of  fat 
from  proteids,  were  indecisive. 

Pettenkofer  &  Voit's  Experiments. — Carl  Voit,  however, 
was  the  first  to  distinctly  champion  the  new  theory,  and  aside  from 
certain  confirmatory  facts,**  such  as  the  formation  of  fatty  acids  in 
the  oxidation  of  proteids,  the  formation  of  adipocere,  the  alleged 
formation  of  fat  from  proteids  in  the  ripening  of  cheese  and  in  the 
fatty  degeneration  of  muscular  tissue,  especially  in  cases  of  phos- 
phorous poisoning, — facts  not  all  of  which  are  fully  established  and 
whose  importance  in  this  connection  has  probably  been  over- 
estimated,— the  evidence  bearing  on  the  question  of  the  formation  of 
fat  from  proteids  has  been  until  recently  largely  that  supplied  by 
the  famous  researches  of  Pettenkofer  &  Voit  ft  at  Munich. 

In  these  experiments  a  dog  weighing  about  30  kgs.  was  fed 
varying  amounts  of  prepared  lean  meat  from  which  fat,  connective 
tissue,  etc.,  had  been |  removed  as  completely  as  was  possible  by 
mechanical  means.  The  material  thus  prepared,  while  still  con- 
taining small  amounts  of  fat,  etc.,  was  as  near  an  approach  to  an 
exclusively  proteid  diet  as  was  practicable,  it  having  been  found 
impossible  to  successfully  carry  out  feeding  experiments  with  pure 

*  Ann.  Chem.  Pharm.,  61,  228.  %  Virchow's  Archiv,  17,  417. 

t  Virchow's  Archiv,  10,  144  %Ibid.,  36,  5G1. 

||  Wolff,  Ernahrung  Landw.  Nutzthiere,  p.  351. 

f  Zeit.  f.  Biol.,  5,  136. 

**  Compare  Voit's  summary  in  1869,  Zeit.  f.  Biol.,  5,  79-169. 

ft  Am.  Chem.  Pharm.,  II,  Suppl.  Bd.,  pp.  52  and  361  ;  Zeit.  f.  Biol.,  5, 
106;  7,  433. 


THE  RELATIONS  OF  METABOLISM   TO  EOOD-SUPPLY.       109 


proteids.  The  experiments  were  conducted  with  the  aid  of  a  respi- 
ration apparatus,  the  gain  or  loss  of  proteids  and  fat  being  com- 
puted from  the  nitrogen  and  carbon  balance  in  the  manner  described 
in  Chapter  III. 

The  following  is  a  condensed  summary  of  the  average  results 
of  these  experiments,  as  given  by  the  authors,*  but  includes  also 
the  average  of  all  the  experiments  with  1500  grams  of  meat.     On 


Meat  Eaten  per  Day, 
Gratis. 

Gain  (+)  or  Loss  (-)  by  Animal. 

Experiments. 

Flesh. 

Grins. 

Fat, 
Ginis. 

3t 
22 
1 
2 
1 

0 
500 
1000 
1500 
1500 
1800 
2000 
2500 

-165 

-  99 

-  79 

0 
+   18 
+  43 

-  44 

-  12 

-95 

-47 
-19 

+   4 
+   9 

+    1 
+  58 
+  57 

the  smaller  rations,  which  were  obviously  insufficient  for  main- 
tenance, the  animal  lost  both  flesh  and  fat.  A  ration  of  1500 
grains  of  meat  per  day  sufficed  approximately  to  maintain  the  ani- 
mal as  regards  flesh  and  to  cause  a  small  gain  of  fat.  On  the 
heavier  rations  the  excretion  of  nitrogen  kept  pace  with  the  supply 
in  the  food  in  the  manner  illustrated  on  pp.  94-96  but  the  excretion 
of  carbon  fell  considerably  below  the  supply,  indicating  a  produc- 
tion of  fat. 

It  is  to  be  noted  that  only  the  last  three  experiments  in  the  above 
table  actually  show  any  very  considerable  production  of  fat.  The 
insufficient  rations  naturally  do  not,  and  while  among  the  twenty- 
two  trials  with  1500  grams  of  meat  the  majority  appear  to  show  a 
formation  of  fat,  the  amount  is  usually  comparatively  small,  and 
in  two  cases  a  loss  was  observed.  On  the  whole,  however,  the  evi- 
dence of  this  series  of  experiments  has  been  generally  accepted  as 
conclusive  in  favor  of  the  formation  of  fat  from  proteids. 

Pfluger's  Recalculations. — One  very  important  point,  how- 
ever, has  until  recently  been  overlooked.     The  evidence  is  based  on 

*  Zeitschr.  f.  Biol.,  7,  489. 
f  Series  1  only. 


no  PRINCIPLES  OF  ANIMAL  NUTRITION. 

a  comparison  of  the  income  and  outgo  of  carbon  and  nitrogen. 
Pfliiger,*  however,  has  called  attention  to  the  fact  that  while  Pet- 
tenkofer  &  Voit  made  direct  determinations  of  the  outgo  of  these 
elements,  or  at  least  of  the  principal  factors  of  it,  the  income  is  not 
computed  from  actual  analyses  of  the  meat  used,  but  upon  the 
assumption  of  average  composition.  According  to  Pfliiger,  not 
only  are  the  possible  variations  from  the  average  in  individual 
experiments  a  serious  source  of  error,  but  the  average  itself  is 
erroneous,  the  percentage  of  carbon  assumed  in  the  meat  being 
too  high.  Pettenkofer  &  Voit  estimate  the  ratio  of  nitrogen  to 
carbon  in  lean  meat  f  as  1  : 3.684,  while  according  to  Pfliiger  it  is 
not  higher  than  1  : 3.28,  and  probably  lower.  Moreover,  Petten- 
kofer &  Voit  failed  to  take  due  account  of  the  fact  that  a  part  of 
the  gain  of  carbon  which  they  observed  could  be  ascribed  to  the  fat 
still  contained  in  the  prepared  "lean"  meat.  Another,  although 
slight,  source  of  error,  according  to  Pfliiger,  lies  in  the  fact  that 
the  carbon  in  the  urine  was  estimated  from  the  amount  of  nitrogen 
found  by  analysis  on  the  assumption  of  a  ratio  of  1 : 0.60,  while  it 
should  be  1:0.67. 

Using  the  above  corrections,  Pfliiger  has  recalculated  twenty- 
four  of  the  experiments  by  Pettenkofer  &  Voit,  which  have  been 
generally  accepted  as  demonstrating  the  formation  of  fat  from  pro- 
teids,  with  the  results  shown  on  the  opposite  page.J 

In  the  great  majority  of  cases  the  experiments  as  recalculated 
show  a  loss  instead  of  a  gain  of  fat,  and  in  three  of  the  four  cases  in 
which  a  gain  still  appears  it  is  small  in  amount,  and,  as  Pfliiger 
believes,  within  the  limits  of  experimental  error.  Naturally  such 
calculations  as  the  above  can  neither  prove  nor  disprove  the  hypoth- 
esis that  the  proteids  serve  as  a  source  of  fat.  They  simply  show 
that  the  experiments  which  have  served  as  the  principal  support 
for  that  hypothesis  do  not  demonstrate  what  they  were  supposed 
to.  The  question  turns  largely  upon  the  elementary  composition 
of  the  meat  used  by  Pettenkofer  &  Voit,  which  they  failed  to 
determine.     It  is  manifestly  impossible  to  repair  this  error  now, 

*Arch.  ges.  Physiol.,  51,  229. 

f  Including  such  fat  as  cannot  be  removed  by  mechanical  means. 
%Loc.  cit.,  p.  267.     The  experiments  which  shoved  a  loss  of  fat  as  origi- 
nally computed  are  omitted. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY.       nr 


Date  of  Experiment. 


Meat  Eaten 
per  Day, 
Grms. 


Gain  (+)  or  Loss  (— )  of  Fat. 


According  to  Pet- 

tenkofer  &  Voit. 

Grms. 


According  to 
Pfliiger. 
Grms. 


Feb. 19, 1861 , 
Apr.  3, 
Mch.  4,  1862. 

"  21, 
Apr.  7, 

"  12, 

"  14, 

"  16, 
Aug.  6, 

"  8, 
Feb. 20,  1863. 

"  23, 

"  27, 
Mch.  4, 
Apr.  1, 

"  7, 

"  10, 
June  1, 

"  8, 

"  12, 

"  21, 

"  26,* 
July  3, 


1800 
2500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
1500 
2000 
2000 
1500 
1500 


+  1.4 
+  56.7 
+  3.4 
+  7.3 
+  34.4 
+  20.7 
+  35.9 
+  22.9 
+  8.7 
+  17.4 

0.0 
+  9.9 
+  2.1 
+  14.3 

0.0 
+  13.8 
+  9.0 
+  12.7 
+  26.3 
+  29.1 
+  55.9 
+  58.5 
+  11.9 
+  9.4 


-35.8 
+  3.93 
-29.3 
-23.4 
+  3.7 
-11.1 
+  3.8 

-  8.4 
-13.5 

-  8.3 
-31.6 
-22.1 
-24.4 
-16.9 
-31.8 
-17.5 
-22.0 
-13.0 

-  5.4 

-  2.9 
+  13.6 
+  1.6 
-20.6 
-23.7 


and  since  Pfluger's  estimates  seem  to  be  at  least  as  trustworthy  as 
Pettenkofer  &  Voit's,  and  lead  to  exactly  the  opposite  results,  the 
only  verdict  possible,  so  far  as  these  experiments  are  concerned,  is 
"Not  proven." 

Later  Experiments. — Shortly  after  the  publication  of  Pfluger's 
critique,  E.  Voit,f  in  a  preliminary  communication,  presented  the 
results  of  investigations  upon  this  question  undertaken  in  the 
Munich  laboratory.  So  far  as  the  writer  is  aware,  no  complete 
account  of  these  experiments  has  yet  appeared,  but  the  data  given 
by  Voit  in  the  preliminary  account  show  a  retention  in  the  body 
of  8  to  10  per  cent,  of  the  carbon  of  the  metabolized  proteids,  and 
to  this  extent  confirm  the  earlier  results  obtained  by  his  father. 
He  believes  the  observed  gain  of  carbon  to  be  too  great  to  be 
accounted  for  by  the  storage  of  glycogen  and  interprets  it  as 
showing  a  production  of  fat  from  proteids. 

♦Includes  a  correction  of  Pettenkofer  &  Voit's  figures  for  the  urinary 
nitrogen.     Loc.  cit.  p.  263. 

fThier.  Chem.  Ber.,  22,  34. 


"2  PRINCIPLES   OF  ANIMAL    NUTRITION. 

Kaufmann  likewise  interprets  the  results  of  the  respiration  ex- 
periments cited  in  another  connection  on  p.  99  as  demonstrating 
the  production  of  fat  from  proteids,  but  in  view  of  the  brevity  of 
the  experiments  (five  hours),  and  the  fact  that  they  covered  the 
period  of  most  active  nitrogen  cleavage,  this  conclusion  seems 
hardly  justified. 

Cremer  *  has  reported  the  results  of  an  experiment  upon  a  cat 
which  he  regards  as  showing  a  formation  of  fat  from  proteids.  The 
animal,  weighing  3.7  kgs.,  passed  eight  days  continuously  in  the 
respiration  apparatus  and  received  per  day  450  grams  of  lean 
meat.  No  complete  nitrogen  and  carbon  balance  is  reported.  The 
average  daily  excretion  of  nitrogen  was  13  grams.  Assuming  the 
ratio  of  nitrogen  to  carbon  in  fat  and  glycogen-free  flesh  to  be  1 :3.2,f 
this  corresponds  to  41.6  grams  of  carbon  in  the  form  of  proteids, 
while  the  total  excretion  of  this  element  was  only  34.3  grams,  thus 
showing  a  retention  by  the  organism  of  7.3  grams  per  day.  The 
body  of  the  animal  at  the  close  of  the  experiment  was  found  to  con- 
tain not  more  than  35  grams  of  glycogen  and  sugar,  while  the  ob- 
served gain  of  carbon  during  the  eight  days  was  equivalent  to 
about  130  grams  of  glycogen.  It  is  therefore  concluded  that  fat 
was  formed  from  proteids.  In  three  other  experiments,  with  an 
abundant  meat  diet,  it  is  computed  that  from  12.6  to  1 7.0  per  cent, 
of  the  carbon  of  the  metabolized  proteids  was  stored  in  the  body. 

Gruber  \  has  recently  reported  two  experiments,  dating  from 
the  year  1882,  in  which  a  dog  was  fed  1500  grams  per  day  of  lean 
meat.  The  nitrogen  of  feces  and  urine  were  determined  daily 
for  six  and  eight  days  respectively  and  the  carbon  dioxide  of  the 
respiration  on  five  days  in  each  experiment;  the  carbon  of  urine 
and  feces  and  of  the  metabolized  proteids  was  computed  from  the 
nitrogen,  using  for  the  carbon  of  the  proteids  the  factor  3.28.  The 
excretion  of  nitrogen  approximately  equaled  the  supply,  especially 
on  the  later  days  of  the  experiments,  but  from  10  to  15  per  cent, 
of  the  carbon  was  unaccounted  for  in  the  excreta.  The  total  reten- 
tion of  carbon  during  the  experiments,  together  with  the  equivalent 
quantities  of  glycogen,  were: 

*Jahresb.  Agr.  Chem.,  40,  538;  Zeit.  f.  Biol.,  38,  309. 

fKShler  (Zeit.  f  physiol.  Chem.,  31,  479)  found  an  average  of  1:3.16  for 
the  fat-free  flesh  of  cattle,  swine,  sheep,  rabbits,  and  hens.     See  p.  64. 

%  Zeit  f  Biol.,  42,  409. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       113 


Experiment  1,  seven  davs. 
2,  eight      "    . 


113.9  Grms. 
195.9      " 


Equivalent  Glycogen. 


256.3  Grms. 
441.0      " 


These  amounts  of  glycogen  are  much  greater  than  have  ever 
been  found  in  the  body  of  a  dog  of  this  weight  (about  20  kgs.),  and 
the  larger  part  of  the  storage  of  carbon  must  therefore  have  been 
in  the  form  of  fat. 

In  addition  to  the  above  results  on  normal  animals,  Polimanti  * 
has  reported  experiments  apparently  showing  a  formation  of  fat 
from  proteids  in  phosphorous  poisoning.  The  latter  investigation, 
as  well  as  those  of  Cremer  and  of  E.  Voit,  have  been  the  subjects  of 
searching  criticism  by  Pfluger,t  who  claims  to  have  shown  the  in- 
sufficiency of  the  experimental  evidence  adduced,  but  it  is  impos- 
sible here  to  enter  into  the  details  of  these  controversial  articles. 
Negative  results  have  also  been  reported  by  Kumagawa  & 
Kaneda.t  Rosenfelt,§  TaylorJ  Athanasiu,!"  and  Lindemann,**  but 
in  a  matter  of  this  sort  negative  evidence  naturally  carries  much 
less  weight  than  positive,  and  on  the  whole  there  would  appear 
to  be  good  reason  for  still  regarding  the  proteids  as  a  possible 
source  of  fat. 

Difficulty  of  Proof. — A  serious  difficulty  in  the  way  of  an 
unquestionable  demonstration  of  this  possibility  lies  in  the  limited 
amount  of  proteids  which  an  animal  can  consume.  As  we  have 
seen,  a  relatively  large  supply  of  them  is  necessary  even  to. produce 
nitrogen  equilibrium,  and  a  still  further  large  addition  is  required  to 
supply  the  demands  of  the  organism  for  energy.  Only  the  proteids 
supplied  in  excess  of  this  latter  amount  are  available  for  fat  produc- 
tion, and  thus  it  comes  about  that  the  limit  of  consumption  and 
digestion  by  the  animal  is  reached  before  any  very  large  produc- 
tion of  fat  can  take  place. 

On  the  other  hand,  if  non-nitrogenous  nutrients  (carbohydrates 

*Arch.  ges.  Physiol.,  70,  349. 

■\Ibid.,  68,  176;  71,  318. 

j  U.  S.  Dept.  Agr.,  Expt.  Station  Record,  8,  71. 

§Jahresb.  Physiol.,  6,  260. 

II  Ibid..  8,  249,  Jour  Exper.  Medicine,  4,  399. 

T[Arch.  ges.  Physiol.,  74,  511. 

**Zeit.  f.  Biol.,  39,  1. 


114  PRINCIPLES   OF  ANIMAL   NUTRITION. 

for  example)  are  employed  to  supply  a  part  of  the  necessary  energy, 
a  more  abundant  fat  production  may  be  caused  but  the  results 
are  ambiguous,  since  it  is  possible  that  the  non-nitrogenous  residue 
of  the  proteids  may  be  metabolized  to  furnish  energy  otherwise 
supplied  by  the  non-nitrogenous  nutrients  and  that  the  actual 
material  for  the  formation  of  fat  may  come  from  the  latter. 

That  proteids  added  to  a  mixed  ration  may  give  rise  to  a  large 
amount  of  fat  has  been  strikingly  shown  by  Kellner  *  in  experi- 
ments on  oxen  in  which  wheat  gluten  was  added  to  a  fattening 
ration.  Approximately  198  grams  of  fat  were  produced  for  each 
kilogram  of  protein  fed,  but  to  the  writer  the  reasoning  by  which 
Kellner  seeks  to  prove  that  this  fat  must  have  been  derived  directly 
from  the  proteids  seems  inconclusive. 

Finally,  as  was  indicated  in  Chapter  II  (p.  50),  the  apparently 
well-established  fact  that  the  metabolism  of  proteids  in  the  body 
gives  rise  to  the  formation  of  carbohydrates  (or  at  least  may  do  so), 
together  with  the  further  fact  that  fat  is  undoubtedly  formed  from 
carbohydrates,  renders  it  difficult  to  assign  any  reason  why  the  non- 
nitrogenous  residue  of  the  proteids  should  not  supply  material  to 
the  cells  of  the  adipose  tissue  for  the  production  of  fat. 
§  2.  The  Non-nitrogenous  Nutrients. 
Effects  on  the  Proteid  Metabolism. 

The  relations  between  proteid  metabolism  and  proteid  supply 
which  have  been  outlined  in  the  preceding  section,  while  deduced 
mainly  from  experiments  in  which  the  food  consisted  substantially 
of  proteids  only,  are  of  general  applicability,  yet  are  subject  to  im- 
portant modifications  in  the  presence  of  non-nitrogenous  nutrients. 

Tend  to  Diminish  Proteid  Metabolism. — As  was  first  shown 
by  C.  Voit,  the  addition  of  non-nitrogenous  nutrients  to  a  ration 
consisting  of  proteids  tends  to  render  the  proteid  metabolism  less 
than  it  otherwise  would  be.  The  effect  is  common  to  the  fats  and 
carbohydrates,  although  with  some  differences  in  details. 

Fats. — The  following  example,  taken  from  Voit's  experiments,! 
illustrates  in  a  somewhat  marked  way  the  influence  of  the  addition 
of  fat  to  proteid  food  upon  the  excretion  of  nitrogen.  A  dog  con- 
suming daily  1000  grams  of  lean  meat  received  in  addition  on  two 
days  100  and  300  grams  of  fat,  with  the  following  results: 
*  Landw.  Vers.  Stat.,  53,  456. 
f  Zeit.  f.  Biol.,  5,  334. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       T15 


Food  per  Day. 


Meat, 
Grms. 


Fat, 
Grms. 


Urea  per  Day, 
Grms. 


July  31 

Aug.   1 

«      2 

"      3 


1000 
1000 
1000 
1000 


0 
100 
300 

0 


81.7 
74.5 
69.3 

81.2 


In  the  whole  series  of  eight  experiments  with  varying  amounts 
of  meat  and  fat  the  decrease  in  the  excretion  of  urea  ranged  from  1 
per  cent,  to  15  per  cent,  of  the  amount  supplied  in  the  food,  averag- 
ing about  7  per  cent.  With  the  same  amount  of  fat  in  the  food 
the  decrease  in  the  excretion  of  urea  was  not,  as  a  rule,  greater  with 
large  than  with  moderate  rations  of  meat.  On  the  other  hand, 
with  a  small  proteid  supply  in  the  food  the  production  of  urea  was 
sometimes  increased  slightly  by  the  addition  of  much  fat,  and  the 
same  result  was  observed  to  a  more  marked  extent  when  fat  alone 
was  given  to  fasting  animals.  With  medium  rations  of  meat,  in- 
creasing the  fat  supply  had  usually  little  effect,  but  with  heavy 
meat  rations  it  tended  to  further  diminish  the  excretion  of  urea. 

Subsequent  investigation  has  fully  established  this  tendency 
of  fat  to  diminish  the  proteid  metabolism,  and  the  fact  is  too  well 
known  to  require  extended  illustration  here.  As  a  recent  instance 
may  be  cited  the  following  results  obtained  by  Kellner  *  in  experi- 
ments upon  oxen,  in  which  oil  was  added  to  a  basal  ration: 


Nitrogen  Digested. 

Nitroge 

n  in  Urine. 

Basal  Ration, 
Grms. 

Basal  Ration  +  Oil, 
Grms. 

Basal  Ration, 
Grms. 

Basal  Ration  +  Oil, 
Grms. 

OxD 

Ox  F 

135.30 
111.67 
86.27 

134 . 55 
109.17 

87.08 

122.54 
106.03 
86.30 

120.38 
89.27 

Ox  G 

79.83 

Carbohydrates. — The  effects  of  the  readily  soluble  hexose 
carbohydrates  (starch  and  the  sugars)  have  been  quite  fully  inves- 
tigated, while  as  to  those  of  the  less  soluble  carbohydrates,  particu- 
larly of  the  five-carbon  series,  considerable  diversity  of  opinion 
still  prevails. 

*  Landw.  Vers.  Stat.,  53,  121  and  210. 


1x6 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Starch  and  Sugars. — The  investigations  of  C.  Voit  *  show  that 
starch  or  sugar  added  to  a  proteid  diet  causes,  as  does  fat,  a  decrease 
in  the  elimination  of  urea.  Voit  found  an  average  decrease  of 
about  9  per  cent,  in  the  proteid  metabolism,  the  extremes  being  5 
and  15  per  cent,  with  varying  amounts  of  carbohydrates.  An  in- 
crease in  the  carbohydrates,  the  proteid  food  remaining  the  same, 
tended  to  further  diminish  the  excretion  of  urea.  The  following 
examples  illustrate  this  effect  of  the  carbohydrates.  When  given 
to  a  fasting  animal,  carbohydrates  did  not,  as  in  the  case  of  fat, 
cause  an  increase  in  the  proteid  metabolism. 


Food. 

Urea 

Meat, 
Grnis. 

Carbohydrates, 
Grms. 

Grms. 

June  23-July  2,  1859 

500 
500 

300-100      . 
0 

35.4 

Julv     2-5,               "     

39.9 

July      4  10,  1864 

800 
800 
800 

0 

100^00 

0 

59.1 

"        10-19,     "     

54.5 

"        19-20,     "     

63.8 

July  23-26,  1864 

1000 
1000 
1000 

0 

100-400 

0 

73.5 

"     26-28,     "          

64.4 

"     28-Aug.  1,  1864 

79.6 

June  29  July  8,  1863 

1500 
1500 

0 
200 

114.9 

July     8  13,            "     

103.3 

Jan    6   1859                     

2000 
2000 

o 

200-300 

143.7 

"     7  11    1859  

131.0 

This  effect  of  the  carbohydrates,  like  that  of  fat,  has  been  abun- 
dantly confirmed  by  later  investigators  and  is  one  of  the  well-estab- 
lished facts  of  physiology.  Weiske  t  in  particular  has  investigated 
the  effect  of  the  non-nitrogenous  nutrients  upon  the  metabolism 
of  sheep,  while  Miura  J  and  Lusk  §  have  shown  that  the  abstraction 
of  carbohydrates  from  the  diet  of  a  man  results  in  a  marked  increase 
in  the  proteid  metabolism.  The  following  data,  taken  from  Kell- 
ner's  extensive  respiration  experiments  at  Mockern,  illustrate  the 
same  effect  of  starch  in  the  case  of  cattle : 

*  Zeit.  f.  Biol.,  5,  434. 

t  Zeit.  physiol.  Chem.  21,  42;  22,  137  and  265. 

%  v.  Noorden,  Pathologie  des  Stoffwechsels,  p.  117. 

§  Zeit.  f.  Biol.,  27,  459. 


THE  RELATIONS   OF  METABOLISM    TO   FOOD  SUPPLY.      117 


Nitrogen  Digested. 

Nitrogen  in  Urine. 

Basal  Ration, 
Grms. 

Basal  Ration 

+  Starch, 

Grms. 

Basal  Ration 
Grms. 

Basal  Ration 

4-  Starch, 

Grms. 

Ox  D...                  

135.30 
111.67 
86.27 
116.51 
128.11 

118.40 

107.55 

80.92 

94.66 

118.18 

122.54 
106.03 
86.30 
109.28 
122.62 

104.69 

Ox  F 

81.18 

Ox  G  . . 

63.83 

Ox  H  . .                   

81.71 

Ox  J. . .                   

103.13 

Since  the  addition  of  starch  to  the  basal  ration  diminished  the 
apparent  digestibility  of  the  protein,  the  effect  is  most  clearly  seen 
by  comparing  the  daily  gains  of  nitrogen  by  the  animals  on  the 
two  rations,  as  follows: 


On  Basal  Ration, 
Grms. 

With  Addition 

of  Starch, 

Grms. 

Difference. 

Ox  D 

12.76 

5.64 

-0.03 

7.23 

5.49 

13.71 
26.37 
17.09 
12.95 
15.05 

+    0.95 

Ox  F 

+  20.73 

Ox  G 

+  17.12 

Ox  H 

+  5.72 

Ox  J 

+  9.56 

Cellulose. — The  peculiar  position  occupied  by  cellulose,  as  the 
essential  constituent  of  the  "crude  fiber"  of  feeding-stuffs,  in  the 
nutrition  of  domestic  animals  causes  much  interest  to  attach  to  the 
study  of  its  effects  upon  metabolism.  We  shall  consider  here  only 
its  effects  upon  the  proteid  metabolism. 

The  first  to  take  up  this  subject  appears  to  have  been  v.  Knie- 
riem,*  who  experimented  upon  rabbits.  In  a  preliminary  experi- 
ment the  addition  of  prepared  "  crude  fiber "  to  a  basal  fiber-free 
ration  in  which  the  necessary  bulk  was  obtained  by  the  use  of  horn- 
dust  t  gave  the  following  results  for  the  urinary  nitrogen  per  day:. 

I.  Without  fiber 0.9034  grams 

II.  With  9.284  grams  fiber 0 .  7618      " 

III.  Without  fiber »  0.7560      " 

The  low  figure  for  the  third  period  is  ascribed  to  the  effect  of 
the  crude  fiber  still  remaining  in  the  digestive  tract.  In  a  follow- 
ing series,  in  which  respiration  experiments  were  also  made,  the 
following  results  per  day  were  obtained  for  the  nitrogen : 

*  Zeit.  f.  Biol.,  21,  67.  t  Shown  to  have  been  entirely  indigestible. 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


Food  per  Day. 


Nitrogen 

Nitrogen 

of  Food,  * 

of  Excreta* 

Orms. 

Grms. 

2.75 

3.35 

2.75 

2.65 

2.70 

3.03 

2.70  . 

3.02 

2.70 

2.73 

Gain  of 

Nitrogen, 

Grms. 


I.        9  days. 

II  10  days. 

III  5  davs. 

IV.  4  days. 

V.  3  days.f 


Milk  and  horn  dust 

Same  +  22  grms.  crude  fiber 

Milk  and  horn  dust 

Same  +  11  grms.  cane  sugar 
"       +  33      a 


-0.60 
+  0.10 
-0.33 
-0.32 
-0.03 


Weiske  J  disputes  v.  Knieriem's  conclusion  that  cellulose  dimin- 
ishes the  proteid  metabolism.  He  experimented  upon  a  sheep, 
which  was  fed  in  a  first  period  exclusively  on  beans.  In  succeeding 
periods  the  effect  upon  the  proteid  metabolism  of  adding  to  this 
ration,  first,  inferior  oat  straw,  and  second,  starch  was  tested,  the 
bean  ration  being  diminished  slightly  in  these  periods  in  order 
to  keep  the  total  digestible  protein  of  the  ration  as  nearly  uniform 
as  possible.  On  the  basis  of  a  preliminary  digestion  trial  with  the 
straw,  the  quantity  of  starch  was  so  adjusted  as  to  supply,  in  Period 
III,  according  to  computation,  an  amount  of  digestible  carbohy- 
drates equal  to  the  digested  fiber  and  nitrogen-free  extract  of  the 
straw  of  Period  II,  while  in  Period  V  it  equalled  the  digested 
nitrogen-free  extract  only.  Actual  determinations  of  the  digesti- 
bility of  the  mixed  rations  showed  that  this  equality  was  approxi- 
mately, although  not  exactly,  reached,  the  amount  of  digested 
starch  being  rather  less  than  the  computed  amount. 

The  results  as  regards  the  proteid  metabolism  as  originally  re- 
ported by  Weiske  are  given  in  the  first  portion  of  the  table  on 
the  opposite  page.  v.  Knieriem,§  having  criticised  the  results  on 
the  ground  that  the  metabolic  nitrogen  of  the  feces  was  not  taken 
into  account,  and  that  when  this  was  done  the  experiments  made  a 
more  favorable  showing  for  the  digested  crude  fiber,  Weiske  ||  has 
recalculated  his  results  on  the  assumption  that  the  feces  contained 
0.4  grams  of  metabolic  nitrogen  for  each  100  grams  of  dry  matter 
digested,^  with  the  results  shown  in  the  second  half  of  the  table. 

*  Not  including  that  of  the  horn  dust. 

t  Results  regarded  by  the  author  as  of  doubtful  value 

%  Zeit,  f.  Biol.,  22,  373. 

%Ibid.,  24,  293. 

||  Ibid.,  24,  553. 

f  Compare  Kellner,  Landw.  Vers.  Stat.,  24,  434;  and  Pfeiffer,  Jour.  f. 
Landw.,  33,  149. 


THE  RELATIONS   OF  METABOLISM    TO   FOOD-SUPPLY.      119 


Ration. 

Uncorrected. 

Corrected. 

i 
1 

Ah 

Nitro- 
gen 
Appar- 
ently 
Di- 
gested. 

Nitro- 
gen 

of 
Urine. 

Gain. 

Com- 
puted 
Nitro- 
gen 
Di- 
gested. 

Nitro- 
gen 
of 
Urine. 

Gain. 

I. 

500 

grms 

beans .... 

Grms. 
20.51 

Grms. 
20.93 

Grms. 
-0.42 

Grms. 
22.02 

Grms. 
20.93 

Grms. 

+  1.09 

II. 

j  490 
(  515 
J490 
|515 

Averaj 

(< 

beans  ) 
straw  f  '  ' 
beans ) 
straw  (  '  ' 

[I.  and  IV  . 

19.58 

16.82 

+  2.76 

21.78 

16.82 

+  4.96 

IV. 

re  of 

18.81 

17.26 

+  1.55 

21.09 

17.26 

+  3.83 

+  2.16 

+  4.40 

III. 

(510 
\  180 
(    20 
(  500 

2;rn 

is 

beans     ) 
starch   >  .. 
sugar     ) 
beans    ) 

20.03 

14.94 

+  5.09 

22.16 

14.94 

+  7.22 

V. 

{    90 

' 

starch   >■  . . 

20.64 

17.75 

+2.89 

22.43 

17.75 

+4.68 

(  10 

" 

sugar     ) 

It  will  be  seen  that  the  experiments  make  substantially  the 
same  showing  for  the  relative  effects  of  cellulose  and  starch  whether 
we  take  the  uncorrected  results  or  eliminate  so  far  as  possible  the 
effects  of  the  greater  amount  of  food  in  the  later  periods  upon  the 
excretion  of  metabolic  products  in  the  feces.  The  addition  of  starch 
and  sugar  in  Period  III  produced  about  twice  as  great  an  effect  in 
reducing  the  proteid  metabolism  as  did  a  somewhat  larger  amount 
of  digestible  fiber  and  nitrogen-free  extract  from  straw  in  Periods 
II  and  IV.  In  Period  V  the  starch  added  was  only  equal  to  the 
digested  nitrogen-free  extract  of  the  straw  in  Periods  II  and  IV. 
Since  the  effect  upon  the  proteid  metabolism  is  substantially  the 
same,  Weiske  concludes  that  the  nitrogen-free  extract  of  the 
straw,  which  has  the  elementary  composition  of  starch,  is  equal  to 
it  in  its  effect  upon  the  proteid  metabolism,  and  that  the  digested 
crude  fiber  is  valueless  in  this  respect.  It  must  be  said,  however, 
that  this  latter  conclusion  is  not  warranted  by  the  facts,  since  it 
rests  upon  the  unproved  assumption  of  equality  of  nutritive  value 
(in  respect  of  the  proteid  metabolism,  at  least)  of  starch  and  the 
nitrogen-free  extract  of  the  straw.  Weiske  also  experimented 
with  rabbits,  finding  in  one  case  no  effect  upon  the  proteid  metab- 
olism and  in  the  second  an  increase  of  it,  as  a  result  of  adding  crude 
fiber  to  a  fiber-free  ration. 


120  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Lehmann  *  experimented  upon  a  sheep  by  adding  respectively 
crude  fiber,  prepared  from  wheat  straw,  and  starch  to  a  basal  ration. 
The  results  were  not  entirely  sharp  but  showed  plainly  a  decrease 
of  the  proteid  metabolism  on  the  crude  fiber  ration  which  was 
equal  approximately  to  61  per  cent,  of  that  secured  by  the  use  of 
starch.  In  a  second  series  of  experiments,  Lehmann  and  Vogel  f 
compared  the  effects  upon  the  proteid  metabolism  of  sheep  of  cane- 
sugar  and  of  the  digestible  non-nitrogenous  matters  of  oat  straw. 
On  the  basis  of  a  very  careful  discussion  of  the  experimental  errors, 
they  show  that  the  latter  substances  have  a  marked  effect  in  diminish- 
ing the  proteid  metabolism,  and  in  particular  that  if  we  ascribe  this 
effect  exclusively  to  the  digested  nitrogen-free  extract,  as  Weiske 
does,  we  must  admit  that  the  latter  produced  an  effect  from  two 
to  nine  times  as  great  as  that  of  cane-sugar.  They  therefore  con- 
clude that  their  results  show  qualitatively  an  effect  of  the  digested 
cellulose  upon  the  proteid  metabolism.  Reckoning  the  digested 
nitrogen-free  extract  of  the  straw  as  equivalent  to  sugar,  they  com- 
pute from  the  average  of  all  their  experiments  that  the  cellulose 
produced  75.7  per  cent,  as  great  an  effect  as  the  sugar,  but  they  do 
not  regard  this  quantitative  result  as  well  established. 

Holdefleiss  \  experimented  upon  two  sheep,  feeding  in  a  first 
period  meadow  hay  exclusively.  In  the  second  period  one  half  of  the 
hay  was  replaced  by  a  mixture  of  peanut  cake,  starch,  and  a  little 
sugar,  while  in  the  third  period  the  starch  was  replaced  by  paper 
pulp.  In  one  case  a  fourth  period  was  added  in  which  the  paper 
pulp  and  sugar  were  simply  omitted  from  the  ration.  The  digested 
nutrients  and  the  proteid  metabolism  per  day  are  tabulated  on  p.  121. 

Converting  the  small  differences  in  the  amount  of  crude  fat 
digested  into  their  equivalent  in  nitrogen-free  extract  by  multipli- 
cation by  the  factor  2.5,  Holdefleiss  computes  from  a  comparison 
of  the  second  and  third  periods  that  the  digested  crude  fiber  pro- 
duced on  the  first  animal  80.1  per  cent,  and  on  the  second  animal 
84.2  per  cent,  of  the  effect  of  the  starch.  A  somewhat  higher  value 
would  be  obtained  from  a  comparison  of  the  first  and  second  periods 
in  the  case  of  Sheep  II,  while  on  the  other  hand  a  comparison  of  the 
corresponding  periods  with  Sheep  I  gives  a  much  lower  value,  and  is 

*  Jour.  f.  Landw.,  37,  267.  t  Ibid.,  37,  281. 

X  Bied.  Centr.  Bl.  Ag.  Chem.,  25,  372. 


THE  RELATIONS   OF  METABOLISM   TO  FOOD-SUPPLY.      121 


Sheep  I. 

Period  1 

"      2 


"  4 
Sheep  II. 
Period  1 

"      2 


Apparently  Digested. 


Crude 
Fat, 
Grms. 


Hay  only 

Hay,  peanut  cake,  sugar, 

and  starch 

Hay,   peanut  cake,   sugar, 

and  paper  pulp 

Hay  and  peanut  cake 

Hay  only 

Hay,  peanut  cake,  sugar, 

and  starch 

Hay,  peanut  cake,  sugar, 

and  paper  pulp 


13.55 
15.27 

13.67 

18.57 

11.76 
13.07 
15.14 


Crude 
Fiber, 
Grms. 


315.72 

134.11 

439.32 
171.12 

171.92 

77.48 
235.31 


N.  fr. 
Extract, 
Grms. 


470.85 

560.71 

320.21 
345.92 

276.42 

336.62 

198.46 


Nitro- 
gen, 
Grms. 


15.02 

13.55 

13.76 
16.25 

10.88 

9.54 

8.82 


Nitro- 
gen of 
Urine, 
Grms. 


13.83 

11.31 

11.26 
14.45 

8.45 

7.85 

7.62 


Gain 

of 
Nitro- 


1  19 

2.24 

2.50 
1.80 

2.43 

1.69 

1.20 


even  consistent  with  the  view  that  cellulose  has  no  effect  upon 
the  proteid  metabolism.  In  other  words,  the  results  on  Sheep  I,  in 
the  first  period,  appear  inconsistent  with  the  other  results. 

Kellner  *  has  experimented  with  rye  straw  extracted  with  an 
alkaline  liquid  under  pressure  in  the  same  manner  as  in  paper- 
making  and  containing  76.78  per  cent,  of  "crude  fiber"  and  19.96 
per  cent,  of  nitrogen-free  extract.  The  results  as  regards  the  pro- 
teid metabolism,  compared  with  those  on  starch,  are  given  in  the 
upper  table  of  p.  122. 

Taking  the  figures  as  they  stand,  and  attempting  no  correction 
for  the  marked  depression  in  the  apparent  digestibility  of  the  nitro- 
gen resulting  from  the  addition  of  the  extracted  straw  or  starch, 
they  show  a  considerable  effect  by  both  in  diminishing  the  proteid 
metabolism  relatively  to  the  supply  in  the  food  and  thus  causing 
an  increased  gain  of  nitrogen  by  the  body.  Any  correction  for  the 
metabolic  nitrogen  of  the  feces,  as  in  Weiske's  experiments,  would, 
of  course,  tend  to  make  the  effect  appear  still  greater.  With  the 
first  animal,  after  taking  account  as  well  as  possible  of  the  slight 
differences  in  the  fat  digested  in  both  periods  and  of  the  slight 
effect  of  the  starch  upon  the  digestibility  of  the  fiber  of  the  basal 
ration,  the  digestible  matter  of  the  extracted  straw,  five  sixths  of 
which  was  cellulose,  appears  to  have  produced  more  than  twice 
as  great  an  effect  as  an  equal  amount  of  starch.  With  the  second 
*  Landw.  Vers.  Stat.,  53,  278. 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Apparently  Digested. 

Nitrogen 

of  Urine, 

Grms. 

Gain  of 

Crude 
Fat, 
Grms. 

Crude 
Fiber, 
Grms. 

N.  fr. 

Extract, 
Grms. 

Nitrogen, 
Grms. 

Nitrogen 
Grms. 

OxH. 
Period  5 
"      4 

Extracted  straw . . 
Basal  ration 

Difference 

116 
101 

3129 
1083 

3351 
2912 

102.47 
116.51 

76.31 
109.28 

26.16 
7.23 

"      3 

15 

92 
101 

2047 

1057 
1083 

439 

4773 
2912 

-14.04' 

94.66 
116.51 

-32.97 

81.71 
109.28 

18.93 
12  95 

"      4 

Basal  ration 

Difference 

Extracted  straw . . 
Basal  ration 

Difference 

Starch. . .  . 

7.23 

Ox  J. 
Period  5 
«      4 

-9 

110 

107 

-26 

3101 
1114 

1861 

• 

3344 

2895 

-21.85 

112.19 
128.11 

-27.57 

95.80 
122.62 

5.72 

16.39 
5.49 

"      3 

3 

85 
107 

1987 

1105 
1114 

449 

4396 
2895 

-15.92 

118.18 
128.11 

-26.82 

103.13 
122.62 

10.90 
15  05 

«      4 

Basal  ration 

Difference 

5.49 

-22 

-9 

1501 

-9.93 

-19.49 

9.56 

animal,  on  the  contrary,  the  effect  of  the  digested  matter  of  the  ex- 
tracted straw  was  but  little  more  than  two  thirds  that  of  the  starch. 
Ustjantzen  *  has  recently  reported  the  results  of  an  experiment 
upon  a  sheep  substantially  like  those  of  Weiske  (p.  118),  a  basal 
ration  of  beans  receiving,  in  succeeding  periods,  additions  of  meadow 
hay,  rice,  or  sugar,  the  two  latter  being  computed  to  supply  an 
amount  of  digestible  carbohydrates  equal  to  the  digestible  nitrogen- 
free  extract  supplied  by  the  hay.  The  increased  amounts  of  crude 
fiber  and  nitrogen-free  extract  digested  and  the  resulting  increases 
in  the  gain  of  nitrogen  by  the  animal  were  as  follows : 


Crude  Fiber, 
Grms. 

Nitrogen-free 

Extract, 

Grms. 

Gain  of 

Nitrogen, 

Grms. 

108.60 

-2.53 

5.07 

95.55 
107.15 
109.20 

3.33 

2.90 

2.59 

It  appears  that,  as  in  Weiske's  experiments,  the  carbohydrates 
of  the  rice  and  sugar  produced  nearly  as  great  an  effect  upon  the 
*  Landw.  Vers.  Stat.,  56,  463. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       123 


gain  of  nitrogen  as  the  total  non-nitrogenous  matter  digested  from  the 
hay,  and  the  author  follows  Weiske  in  concluding  that  the  digested 
crude  fiber  producd  but  little  effect  on  the  proteid  metabolism. 

The  same  author  also  reports  experiments  upon  a  rabbit  similar 
to  those  of  v.  Knieriem,  crude  fiber  prepared  from  hay  being  added 
to  a  basal  ration  of  peas,  with  the  following  results,  which  show 
practically  no  effect  of  the  crude  fiber  upon  the  proteid  metabolism : 


Nitrogen 
of  Food, 
Grms. 

Nitrogen  Excreted. 

Gain  of 

Food. 

Urine, 
Grms. 

Feces, 
Grms. 

Total, 
Grms. 

Nitrogen. 
Grms. 

0.845 
0.857 
3.845 
0.860 

0.855 
0.821 
0.701 
0.899 

0.016 
0.120 
0.080 
0.170 

0.871 
0.941 
0.781 
1.069 

-0  026 

' '    and  5  grams,  crude  fiber .... 

"      "    5       "        sugar 

"      "    6.5"       crude  fiber ...  . 

-0.084 
+  0.064 
-0.209 

While  it  is  obviously  unsafe  to  draw  any  positive  conclusions 
regarding  the  relative  effect  of  cellulose  and  the  more  soluble  carbo- 
hydrates from  the  various  experiments  cited  above,  the  balance 
of  evidence  seems  clearly  to  show  that  their  influence  upon  the  pro- 
teid metabolism  is  qualitatively  the  same,  while  it  appears  on  the 
whole  probable  that  digested  cellulose  is  at  least  not  greatly  in- 
ferior quantitatively  to  digested  starch. 

Organic  Acids. — (pertain  methods  of  preparing  or  preserving 
fodder,  notably  ensilage,  result  in  the  formation  of  not  inconsider- 
able amounts  of  organic  acids.  Moreover,  it  appears  that  these  acids 
are  normally  produced  in  considerable  quantities  in  the  herbivora 
by  the  fermentation  of  cellulose  and  other  carbohydrates,  and  that 
fact  naturally  leads  to  a  consideration  of  their  effects  upon  meta- 
bolism as  compared  with  the  latter  substances. 

We  have  seen  (p.  27)  that  the  organic  acids  are  oxidized  in  the 
body,  and  it  therefore  seems  natural  to  suppose  that  they  may 
influence  the  proteid  metabolism.  This  question  has  been  investi- 
gated by  Weiske  &  Flechsig.*  After  some  only  partially  success- 
ful experiments  on  a  rabbit,  they  fed  a  sheep  with  a  basal  ration 
(of  hay,  starch,  cane-sugar  and  peanut  cake)  containing  a  liberal 
supply  of  protein  and  having  a  nutritive  ratio  of  1 : 3.4.  To  this 
ration  there  was  added  in  succeeding  periods  lactic  acid  as  calcium 
lactate,  acetic  acid  as  sodium  acetate,  and  for  comparison  dextrose. 
*Jour.  f.  Landw.,  37,  199. 


T24 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


Disregarding  for  our  present  purpose  the  slight  effect  of  these  sub- 
stances upon  the  digestibility  of  the  non-nitrogenous  ingredients 
of  the  ration  the  results  were: 


Basal  ration 

"  "      +   60  grms.  lactic  acid 

"      +120     "  "       "    I 

(Three  days  only.)  j 

Basal  ration 

"  "      +   60  grms.  dextrose   .  . 

"      +120     «  «  ) 

(Three  days  only.)  f 

Basal  ration 

"  "      +60  grms.  acetic  acid  | 

(Three  days  only.)  ) 


Nitrogen 

Digested, 

Grins. 

Nitrogen 

of  Urine, 

Grms. 

18.06 

17.56 

17.83 

15.60 

18.03 

15.72 

18.69 

16.85 

17.69 

15.29 

17.93 

12.86 

18.70 

16.54 

[18.70*] 

17.04 

Gain  of 

NUrogen, 

Grms. 


0.50 
2.23 

2.31 

1.84 
2.40 

5.07 

2.16 

1.66 


The  smaller  amount  of  lactic  acid  seems  to  have  produced  as 
great  an  effect  in  reducing  the  proteid  metabolism  as  an  equal 
weight  of  dextrose,  but  no  further  effect  was  noted  from  an  increase 
in  its  amount,  as  was  the  case  with  the  dextrose.  The  acetic  acid, 
on  the  contrary,  seems  to  have  had  a  tendency  to  increase  rather 
than  to  diminish  the  proteid  metabolism,  and  the  same  effect  was 
indicated  in  one  of  the  experiments  on  a  rabbit.  It  is  to  be  re- 
marked, however,  that  the  sodium  acetate  appeared  to  be  particu- 
larly obnoxious  to  the  animals.  In  the  case  of  the  sheep  it  was  in- 
troduced into  the  stomach  in  solution  by  means  of  a  funnel,  and 
besides  causing  the  animal  considerable  discomfort  had  a  very 
marked  diuretic  action.  It  may  perhaps  be  questioned  whether 
the  results  obtained  under  such  conditions  represent  the  normal 
effects  of  acetic  acid. 

Pentose  Carbohydrates. — While  the  fate  of  the  pentose  carbo- 
hydrates in  the  body  has  been  the  subject  of  considerable  research 
(compare  Chapter  II,  p.  24),  their  effect  upon  the  proteid  meta- 
bolism does  not  seem  to  have  been  specifically  investigated,  although 
Pfeiffer  &  Eber,f  in  the  course  of  experiments  upon  the  origin  of 
hippuric  acid,  observed  that  after  the  consumption  of  500  grams  of 

*  Assumed  to  be  the  same  as  with  the  basal  ration.  The  actual  nitrogen 
of  the  feces  for  these  three  days  was  4.78  grms.,  making  the  apparently 
digested  nitrogen  19.33  grms. 

+  Landw.  Vers.  Stat.,  49,  137. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       125 

cherry  gum,  containing  41.98  per  cent,  of  pentose  carbohydrates, 
the  urinary  nitrogen  of  a  horse  decreased  by  over  6  per  cent.  They 
leave  it  uncertain,  however,  whether  the  effect  was  due  to  the 
pentosans  or  to  other  ingredients  of  the  gum. 

Among  the  early  experiments  of  Grouven  *  are  also  four  in  which 
gum  arabic,  added  to  an  exclusive  straw  ration,  materially  reduced 
the  proteid  metabolism,  but  the  methods  of  these  early  experiments 
were  naturally  somewhat  defective.  On  the  other  hand,  Cremer's 
experiments  f  with  rhammose  on  rabbits  showed  no  marked  effect 
of  this  substance  upon  the  proteid  metabolism. 

Total  A 'on-nitrogenous  Matter  of  Feeding-stuffs. — The  digestible 
non-nitrogenous  matters  of  feeding-stuffs,  aside  from  a  small  pro- 
portion of  fat,  are  commonly  although  loosely  grouped  together  as 
carbohydrates.  They  include  both  hexose  and  pentose  carbohy- 
drates, such  organic  acids  as  may  be  present  or  as  are  formed  during 
digestion,  and  a  variety  of  other  less  well-known  substances. 

As  has  already  appeared  in  discussing  the  effect  of  crude  fiber,  the 
mixture  of  material  included  in  the  digestible  crude  fiber  and  nitro- 
gen-free extract  shows  the  same  tendency  as  starch  and  sugar  to 
diminish  the  proteid  metabolism.  In  other  words,  while  the  com- 
mon designation  of  digestible  carbohydrates  may  be  of  questionable 
accuracy  from  a  chemical  point  of  view,  nevertheless  the  some- 
what heterogeneous  mixture  to  which  it  is  applied  behaves,  in  this 
respect  at  least,  qualitatively  like  the  pure  hexose  carbohydrates. 
Numerous  instances  of  this  are  cited  by  v.  Wolff  %  m  his  discus- 
sion of  the  data  prior  to  1876.  Of  more  recent  results,  attention 
may  be  specially  called  to  those  of  Kellner,  some  of  which  have 
been  cited  above.  The  results  upon  coarse  fodders  are  those  which 
are  of  particular  interest,  since  it  is  these  whose  ingredients  are 
least  known  chemically.  They  are  presented  on  the  following 
page  in  the  same  form  as  those  upon  extracted  straw  above. 

Although  the  addition  of  hay  or  straw  to  the  basal  ration  in- 
creased the  supply  of  digestible  nitrogenous  matter,  the  proteid 
metabolism  was  not  proportionately  increased,  but  in  every  instance 

*  Wolff,  Ernahrung  Landw.  Nutzthiere,  p.  289. 

fZeit.  f.  Biol.,  42,451. 

%  Ernahrung  Landw.  Nutzthiere,  pp.  288-309. 


126 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Apparently  Digested. 


S  S  M 


Bug 

■Sfe-d 


Nitrogen 

of 

Urine. 


Gain 
of 

Nitro- 
gen. 


Meadow  Hay. 
Basal  ration  +  hay .  . 
<<  (i 

Difference  

Basal  ration  +  hay .  . 

Difference  

Basal  ration  +  hay .  . 

Difference 

Basal  ration  +  hay .  . 

Difference 

Basal  ration  +  hay .  . 

Difference 

Oat  Straw. 
Basal  ration  +  straw 

Difference 

Basal  ration  +  straw 

Differecne  

Wheat  Straw. 
Basal  ration  +  straw 

Difference  

ration  +  straw 
<<  <( 

Difference , 


123 
90 


33 


38 


150 
101 


49 


165 
101 


141 
107 


34 


139 
90 


49 


20 


15531  3850 
10071  3014 


Grms.  I  Grms.       Grms 
1383  133.84      97.19 
1069.111.67    106.03 


J6 .  65 
5.64 


546 


1675 
1137 


538 


1786 
1083 


703 

1822 
1083 


739 


1797 
1114 


683 


1701 
1007 


694 


1732 
1137 


115 
101 


595 


1904 
1083 


111 
107 


821 


1943 
1114 


829 


4006 
3120 


314    22.17 


1498  108.96 
1143    86.27 


4037 
2912 


355  22.1 


1487  145.94 
1071116. 51 


1125 


4148 
2912 


416'  29.43 


1531  146.84 
1071  116.51 


1236 


4108 

2S1I5 


460    30.33 


1542163.37 
1059  128.11 


-8.84 

91.30 
86.30 


5.00 


122.19 
109.28 


12.91 


130.78 
109.28 


1213 


3735 
3014 


21.50 


137.97 
122.62 


31.01 


17.66 
-0.03 


17.1 


23 .  75 
7.23 


16.52 


16.06 
7.23 


8.83 


25.40 
5.49 


483    35.26 


1553119.15 

1069  111.67 


721 


3804 
3120 


4S4 


1574 
1143 


684 


3436 
2912 


431 


1485 
1071 


524 


3511 
2895 


414 


1555 

1059 


616      496 


7.48 


91.77 
86.27 


5.50 


110.80 
116.51 


5.71 


128.94 
128.11 


0.83 


15.35 


99.40 
106.03 


-6.63 


71.36 
86.30 


-14.94 


106.32 
109.28 


•2.96 


119.89 
122.62 


■2.73 


19.91 


19.75 
5.64 


14.11 


20.41 
-0.03 


20.44 


4.48 
7.23 


-2.75 


9.05 
5.49 


3.56 


THE  RELATIONS   OF  METABOLISM   TO   FOOD-SUPPLY.      127 


save  one  the  gain  of  nitrogen  by  the  body  showed  a  marked  increase, 
and  this,  it  is  to  be  noted,  after  the  feeding  had  been  continued  for 
a  considerable  time.  The  one  exceptional  case,  on  wheat  straw, 
is  readily  explained  by  the  obvious  effect  of  this  material  in  increas- 
ing the  metabolic  nitrogen  of  the  feces  and  thus  diminishing  the 
apparent  digestibility  of  the  protein  of  the  ration.  Had  account 
been  taken  of  these  metabolic  products,  the  increased  gain  of 
nitrogen  by  the  animals  would  doubtless  have  been  more  marked 
in  all  cases.  This  gain,  it  would  seem,  may  fairly  be  ascribed  to  the 
large  additions  of  digestible  non-nitrogenous  matter  derived  from 
the  hay  or  straw  added. 

Comparative  Effects  of  Fat  and  Carbohydrates. — C.  Voit  * 
found  the  hexose  carbohydrates  to  be  superior  to  fat  in  diminishing 
the  proteid  metabolism.     He  gives  the  following  comparisons: 


Food  per  Day. 

Urea 

Meat, 
Grms. 

Carbohydrates  or  Fat, 
Grms. 

per  Day, 
Grms. 

Nov.  16-22,  1857 

"    22-Dec.  2,  1857 

150 
150 

176 
176 

400 
400 
400 

500 
500 
500 
500 

800 
800 

1000 
1000 
1000 
1000 
1000 
1000 
1000 

2000 
2000 

150-350  sugar 
250  fat 

100-364  starch 
250  fat 

200  fat 
250  starch 
250  sugar 

250  fat 
300  sugar 
200     " 
100     " 

250  starch 
200  fat 

0 
100  starch 
400      " 

0 
100  fat 
300   " 

0 

200-300  starch 
250  fat 

13.4 
15  6 

Oct.  28-Nov.  8,  1857 

15  1 

Nov.    8-15,             "     

16  2 

Feb.  23-25,  1861 

31  9 

"     25-28,      "    

30  5 

"    28-Mch.  3,  1861 

30  3 

June  19-23,  1859 

38  5 

"    23-26,     "     

32  7 

"    26-29,     "     

35  6 

"    29-Julv  2,  1859 

37  9 

Feb.  17-22,  1865 

52  8 

"    22-25,     "     

54  7 

July  23-26,  1864 

73  5 

"     26,           "     

68  5 

"    27,           "       

60  2 

"    27-Aug.  1,  1864 

79  6 

Aug.  1,  1864 

74  5 

"    2,     "     

69  3 

"    3,     "     

'      80  2 

Jan    7-12,  1859 

128  4 

"   12-15,     "    

135.9 

*  Zeit.  f.  Biol.,  5,  447. 


J28  PRINCIPLES   OP  ANIMAL   NUTRITION. 

Subsequent  investigations  have  substantially  confirmed  this 
conclusion.  Thus  Kayser  *  in  an  experiment  upon  himself  found 
that  the  replacements  of  the  carbohydrates  of  his  diet  by  an  amount 
of  fat  equivalent  to  them  in  heat  value  caused  a  marked  increase 
in  the  urinary  nitrogen,  resulting  in  a  loss  of  this  element  by  the 
body  in  place  of  the  previous  small  gain.  The  possible  effect  upon 
the  apparent  digestibility  of  the  proteids  of  the  food  does  not  appear 
to  have  been  considered. 

Wicke  &  Weiske  f  report  two  series  of  experiments  upon  sheep 
in  which  equivalent  ("isodynamic  ")  quantities  of  fat  and  of  starch 
were  added  to  a  basal  ration.  In  the  first  series  the  basal  ration 
was  comparatively  poor  in  proteids  and  fat,  having  a  nutritive  ratio 
of  about  1 : 8.3  ;  in  the  second  series  it  was  richer  in  both  these 
substances  and  had  a  nutritive  ratio  of  1  :5.1  and  1  :  6.3  for  the 
two  animals  respectively.  As  is  usually  the  case,  the  starch  dimin- 
ished the  apparent  digestibility  of  the  protein  of  the  basal  ration, 
while  the  fat  produced  but  a  slight  effect  in  this  direction.  Not- 
withstanding this  complication,  however,  the  effect  of  the  starch 
in  diminishing  the  proteid  metabolism  was  clearly  greater  than 
that  of  the  fat,  and  if  the  results  were  corrected  for  the  increase 
in  the  nitrogenous  metabolic  products  in  the  feces  they  would  be 
still  more  decisive. 

The  investigations  of  E.  Voit  &  Korkunoff  upon  the  minimum 
of  proteids,  which  will  be  considered  in  a  subsequent  paragraph, 
also  show  a  superiority  in  this  respect  of  the  carbohydrates  over 
the  fats  which  these  authors  ascribe  to  the  greater  lability  of 
their  molecular  structure  which  enables  them  to  enter  into  reactions 
in  the  body  more  readily  than  the  fats. 

Magnitude  and  Duration  of  the  Effect. — The  pre-eminent 
position  of  the  proteids  in  nutrition  has  perhaps  led  investigators 
to  attach  undue  importance  to  this  power  of  the  non-nitrogenous 
nutrients  to  diminish  the  proteid  metabolism.  It  is  well  to  note 
that  it  is  relatively,  small.  C.  Voit,  as  already  stated,  found  an 
average  decrease  of  about  7  per  cent,  with  fats  and  about  9  per 
cent,  with  carbohydrates,  and  subsequent  investigators  have  ob- 
tained results  entirely  comparable  with  these. 

Proteid  Metabolism  Determined  by  Supply. — In  the  presence 
*v.  Noorden,  Pathologic  des  Stoffwechsels,  p.  117. 
fZeit.  physiol.  Chem.,  21,  42;  22,  137. 


THE  RELATIONS   OF  METABOLISM    TO   FOOD-SUPPLY.       129 


ot  non-nitrogenous  nutrients  it  is  still  true  that  the  proteid  meta- 
bolism, or  more  exactly  the  excretion  of  nitrogen,  is  mainly  deter- 
mined by  the  supply  of  it  in  the  food  just  as  it  is  upon  an  exclusive 
proteid  diet.  Fat  or  carbohydrates  simply  produce  a  relatively 
small,  and  probably  more  or  less  transitory,  diminution  of  it  with- 
out affecting  the  substantial  truth  of  the  above  statement. 

Lawes  &  Gilbert,*  in  discussing  the  results  of  their  fattening 
experiments  upon  sheep  and  pigs,  called  attention  to  the  very  wide 
variations  in  the  amount  of  protein  consumed,  both  per  unit  of 
weight  and  especially  per  unit  of  gain,  and  concluded  that  the  ap- 
parent excess  of  protein  in  some  cases  must  have  served  substan- 
tially for  respiratory  purposes.  The  subsequent  investigations  of 
Bischoff,  Voit,  and  v.  Pettenkofer  upon  the  proteid  metabolism  of 
carnivora  showed  clearly  that  the  dependence  of  the  latter  upon 
the  proteid  supply,  which  is  so  marked  upon  a  purely  proteid  diet, 
is  equally  evident  upon  a  mixed  diet,  and  thus  supplied  a  scientific 
explanation  of  the  facts  observed  by  Lawes  &  Gilbert.  The 
effect  of  the  proteid  supply  upon  the  nitrogen  excretion  is  clearly 
shown  by  the  following  summary  of  Voit's  experiments :  f 


Food. 

Fat. 

Lean  Meat. 

Grms. 

Grms. 

250 

150 

17.0 

300 

176 

18.9 

250 

250 

19.7 

200 

500 

36.6 

200 

800 

56.7 

250 

1500 

100.7 

Since  Voit's  researches,  very  many  experiments,  among  the 
earliest  of  which  were  those  of  Henneberg  &  Stohmann  %  upon 
cattle,  have  confirmed  his  results,  both  for  carnivora,  herbivora 
and  omnivora.  A  somewhat  striking  example  is  afforded  by  Stoh- 
mann's  §  experiments  upon  milch  goats  which  are  summarized  in 
the  following  table : 

*Rep.  Brit  Asso.  Adv.  Sci.,  1852;  Rothamsted  Memoirs,  Vol.  II. 

t  Zeit.  f.  Biol ,  5,  329. 

%  Beitriige,  etc.,  Heft  2,  p  412. 

§Biologische  Studien,  121. 


5° 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Eaten  per  Day. 

Protein 

Digested 

Hay, 
Grms. 

Linseed 
Meal, 
Grms. 

per  Day, 
Grms. 

1500 

100 

111.6 

1450 

150 

125.0 

1400 

200 

132.2 

1350 

250 

150.9 

1250 

350 

170.5 

1100 

500 

193.8 

950 

650 

221.4 

800 

800 

257.2 

1600 

0 

92.9 

1600 

0 

74.1 

Protein 

Metabolized 

per  Day,* 

Grms. 


May 
June 

July 

Aug. 
Sept 
Oct. 


23-29 

6-12 
20-26 

4-10 
25-31 

8-14 
22-28 

5-11 
19-25 

3-9. 


66.6 

79.4 

90.6 

90.1 

101.6 

117.9 

143.1 

173.7 

56.3 

41.9 


A  full  compilation  of  these  earlier  results  has  been  made  by 
v.  Wolff ,f  and  the  fact  is  now  so  well  established  that  further  cita- 
tions would  be  superfluous. 

Rate  of  Nitrogen  Excretion. — Some  interesting  hints  as  to 
the  manner  in  which  the  non-nitrogenous  nutrients  produce  the 
effect  upon  the  proteid  metabolism  which  has  just  been  described 
are  afforded  by  a  consideration  of  the  rate  of  nitrogen  excretion 
under  their  influence. 

It  was  shown  in  the  preceding  section  that  the  effect  of  a  meal 
of  proteids  was  a  sudden,  almost  explosive,  increase  in  the  nitrogen 
cleavage  and  excretion,  reaching  its  maximum  within  a  few  hours 
after  the  meal.  If,  however,  non-nitrogenous  nutrients  are  given 
along  with  the  proteids,  the  character  of  the  curve  is  essentially 
altered,  the  maximum  rate  of  excretion  being  less  and  being  reached 
somewhat  later,  while  the  fall  from  this  maximum  is  less  rapid. 
In  other  words,  the  rate  of  excretion  becomes  more  uniform — the 
curve  is  flattened  out.  The  influence  of  fat  in  this  respect  is  clearly 
shown  in  the  experiments  of  Panum  \  and  of  Feder  \  cited  pre- 
viously, and  appears  evident  also  in  those  of  Graff enberger.  §  In 
the  latter  experiments  the  nitrogenous  substances  to  be  tested  were 
added  to  a  mixed  diet.  The  results  show  a  distinct  maximum,  but 
the  rate  of  decrease  after  the  maximum  was  reached  was  not  rapid, 

*  Exclusive  of  the  protein  of  the  milk. 

f  Ernahrung  Landw.  Nutzthiere,  pp.  285-309 

%  Thier.  Chem.  Ber.,  12,  402. 

§Zeit.  f.  Biol.,  28,  318. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY.       131 

and  only  a  part  of  the  nitrogen  appeared  during  the  twenty-four 
hours  following  its  ingestion,  viz.: 

With  fibrin 49 . 2  per  cent. 

With  gelatin 37.6    "      " 

With  peptone 67.6    "      " 

With  asparagin 79.0    "      " 

Rosemann's*  results  upon  the  rate  of  nitrogen  excretion  by 
man,  likewise  cited  above,  indicate  a  similar  effect  of  the  non- 
nitrogenous  nutrients,  the  fluctuations  due  to  the  ingestion  of  mixed 
food  being  much  less  sharp  than  those  found  by  other  experi- 
menters with  proteids  alone. 

If  we  accept  Rosemann's  view  (p.  101),  that  the  sudden  increase 
in  the  nitrogen  cleavage  is  due,  in  part  at  least,  to  a  direct  stimulus 
to  the  metabolic  activity  of  the  cells,  arising  from  the  presence  in 
the  fluids  of  the  body  of  an  increased  percentage  of  proteids,  we 
may  perhaps  suppose  that  the  simultaneous  resorption  of  non- 
nitrogenous  matter  renders  this  stimulus  less  and  so  reduces  the 
maximum  rate  of  nitrogen  cleavage.  This  conjecture  possibly 
receives  some  support  also  from  the  results  of  Krummacher,f  who, 
contrary  to  Adrian  and  Munk,  finds  that  the  division  of  the 
proteid  ration  into  several  meals  not  only  renders  the  rate  of  nitro- 
gen excretion  more  uniform,  but  reduces  somewhat  the  total  amount 
excreted.     Gebhardt  X  has  also  obtained  similiar  results. 

There  is  also  the  possibility,  however,  that  the  non-nitrogenous 
nutrients  may  modify  the  rate  at  which  the  proteids  are  resorbed, 
or  perhaps,  as  has  been  suggested  by  various  investigators,  the 
extent  to  which  the  proteids  are  converted  into  amide-like  bodies 
by  the  pancreatic  juice  or  the  extent  of  proteid  putrefaction  in  the 
intestines.  Suggestive  in  this  regard  is  the  fact  found  by  Gruber  § 
that  common  salt,  which  acts  as  a  stimulant  to  thesecretion  of  hydro- 
chloric acid  by  the  stomach,  and  would  thus  tend  to  favor  gastric 
as  compared  with  intestinal  digestion  of  the  proteids,  produces  an 
effect  on  the  nitrogen  excretion  similar  to  that  of  the  non-nitroge- 
nous nutrients. 

*Arch.  ges.  Physiol.,  65,  343. 
fZeit.  f.  Biol.,  35,481. 
%  Arch.  ges.  Physiol.,  65,  611. 
§Zeit.  f.  Biol.,  42,425. 


132  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Extent  of  Protein  Storage. — Whatever  may  be  the  expla- 
nation of  the  action  of  the  non-nitrogenous  nutrients,  its  effect  is 
obvious.  Attention  has  already  been  called  (p.  102)  to  Gruber's 
hypothesis  that  the  transitory  storage  of  nitrogen  following  an 
increase  in  the  proteid  supply  is  the  result  of  a  superposition  of  the 
daily  curves  of  nitrogen  excretion.  The  effect  of  the  non-nitroge- 
nous nutrients  appears  to  be  to  diminish  the  rate  of  nitrogen  cleavage 
and  to  protract  it,  in  the  case  of  a  single  meal  of  proteids,  over  a 
longer  time.  Evidently,  then,  an  increase  of  the  proteid  supply  in 
a  mixed  diet,  or  the  addition  of  non-nitrogenous  nutrients  to  a  pro- 
teid diet,  will  extend  its  effect  over  a  considerably  longer  period 
than  in  case  of  an  exclusive  proteid  diet — that  is,  nitrogen  equi- 
librium will  be  reached  more  slowly,  and  there  will  be  a  longer  *br 
shorter  time  after  the  change  during  which  the  nitrogen  excretion 
will  be  less  than  in  the  absence  of  the  non-nitrogenous  matters. 

This  explanation  also  implies,  however,  that  the  storage  of 
nitrogenous  matter  in  the  body  of  the  mature  animal  is  of  limited 
duration  and  that  no  long-continued  gain  of  protein  can  occur;  in 
other  words,  that  it  is  impossible  to  materially  increase  the  proteid 
tissue  (lean  meat)  of  a  mature  animal. 

Numerous  comparative  fattening  experiments  with  domestic 
animals,  notably  those  of  Henneberg,  Kern,  &  Wattenberg  *  upon 
sheep,  fully  sustain  this  conclusion.  On  the  other  hand,  metabo- 
lism experiments  with  domestic  animals  rarely  show  an  equality 
between  the  income  of  nitrogen  and  its  outgo  in  feces  and  urine, 
but  almost  always  indicate  a  gain  of  nitrogenous  matter  by  the 
body.  As  regards  the  significance  of  this  fact,  however,  several 
considerations  must  be  borne  in  mind. 

First,  the  normal  growth  of  the  epidermal  tissues — hair  or  wool, 
hoofs,  horns,  etc. — as  pointed  out  in  Chapter  III,  consumes  a  por- 
tion of  the  nitrogen  of  the  food  and  contributes  its  share  to  the 
storage  of  nitrogen  in  the  body. 

Second,  the  adipose  tissue  itself  contains  a  small  percentage  of 
proteid  matter,  and  a  storage  of  fat  in  considerable  amounts  in- 
volves the  production  of  new  adipose  tissue  in  which  to  store  it. 

Third,  in  many  cases  the  metabolism  experiments  which  show 
a  storage  of  nitrogen  have  been  made  within  a  rather  short  time 
*Jour.  f.  Landw.,  26,  549. 


THE  RELATIONS   OF  METABOLISM   TO  FOOD-SUPPLY.       133 

after  a  change  in  the  ration,  and  can  therefore  be  interpreted  as 
showing  simply  that  sufficient  time  had  not  elapsed  to  reach  nitro- 
gen equilibrium. 

If  we  consider  also  the  somewhat  indefinite  nature  of  the  term 
mature,  and  likewise  the  possibilities  of  error  due  to  mechanical 
losses  of  excreta  and  to  escape  of  nitrogen  from  the  latter  by  fermen- 
tation and  decomposition,  we  can  readily  see  why  the  results  of  a 
short  metabolism  experiment  may  not  agree  with  those  of  a  long 
fattening  experiment;  yet,  nevertheless,  it  must  be  confessed  that 
the  impression  left  by  a  comparison  of  the  whole  mass  of  evidence 
is  that  the  discrepancy  is  as  yet  but  partially  explained. 

In  conclusion,  we  may  anticipate  a  discussion  in  Chapter  VI, 
and  call  attention  to  the  fact  that  muscular  exertion  may,  to  a 
limited  extent  at  least,  stimulate  those  constructive  processes  which 
result  in  a  storage  of  protein  in  the  body. 

The  Minimum  of  Proteids. — In  the  preceding  section  it  ap- 
peared that  the  administration  of  proteid  food  to  a  previously  fast- 
ing animal  caused  a  prompt  and  large  increase  in  the  nitrogen 
cleavage  and  excretion,  while  but  a  comparatively  small  portion 
of  the  proteids  was  applied  to  constructive  purposes,  the  result 
being  that  two  to  three  times  as  much  proteids  must  be  given  as 
are  metabolized  during  fasting  before  nitrogen  equilibrium  is 
reached.  This  effect  was  there  ascribed  to  the  stimulating  effect 
of  the  rapid  digestion  and  resorption  of  the  proteids  upon  the  nitro- 
gen cleavage,  much  of  the  proteids  being  apparently  destroyed 
as  such  before  they  can  serve  for  tissue-building. 

We  have  just  seen  that  the  effect  of  the  non-nitrogenous  nutri- 
ents is  to  diminish  somewhat  the  nitrogen  cleavage,  apparently 
by  moderating  this  stimulating  effect.  The  necessary  result  is 
that,  as  the  nitrogen  supply  is  increased,  it  and  the  nitrogen  excretion 
will  start  more  nearly  together  and  approach  each  other  more  rapidly 
upon  a  mixed  diet  than  upon  one  consisting  of  proteids  only.  Conse- 
quently, while  the  percentage  decrease  in  the  proteid  metabolism 
is,  as  we  have  seen,  relatively  small,  nitrogen  equilibrium  may  be 
reached  with  a  much  smaller  supply  of  proteids  than  is  the  case  in 
the  absence  of  the  non-nitrogenous  nutrients.  Indeed,  it  is  con- 
ceivable that  a  sufficient  supply  of  carbohydrates  or  fats  in  the  diet 
should  practically  destroy  the  stimulative  effects  of  the  proteids  in 


34 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


which  case  we  might  expect  a  proteid  supply  equal  to  the  fasting 
proteid  metabolism  to  be  sufficient  to  produce  nitrogen  equilibrium. 
Seen  in  this  light,  the  apparently  insignificant  effect  of  the  non- 
nitrogenous  nutrients  becomes  a  very  important  factor  in  nutrition. 
The  effect  of  the  non-nitrogenous  nutrients  in  largely  diminish- 
ing the  necessary  proteid  supply  was  pointed  out  by  C.  Voit  *  and 
appears  clearly  in  many  of  his  experimental  results.  Thus  from  the 
summary  on  p.  95  it  appears  that  from  1200  to  1500  grams  of  lean 
meat  per  day  was  required  to  maintain  the  animal  experimented 
upon  in  nitrogen  equilibrium.  When  fat  or  carbohydrates  were 
added  to  the  ration,  however,  strikingly  different  results  were 
reached,  as  appears  from  the  following  comparative  statement, 
the  results  being  expressed  as  "  flesh  "  with  3.4  per  cent,  of  nitrogen : 


Meat. 


Fat  or  Carbo- 
hydrates. 


Flesh 
Meta- 
bolized. 


Gain  of 
Flesh. 


Meat  only  (average  of  both  series) 


Meat  and  fat 


Meat  and  carbohydrates  (compare  j 
p.  116) 1 


300 
600 
900 
1200 
1500 

500 

800 

1000 

500 

800 
1000 


250 
200 
250 

300-100 
100-400 
100-400 


416 
674 
943 
1207 
1478 

444 
720 

875 

502 
763 
902 


-116 

-  74 

-  43 

-  7 
+  22 

+  56 
+  80 
+  125 

-  2 
+  37 

+  88 


In  the  presence  of  non-nitrogenous  nutrients,  nitrogen  equi- 
librium was  reached  with  quantities  of  proteids  from  one  third  to 
one  half  as  great  as  the  amount  required  when  fed  alone.  In  other 
words,  the  non-nitrogenous  nutrients  materially  reduced  the  mini- 
mum of  food  proteids  required  to  maintain  the  proteid  tissues  of 
the  body. 

In  view  of  the  peculiar  importance  of  the  proteids  in  nutri- 
tion, as  well  as  of  their  relative  scarcity  and  high  cost,  particu- 
larly in  the  food  of  our  domestic  animals,  great  interest  attaches 
to  a  determination  of  the  least  amount  required  to  sustain  a  mature 
*  Zeit.  f.  Biol.,  5. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       135 


animal.  The  results  obtained  by  E.  Voit  &  Korkimoff  *  regard- 
ing the  minimum  requirement  upon  an  exclusive  proteid  diet  have 
already  been  stated  in  the  first  section  of  this  chapter  (p.  95).  The 
same  investigators  have  also  studied  the  more  interesting  question 
of  how  far  the  necessary  proteid  supply  can  be  reduced  in  the 
presence  of  non-nitrogenous  nutrients. 

Proteids  and  Fat. — The  experiments  were  upon  the  same 
general  plan  as  those  just  referred  to  on  proteids  alone.  Beginning 
with  an  insufficient  quantity  of  proteids,  the  amount  was  gradually 
increased,  that  of  the  fat  remaining  constant,  until  nitrogen  equi- 
librium was  reached.  As  in  those  experiments,  too,  the  nitrogen 
of  the  food  was  practically  all  in  the  proteid  form,  and  its  amount 
is  compared  with  the  proteid  nitrogen  excreted,  it  being  assumed 
that  18.45  per  cent,  of  the  urinary  nitrogen  was  derived  from  the 
extractives  of  the  flesh  metabolized  in  the  body.  To  the  writer  it 
would  seem  that  a  more  suitable  unit  would  be  the  total  excretory 
nitrogen,  since  the  proteids  of  the  food  had  to  make  good  the  loss 
of  extractives  as  well  as  of  true  proteids  from  the  body,  and  the 
former  loss  is  as  unavoidable  as  the  latter.  Accordingly,  the  results 
have  been  stated  in  the  table  below  in  both  ways. 

Two  series  of  experiments  were  made:  one  in  which  the  total 
food-supply  was  less  than  was  required  to  supply  the  estimated 
demands  of  the  body  for  energy,  and  one  in  which  it  considerably 
exceeded  that  demand,  with  the  following  results: 


Series  I: 

Experiment  1 
2 

Series  II: 

Experiment  3 
4 
5 


Total 

Excretion 

Fasting, 

Grms. 


4.85 
4.22 


4.98 
4.01 
3.86 


Per  Cent,  of  Energy 
Demand  Supplied  by 


Fat, 
Per  Cent. 


116 
127 
137 


Total 

Food, 

Per  Cent. 


90 


128 
140 
150 


MiDimum  of  Food  Nitrogen. 


7.63 
>5.61 


>6.61 
5.12 
5.07 


Per  Cent  or  Fasting 
Metabolism. 


Total,      Proteid, 
Per  Cent.  Per  Cent 


157 
>133 


>133 
128 
131 


193 
>163 


>162 
157 
161 


The  authors  also  compute  from  experiments  by  C.  Voit  and  by 
Rubner  percentages  lying  between  162  and  207,  and  state  as  their 
*  Zeit.  f.  Biol.,  32,  58. 


136  PRINCIPLES   OF  ANIMAL    NUTRITION. 

final  result  that  the  minimum  of  proteid  nitrogen  on  a  diet  of  pro- 
teids  and  fat  lies  between  160  and  200  per  cent,  of  the  proteid  nitro- 
gen excreted  during  fasting.  These  figures  when  computed  on  the 
total  excretory  nitrogen  would  become  131  per  cent,  and  163  per 
cent,  respectively. 

Proteids  and  Carbohydrates. — We  have  seen  (p.  127)  that 
the  carbohydrates  diminish  the  proteid  metabolism  to  a  greater 
extent  than  the  fats.  The  results  which  have  been  reached  as  to 
their  effect  in  lowering  the  minimum  demand  for  proteids  are  on 
the  whole  in  accord  with  this  fact.  With  a  liberal  supply  of  carbo- 
hydrates in  the  food,  a  much  smaller  quantity  of  proteids  would 
'  seem  to  suffice  to  maintain  nitrogen  equilibrium  than  when  the 
non-nitrogenous  matter  of  the  ration  consists  of  fat.  Indeed,  ac- 
cording to  some  investigators,  the  proteid  metabolism  may  eveL 
be  thus  reduced  much  below  that  during  fasting. 

Munk  *  appears  to  have  been  the  first  to  advance  the  view  last 
mentioned.  In  an  investigation  upon  the  formation  of  fat  from 
carbohydrates  a  dog  was  fasted  for  thirty-one  days  and  then  re- 
ceived a  diet  consisting  of  a  little  meat  with  large  amounts  of 
carbohydrates  (starch  and  sugar)  and  also,  during  the  first  twelve 
days,  gelatin.  Omitting  these  twelve  days  and  also  the  earlier  days 
of  the  fasting  period,  the  average  daily  excretion  of  nitrogen  in  the 
urine  was 

Twelfth  to  thirty-first  days  of  fasting 5.38  grams 

Thirteenth  to  twenty-fourth  days  of  feeding  (200 

grams  meat,  500  grams  carbohydrates) 5.79      " 

On  the  seventeenth  day  of  the  feeding  the  urinary  nitrogen 
reached  the  minimum  of  4.133  grams,  and  Munk  regards  this  as 
showing  the  possibility  of  a  reduction  of  the  proteid  metabolism 
considerable  below  the  fasting  level.  It  is  to  be  noted,  however, 
that  the  nitrogen  excretion  varied  considerably  from  day  to  day, 
and  a  selection  of  a  single  day  for  comparison  seems  hardly  justified. 

Hirschfeld  f  and  Kumagawa  \  found  that  the  nitrogen  equili- 

*  Arch.  path.  Anat.  u.  Physiol.,  101,  91. 
t  Ibid.,  114,  301. 
%  Ibid.,  116,  370. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       137 

brium  of  man  could  be  maintained  on  a  diet  containing  little  nitro- 
gen but  abundance  of  non-nitrogenous  nutrients.  Under  these 
conditions  the  urinary  nitrogen  was  reduced  to  5.87  grams  and  6.07 
grams  per  day  respectively,  and  the  total  nitrogen  excretion  to 
7.45  grams  and  8.10  grams,  amounts  much  lower  than  have  been 
observed  for  fasting  men.  Thus  in  the  extensive  investigations  by 
Lehmann,  Miiller,  Munk,  Senator,  &  Zuntz  *  of  the  metabolism 
of  two  fasting  men,  much  higher  figures  than  the  above  were  ob- 
tained for  the  urinary  nitrogen,  and  Munk  (loc.  cit.,  p.  225)  calls 
attention  to  the  fact  that  in  one  case  the  urinary  nitrogen  on  the 
second  day  succeeding  the  fasting  period  was  materially  less  than 
on  the  last  day  of  the  fasting,  viz.,  8.26  grams  as  compared  with 
9.88  grams. 

In  a  subsequent  series  of  experiments  upon  dogs,  Munk  f  showed 
that  by  very  liberal  feeding  with  food  poor  in  proteids  (rice  with 
small  amounts  of  meat)  the  nitrogen  balance  could  be  maintained 
for  a  considerable  time  at  an  amount  very  much  lower  than  pre- 
vious observers  had  found  for  the  proteid  metabolism  of  fasting 
dogs  of  similar  weight. 


Length 
of  Exper- 
iment, 
Days. 

Average 
Live 

Weight, 
Kgs. 

Food  per  Day. 

Urinary 

Fat, 
Grms. 

Starch, 
Grms. 

Nitrogen, 
Grms. 

Nitro- 
gen, 
Grms. 

With  food:    <  jjj 

[iv:::::: 

■n,     , .          <  Munk 

Fasting:   -jFalck 

5 
5 
4 
4 

11.20 
10.21 
9.88 
8.25 

14.4 
8.9 

55 

38 
53 
47 

116 
96 
108 
100 

2.63 

2.48 
2.66 
2.60 

2.61 
2.40 
2.67 
2.62 

3.65 

5.10 

Munk  also  cites  in  support  of  his  conclusions  Rubner's  results 
on  a  clog  fed  exclusively  on  carbohydrates.  A  reference  to  these 
results  as  tabulated  on  a  subsequent  page  does  in  fact  show  in  most 
cases  a  decrease  in  the  proteid  metabolism  as  compared  with  the 
fasting  values,  but  how  much  of  this  is  due  to  the  normal  decrease 
during  the  first  few  days  of  abstinence  from  proteid  food  it  is  dif- 

*Arch.  path.  Anat.  u.  Physiol.,  131,  Supp. 
\Ibid.,  132,91. 


138 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


ficult  to  decide.  Munk  also  cites  results  obtained  by  Salkowski,* 
who  observed  the  nitrogen  excretion  of  a  dog  on  a  light  ration  con- 
taining but  little  proteids  to  be  scarcely  greater  than  in  the  absence 
of  all  food. 

E.  Voit  &  Korkunoff  (loc.  cit.)  also  included  the  carbohydrates 
in  their  investigation  upon  this  subject,  following  the  same  general 
method  as  in  the  experiments  with  fat.  The  following  are  their 
results  compared  with  the  fasting  proteid  metabolism  exactly  as 
in  the  former  case : 


Total 

Per  Cent,  of 
Energy  Demand 

Minimum 

of  Food  Nitrogen. 

Live 

Nitrogen 
Excre- 

Supplied by 

Per  Cent.of  Fasting 
Metabolism. 

Weight, 
Kgs. 

tion, 
Fasting, 

Carbo- 

Total 
Food, 

Amount, 
Qrms. 

Grms. 

hydrates, 
Per  Cent. 

Per 
Cent. 

Total, 
PerCent. 

Proteid, 
Pt-rCent. 

Series  I: 

- 

Experiment  3o 

24.0 

4.93 

78 

91 

>5.43 

>no 

>133 

" 

2 

24.6 

4.94 

79 

92 

5.00 

101 

124 

Series  II: 

Experiment 

5 

27.7 

4.98 

111 

122 

5.11 

103 

126 

" 

1 

24.1 

5.25 

115 

126 

>4.91 

>94 

>123 

" 

2 

24.7 

4.94 

118 

131 

<4.35 

<88 

<108 

" 

4 

30.0 

4.08 

122 

136 

<4.47 

<110 

<134 

3b 

24.0 

4.93 

155 

168 

<4.48 

<91 

<111 

The  authors  also  compute  from  a  few  experiments  by  C.  Voit 
and  by  Rubner  values  not  inconsistent  with  the  above. 

When  compared  with  the  total  nitrogen  excretion,  the  results  of 
Voit  &  Korkunoff  show  in  but  a  single  case  a  minimum  unmistak- 
ably greater  than  the  fasting  proteid  metabolism.  In  three  cases 
the  minimum  falls  below  this  amount,  while  in  the  remaining  cases 
it  is  either  substantially  equal  to  it  or  doubtful.  Regarded  in  this 
way,  they  seem  on  the  whole  in  accord  with  Munk's  claim  that  the 
proteid  metabolism  may  be  reduced  below  the  fasting  limit.  Voit 
&  Korkunoff,  however,  dispute  this  and  subject  Munk's  experi- 
ments to  a  detailed  criticism,  the  principal  points  of  which  are  that 
in  the  earlier  experiments,  as  noted  above,  the  nitrogen  excre- 
tion was  irregular  and  that  the  result  of  a  single  day  is  arbi- 
trarily selected  for  comparison,  while  in  the  later  experiments  no 
*  Zeit.  physiol.  Chem.,  1,  44. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY.       139 

determinations  of  the  fasting  metabolism  of  the  animals  actually 
used  for  the  experiments  were  made.  By  a  re-computation  of 
Munk's  experiments  they  obtain  results  varying  but  little  from 
100  per  cent.  A  computation  from  the  average  figures  given  on 
p.  136,  assuming  3.4  per  cent,  of  nitrogen  in  the  meat  and  0.51 
grams  of  nitrogen  per  day  in  the  feces,  shows  that  the  minimum  is 
probably  less  than  107  per  cent,  of  the  fasting  nitrogen  excretion. 

Much  depends,  however,  upon  whether  we  take  as  the  unit  of 
comparison  the  total  nitrogen  excretion  or,  like  Voit  &  Korkunoff, 
eliminate  that  portion  derived  from  the  extractives.  If  we  select 
the  former,  then  it  appears  that  with  a  liberal  supply  of  carbohy- 
drates in  the  food  the  supply  of  proteids  certainly  need  not  exceed 
the  fasting  metabolism  in  order  to  maintain  nitrogen  equilibrium, 
and  perhaps  may  be  reduced  materially  below  it. 

Finally,  it  must  be  remembered  that  the  fasting  proteid  meta- 
bolism itself  is  not  a  constant.  In  Chapter  IV  it  was  shown  that 
as  the  store  of  fat  in  the  body  of  a  fasting  animal  becomes  depleted 
the  body  proteids  are  drawn  upon  to  an  increasing  extent  to  supply 
energy  to  the  animal.  It  is  not  possible  to  show  that  the  experi- 
mental results  which  have  been  cited  are  materially  affected  by  this 
variability  of  the  fasting  proteid  metabolism — indeed,  it  seems 
doubtful  whether  they  are — but  the  fact  that  the  demands  of  the 
organism  for  energy  may  affect  the  proteid  metabolism  is  of  itself 
sufficient  to  show  that  our  unit  of  comparison,  while  practically 
convenient  and  perhaps  sufficiently  accurate,  is  not  invariable. 

Amount  of  Non-nitrogenous  Nutrients  Required. — In 
most  of  the  experiments  which  have  been  cited,  the  very  low  figures 
for  the  necessary  proteid  supply  have  been  obtained  by  the  em- 
ployment of  an  amount  of  non-nitrogenous  nutrients  materially 
in  excess  of  the  estimated  requirements  of  the  animal  for  energy, 
although  in  no  case  was  this  latter  factor  actually  determined. 

Siven,*  however,  experimenting  upon  himself  with  a  diet  equal 
in  amount  to  that  ordinarily  required  to  maintain  his  weight,  was 
able  to  gradually  reduce  the  total  nitrogen  of  his  food  to  4.52  grams 
and  maintain  nitrogen  equilibrium.  He  did  not  determine  his  fast- 
ing metabolism,  but  the  above  figure,  which  is  equivalent  to  0.08 
gram  of  nitrogen  per  kilogram  live  weight,  is  lower  than  the  low- 
*  Skand.  Arch.  f.  Physiol.,  10,  91. 


140 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


est  fasting  values  previously  obtained,  Moreover,  much  of  the 
nitrogen  of  his  food  was  in  the  non-proteid  form,  the  proteid  nitro- 
gen being  estimated  at  only  0.03  gram  per  kilogram  live  weight. 

Cremer  &  Henderson  *  have  attempted  to  reproduce  Siven'a 
results  in  two  experiments  upon  a  dog,  the  total  amount  of  food 
being  equal  to  or  slightly  less  than  the  estimated  requirements  of 
the  animal.  Under  these  conditions  they  were  unable  to  reach 
even  as  low  a  minimum  as  did  Voit  &  Korkunoff.  On  the  other 
hand,  Jaffa,f  in  a  dietary  stud}'  of  a  child  on  a  diet  of  fruits  and  nuts 
(so-called  frutarian  diet),  observed  a  gain  of  nitrogen  by  the  sub- 
ject with  only  0.041  gram  of  food  nitrogen  per  kilogram  body  weight. 

The  Minimum  for  Herbivora. — The  ordinary  food  of  our 
domestic  herbivora  contains  an  abundance  of  non-nitrogenous 
matter  and  relatively  little  protein.  It  is  impossible,  for  obvious 
reasons,  to  determine  the  fasting  metabolism  of  ruminants,  and 
the  basis  for  comparisons  like  those  made  above  is  therefore 
largely  lacking.  There  is,  however,  abundant  evidence  to  show  that 
only  a  comparatively  small  amount  of  proteids  is  necessary  to 
maintain  the  nitrogen  equilibrium  of  cattle  in  particular,  although 
exact  data  as  to  the  least  amount  required  are  still  lacking. 

The  early  experiments  of  Henneberg  &  Stohmann  %  upon  the 
maintenance  ration  of  cattle  furnish  the  following  examples  of  the 
sufficiency  of  a  very  small  proteid  supply,  the  results  being  com- 
puted per  500  kgs.  live  weight  per  day : 


Digested. 

Gain  of 

Protein, 
Grnis. 

Non-nitrogenous 

Nutrients, 

Grms. 

Nitrogen 

by  Animal, 

Grms. 

Ox  J: 

Period  1 

178 
259 
209 

278 

4247 
3546 
3926 

3607 

4.0 

"      2 

21.0 

"      3 

11.0 

Ox  II: 

Period  2 

19.5 

*  Zeit.  f.  Biol.,  42,  612. 

f  U.  S.  Dept  Agr.,  Office  of  Expt.  Stations,  Bull.  107,  21. 
%  Beitnige  zur  Begriindung  einer  rationellen  Filtterung  der  Wiederkaiier, 
Heft  I. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       141 


The  following  figures,  obtained  by  the  same  investigators  *  in 
later  experiments,  are  taken  from  Wolff's  compilation :  f 


Live 

Weight, 

Kgs. 

Digested. 

Gain  of 

Protein, 
Grms. 

Non-nitrogenous 

Nutrients, 

Grins. 

Nitrogen 

by  Animal, 

Grms. 

1860-61. 
Ox  I.    Period    5 

514 
531 
533 
625 
643 

638 
643 
661 
701 
715 

315 
405 
375 
280 
435 

395 
410 
400 
445 
390 

2435 
4090 
4980 
3060 
4590 

4995 
3610 
3620 
5540 
6060 

0 

"       14 

"       16 

Ox  II.  Period     6 

+  9.6 

+  24.8 
+  9.6 
+  14.4 

+  0.5 

-  0.8 
+   4.0 

-  6.4 
+   3.2 

"       15 

1865. 
Ox  I.     Experiment  1 

2 

3 

Ox  II.            "            5 

6 

G.  Kiihn's  extensive  investigations  at  Mockern.J  together  with 
subsequent  ones  by  Kellner,§  afford  the  following  data  for  the 
periods  in  which  the  ration  was  approximately  a  maintenance 
ration : 


Live 

Weight, 

Kgs. 

In  Digested  Food. 

Gain  of 

Protein, 
Grms. 

Metabolizable 

Energy, 

Cals. 

Nitrogen 

by  Animal, 

Grms. 

Kiihn's  Experiments: 

Ox    II.    Period  1 

632 
632 
631 
623 
602 
644 
672 

620 
612 
748 
750 
858 

413 
338 
339 
320 
451 
458 
540 

440 
213 
343 
696 
665 

16388 
17986 
18077 
17125 
15072 
15872 
17416 

16322 
15447 
13716 
18655 
24558 

+    0.1 

"   III.        "       1 

-    2.6 

"    IV.        "       la 

-   0.5 

"    IV.        "       lb.... 

-   5.7 

"     V.        "       1 

+   8.5 

"VI.       "       I 

+  6.3 

"XX.       "       1 

+  3.3 

Kellner's  Experiments: 

Ox    A 

+   6.2 

"     B 

-14.6 

"      I 

-13.8 

"     II 

-  2.8 

"  III 

+   5.1 

*  Beitrage,  etc.,  Heft  II.,  and  Neue  Beitrage,  etc. 
t  Ernahrung  landw.  Nutzthiere,  pp.  406-410. 
I  Landw.  Vers.  Stat.,  44,  257. 
$Ibid.:  47,  275;  50,  245. 


142 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


Experiments  by  the  writer  *  have  shown  that  nitrogen  equi- 
librium may  be  maintained,  for  a  time  at  least,  on  even  smaller 
amounts  of  protein  than  the  above  figures  would  indicate.  The 
figures  in  the  first  column  of  the  following  table  signify  the  proteid 
nitrogen  only  of  the  food  multiplied  by  6.25: 


Digested  Pro- 
teids  per  Day 
and  500  Kgs. 
Live  Weight, 
Grms. 


Meta- 
bolizable 

Energy 

of  Food, 

Cals. 


Average 
Live 

Weight, 
Kgs. 


Gain  or  Loss 

of  Nitrogen 

by  Body, 

Grms. 


Nutritive 
Ratio 

1:  . 


Experiment  I: 

Steer  1 

"     2 

"     3 

Experiment  II: 

Steer  1 

"     2 

"     3 

Experiment  VI: 

Steer  1 

"     2 

"     3 

Experiment  VII: 

Steer  1  

"     2 

"     3 

Experiment  VIII 

Steer  1 

"  2 

"  3 


129 
113 
133 


192 
202 
209 


297 
277 
314 


156 
131 
152 


258 
242 
275 


7956 
7588 
7191 


8144 
9590 
8084 


11130 
11318 
11324 


11955 
11904 
11557 


11634 
12976 
12030 


420 
450 
400 


420 
450 
400 


450 
490 
430 


450 
490 
430 


543 
629 
516 


-2.51 
-0.39 
-1.08 


+  1.76 
+  4.23 
+  4.62 


+  4.67 
+  6.47 
+  2.65 


+  5.68 
+  3.98 
+  4.15 


+  0.26 
-0.20 
-2.31 


20.1 
20.4 
18.6 


13.4 
13.6 
12.8 


10.9 
10.9 
10.6 


23.0 
25.3 
23.9 


10.4 
10.7 
10.6 


While  the  above  data  are  hardly  sufficient  to  fix  absolutely  the 
minimum  of  proteids  for  cattle  on  a  maintenance  ration,  they  indi- 
cate clearly  that  from  200  to  300  grams  of  digestible  protein  per  day 
is  at  least  sufficient  for  a  steer  weighing  500  kgs.,  and  there  is  a 
possibility  that  the  amount  may  be  somewhat  further  reduced. 
Although  we  are  unable  to  compare  this  with  the  fasting  meta- 
bolism, a  comparison  on  the  basis  of  live  weight  with  some  of  the 
results  previously  cited  shows  that  the  minimum  demand  for  pro- 
teids on  the  part  of  cattle  is  relatively  much  less  than  on  the  part 
of  carnivora.  Thus  the  results  obtained  by  Lehmann  et.  al.  and 
Munk  (p.  137),  and  by  Voit  &  Korkunoff  (p.  138),  computed  in 
*Penna.  Expt.  Station,  Bull.  42,  165. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       143 

grams  of  food  nitrogen  per  kilogram  live  weight,  give  the  following 
figures  for  the  minimum  nitrogen  requirements  of  the  dog  and  of 
man  as  compared  with  cattle : 

Experiments  on  Dogs. 


Munk 


Average. 


Voit  &  Korkunoff . 


Experiments  on  Man. 
Lehmann  etal 


f  0.235  gram 

J  0.243 

it 

]  0.269 

tt 

[0.315 

a 

0.266 

it 

>0.226 

it 

0.203 

tt 

0.185 

" 

>0.204 

u 

<0.176 

it 

<0.149 

" 

<0.187 

it 

f 0.190 

tt 

J  0.180 

a 

|  0.090 

it 

10.180 

it 

Experiments  on  Cattle. 
Range  of  experiments  cited 0 .  064-0 .  098  gram. 

Only  one  of  the  results  on  man,  together  with  the  very  low 
figure  obtained  by  Siven  (p.  139),  is  comparable  with  those  reached 
with  cattle.  Whether  we  are  to  ascribe  the  small  demand  of  the 
latter  for  proteids  to  a  specific  difference  in  their  rate  of  meta- 
bolism or  to  the  large  amounts  of  carbohydrate  material  which 
they  habitually  consume  does  not  clearly  appear. 

Effects  upon  Health. — Munk,  in  his  experiments  with  rations 
very  poor  in  proteids,  made  the  observation  that  while  such  rations 
were  adequate  to  maintain  the  nitrogen  balance  of  the  body  they 
nevertheless  appeared  to  produce,  in  time,  profound  functional  dis- 
turbances, sometimes  ending  in  death.  Similar  observations  have 
also  been  made  by  Rosenheim.*      These  experimenters  ascribe 

♦Arch.  ges.  Physiol.,  54,  61. 


144 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


the  ill  effects  directly  to  the  small  supply  of  proteids,  but  some  other 
writers  appear  inclined  to  explain  them  as  due  to  the  long  continu- 
ance of  a  uniform  and  rather  artificial  diet.  The  writer's  experi- 
ments, cited  above,  showed  no  evidence  of  any  ill  effect  in  the  case  of 
cattle  upon  a  ration  containing  but  about  200  grams  digestible  pro- 
tein per  day  and  continued  for  seventy  days,  and  subsequent  obser- 
vations, as  well  as  the  common  experience  of  farmers  in  wintering 
cattle  upon  such  feeding-stuffs  as  inferior  hay,  straw,  etc.,  fully 
confirm  this  result. 

Effects  on  Total  Metabolism. 

Substitution  for  Body  Fat. — We  have  seen  in  the  preceding 
section  that  proteid  food,  or  rather  the  non-nitrogenous  residue 
arising  from  its  cleavage  in  the  body,  may  be  utilized  as  a  source  of 
energy  in  place  of  the  body  fat  which  would  otherwise  be  meta- 
bolized. Similarly,  the  non-nitrogenous  nutrients  supplied  in  the 
food  may  be  thus  substituted  for  body  fat  in  the  metabolism  of  the 
animal.  The  substitution  is  shown  most  clearly  in  experiments 
upon  fasting  animals,  although  it  appears  also  in  those  in  which 
these  nutrients  are  added  to  an  insufficient  ration. 

Fat. — The  following  averages  of  Pettenkofer  &  Voit's  experi- 
ments *  computed  from  Atwater  &  Langworthy's  digest,f  illustrate 
this  substitution  of  food  fat  for  body  fat : 


Food, 
Grms. 

Number  of 
Experiments. 

Gain  or  Loss  by  Body. 

Nitrogen, 
Grms. 

Fat, 

Grms. 

Nothing 
100  fat 
350    " 

5 
2 

-6.64 
-4.90 
-7.70 

-  97.76 

-  16.25 
+  113.60 

The  smaller  amount  of  fat  not  only  diminished  the  proteid  meta- 
bolism but  also  largely  reduced  the  loss  of  fat  from  the  body.  The 
larger  amount  of  fat  showed  the  tendency  noted  on  p.  115  to  increase 
the  proteid  metabolism,  but  at  the  same  time  it  not  only  suspended 
the  loss  of  body  fat  but  caused  a  storage  of  fat  in  the  organism.  Of 
course  we  have  no  means  of  distinguishing  in  such  a  case  between 
*  Zeit.  f.  Biol.,  6,  370. 
t  U.  S.  Dept.  Agr.,  Office  of  Experiment  Stations,  Bull.  45. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY. 


45 


food  fat  and  body  fat,  but  it  is  most  natural  to  suppose  that  the  re- 
sorbed  fat  of  the  food,  being  already  in  circulation  in  the  body,  is 
more  easily  accessible  to  the  active  cells  than  the  stored-up  fat  of 
the  adipose  tissue  and  is,  therefore,  metabolized  in  preference  to  the 
latter. 

Rubner,*  in  his  study  of  the  replacement  values  of  the  several 
nutrients,  has  demonstrated  the  same  effect  of  food  fat.  Fat 
supplied  in  the  food  is  utilized  as  a  source  of  energy  to  the  body  and 
a  corresponding  quantity  of  body  fat  escapes  oxidation,  while  if 
supplied  in  excess  fat  is  stored  up  in  the  body.  The  experiments 
were  made  in  the  same  manner  and  are  computed  on  the  same 
assumptions  as  those  upon  proteids  recorded  on  p.  106.  All  were 
on  dogs  except  the  third,  which  was  on  a  rabbit. 


Total  Nitrogen 

of  Excreta, 

Grms. 


Fat 

Metabolized, 

Grms. 


Gain  or  Lo 
of  Fat, 
Grms. 


Nothing 

200  grms.  bacon 

Nothing 

39 .  75  grms.  of  butter  fat 

Nothing 

26 . 1  grms.  bacon  f 

Nothing 

100  grms.  fat 

Nothing 

40  grms.  bacon 


2.14 
2.44 


0.778 
1.045 


2.56 
2.48 


1.08 
1.32 


60.47 
71.80 


33.78 
33.48 


7.18 
6.44 


42.40 
47.73 


22.88 
28.73 


-  60.47 
+  128.20 


-   33.78 
+     6.27 


-     7.18 
+    19.63 


-  42.40 
+   52.27 


-  22.88 
+   11.27 


In  nearly  every  case  there  was  a  slight  increase  in  the  proteid 
metabolism,  as  in  Pettenkofer  &  Voit's  experiments,  and  a  some- 
what, greater,  although  still  not  very  considerable,  increase  in  the 
fat  metabolism.  In  the  main,  however,  the  food  fat  was  metabolized 
in  place  of  the  body  fat. 

In  those  of  Pettenkofer  &  Voit's  experiments  in  which  fat  was 
added  to  an  insufficient  ration  of  meat  the  same  effect  was  pro- 
duced, as  appears  when  we  compare  the  results  upon  a  ration  of  meat 

*  Zeit.  f.  Biol.,  19,  328-334;  30,  123. 
t  Results  approximate  only. 


146 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


and  fat  given  on  p.  150  with  those  upon  the  same  ration  of  meat 
without  the  fat,  as  in  the  table  below: 


Number 

of 
Experi- 
ments. 

Food  per  Day. 

Gain  or  Loss  by  Body. 

Meat, 
Grms. 

Fat, 
Grms. 

Nitrogen, 
Grms. 

Carbon, 
Grms. 

6 
1 
5 

500 
500 
500 

ioo 

200 

-3.4 
+  0.3 
-0.6 

-49.1 

+  27.1 

+  67.3 

Carbohydrates.  —  The  more  soluble  hexose  carbohydrates 
when  given  to  a  fasting  animal  serve,  like  the  fats,  as  a  source  of 
energy  for  the  organism  in  place  of  the  body  fat  which  would  other- 
wise be  oxidized. 

The  following  is  a  summary  of  the  average  results  obtained  by 
Pettenkofer  &  Voit  *  by  feeding  starch  with  a  small  amount  of 
fat,  the  fasting  metabolism  being  the  same  as  that  just  given  on 
p.  144.  The  averages  are  computed  as  before  from  Atwater  & 
Langworthy's  digest  (loc.  cit.) : 


Number 

of 
Experi- 
ments. 

Food  per  Day. 

Gain  or  Loss  by  Body. 

Starch, 
Grms. 

Fat. 
Grms. 

Nitrogen, 
Grms. 

Carbon, 
Grms. 

5 

II 

450 
597 
700 

16^9 
21.2 
20.2 

-6.64 
-7.20 
-9.40 
-6.20 

-97.76 

+  19.40 
-28.50 

+  61.30 

The  fasting  metabolism  in  this  case  represents  a  series  of  experi- 
ments antedating  by  a  year  or  two  that  upon  starch.  In  only  one 
case  were  the  respiratory  products  of  the  fasting  animal  determined 
during  the  latter  series.  That  determination  immediately  fol- 
lowed a  day  on  which  a  large  amount  of  starch  was  consumed, 
and  the  results  are  believed  by  the  authors  to  be  affected  thereby. 
No  very  strict  comparison  is  therefore  possible,  but  the  general 
effect  of  the  starch  in  diminishing  the  loss  of  body  fat  is  evident. 

♦Zeit.  f.  Biol.,  9,  485. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY.       147 

The  experiments  by  Rubner,*  which  have  been  already  several 
times  referred  to,  include  trials  in  which  sugar  or  starch  was  fed 
alone.  The  results  are  computed  as  previously  described,  with  the 
additional  assumption  that  all  the  carbohydrates  digested  (with  the 
exception  of  small  amounts  of  sugar  found  in  the  urine  in  some 
cases)  were  completely  oxidized  in  the  body.  The  gain  or  loss  of 
carbon  as  fat  is  therefore  computed  by  subtracting  from  the  total 
excretory  carbon,  first,  the  carbon  due  to  the  protein  metabolized, 
and  second,  that  assumed  to  be  derived  from  the  carbohydrates. 
On  this  basis  the  results  are  as  follows,  the  amounts  of  carbohydrates 
given  in  the  table  being  those  believed  to  have  been  actually  oxi- 
dized: 


Total  Nitrogen 

of  Excreta, 

Grms. 


Total  Carbon 

of  Excreta,t 

Grms. 


Gain  or  Los« 
of  Fat, 
Grms 


Nothing 

76 .  12  grms.  cane-sugar 
104.97     "  "       " 

Nothing 

97 . 3  grms.  cane-sugar . 

17.0     "  "        "     . 

143.0     "  "        "     . 

Nothing 

34  grms.  cane-sugar 

45     "  "        "      

50     "  "       "      

Nothing 

Nothing 

42 .  96  grms.  starch 

Nothing , 

57 .  38  grms.  starch , 

Nothing  (second  day) . . . 
94  .  36  grms.  cane-sugar 
67.96     "       starch 
4.70     "       fat 


1.94 
1.45 

1.07 

1.86 
1.92 
1.41 
1.22 

1.32 
1.41 
1.25 
1.57 
1.39 


2.00 
1.52 


2.64 
1.23 


38.18 
43.19 

47.78 

39.22 
50.69 
39.52 
46.45 

21.36 
26.18 
29.14 
27.68 
25.79 

26.47 
33.28 

31.53 
39.67 

27.86 

38.94 


-  40.99 

-  8.41 
+     0.51 

-  42.72 

-  2.95 

-  35.80 
+  23.32 

-  21. S8 

-  9.10 

-  7.46 

-  1.64 

-  27.86 

-  28.10 

-  10.54 

-  32.10 

-  10.74 

-  24.97 
+  116.35| 


In  place  of  the  slight  increase  in  the  proteid  metabolism  fre- 
quently noticed  when  fat  is  consumed,  the  tendency  of  the  carbo- 

*Zeit.  f.  Biol.,  19,  357-379;  22,  273. 

t  Not  including  the  carbon  of  the  carbohydrates  found  in  feces  and  urine. 

%  Total  gain  of  carbon,  computed  as  fat.     Compare,  loc.  cit.,  22,  279. 


[48 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


hydrates  seems  to  be  to  cause  a  slight  decrease,  but  the  chief  effect 
is  upon  the  carbon  metabolism,  increasing  rations  of  carbohydrates 
resulting  in  a  progressive  reduction  of  the  amount  of  body  fat  meta- 
bolized. 

The  effect  of  starch  or  sugar  when  added  to  an  insufficient  pro- 
teid  diet  may  be  illustrated,  as  in  the  case  of  fat,  by  a  comparison 
of  Pettenkofer  &  Voit's  results,  cited  on  p.  150,  with  those  on  pro- 
teids  alone: 


Number 
of 

Food  per  Day. 

Gain  or  Loss 
by  Body. 

Experi- 
ments. 

Meat, 
Grms. 

Fat, 

Grms. 

Starch. 
Grms. 

Dextrose, 
Grms. 

Nitrogen, 
Grms. 

Carbon, 
Grms. 

Proteids  alone 

"       and  starch.  . . 
"          "    dextrose  . 

6 
8 
3 

500 
500 
500 

5^3 

200 

200 

-3.4 
-1.8 
-1.3 

-49.1 
+    9.0 

+   7.2 

Mutual  Replacement  of  Nutrients. — The  facts  which  have  been 
considered  in  the  foregoing  pages  show  a  remarkable  degree  of 
flexibility  in  the  animal  organism  as  regards  the  nature  of  the  mate- 
rial consumed  in  its  vital  processes.  The  amount  of  proteid  mate- 
rial necessarily  required  for  the  metabolism  of  the  mature  animal 
we  have  seen  to  be  relatively  small.  Aside  from  this  minimum,  the 
metabolic  activities  of  the  body  may  be  supported,  now  at  the  ex- 
pense of  the  stored  body  fat,  now  by  the  body  proteids,  and  again 
by  the  proteids.  the  fats,  or  the  carbohydrates  of  the  food.  What- 
ever may  be  true  economically,  physiologically  the  welfare  of  the 
mature  animal  is  not  conditioned  upon  any  fixed  relation  between 
the  classes  of  nutrients  in  its  food-supply,  apart  from  the  minimum 
requirement  for  proteids.  The  possibility  of  a  mutual  replacement 
of  the  several  classes  of  nutrients  in  the  food  follows  almost  neces- 
sarily from  the  power  of  the  organism  to  utilize  them  all  indiffer- 
ently (in  a  qualitative  sense  at  least). 

Replacement  of  Proteids. — It  has  been  shown  that  proteids 
in  excess  of  the  minimum  demand  can  be  used  by  the  organism  to 
take  the  place  of  body  fat  previously  metabolized.  Furthermore, 
as  we  have  just  seen,  the  non-nitrogenous  nutrients  of  the  food  may 
likewise  be  substituted  for  body  fat.  It  is  natural  to  suppose,  there- 
fore, that  that  portion  of  the  proteid  supply  which  serves  substan- 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY.       149 

tially  as  a  source  of  energy  only  may  be  replaced  either  by  body  fat 
or  by  other  food  nutrients,  and  this  supposition  is  borne  out  by  the 
observed  facts. 

By  Body  Fat. — In  considering  the  total  metabolism  of  the  fast- 
ing animal  in  Chapter  IV,  we  saw  that  the  fat  of  the  body  has  a 
marked  effect  in  protecting  the  body  proteids  from  metabolism,  and 
that  with  the  progressive  impoverishment  of  the  body  in  fat,  more 
and  more  of  the  proteids  are  substituted  for  the  latter  as  a  source  of 
energy.  In  §  1  of  the  present  chapter  it  was  further  shown  that 
the  food  proteids,  or  their  non-nitrogenous  residue,  may  be  oxi- 
dized in  the  organism  in  place  of  the  stored  fat  of  the  body. 

It  is  clear,  however,  that  the  same  experiments  may  equally 
well  be  regarded  from  the  converse  point  of  view  as  showing  that 
the  body  fat  may  be  oxidized  and  serve  as  a  source  of  energy  in 
place  of  the  proteids  of  the  food  or  of  the  body.  In  other  words, 
it  is  possible,  within  quite  wide  limits,  for  the  animal  organism  to 
draw  its  supply  of  energy,  according  to  circumstances,  either  from 
food  or  body  proteids  or  from  its  stored-up  fat. 

By  Fats  and  Carbohydrates  of  Food. — When,  in  addition  to  its 
reserve  of  fat,  a  supply  of  non-nitrogenous  nutrients  is  afforded  in 
its  food,  this  range  of  choice  by  the  organism  is  still  further  widened. 
In  considering  the  effects  of  non-nitrogenous  nutrients  upon  the 
proteid  metabolism,  and  particularly  in  the  discussion  of  the  mini- 
mum of  proteids,  it  became  evident  incidentally  that  fat  or  car- 
bohydrates may  to  a  large  extent  be  substituted  for  proteids  in 
the  food.  A  certain  minimum  of  proteids  was  shown  to  be  essential 
to  the  maintenance  of  the  proteid  tissues  of  the  body,  but  proteids 
supplied  in  excess  of  this  amount  undergo  nitrogen  cleavage  and 
serve  substantially  as  a  source  of  energy.  This  excess  of  proteids, 
as  we  have  seen,  can  be  replaced  in  the  food  by  non-nitrogenous 
nutrients,  particularly  the  carbohydrates,  at  least  without  damage 
to  the  proteid  nutrition,  as  is  shown  by  Voit's  results  there 
cited  (p.  134).  The  later  respiration  experiments  of  Pettenkofer 
&  Voit  show  that  this  is  true  also  as  regards  the  total  metab- 
olism. As  appears  from  the  table  on  p.  109.  a  ration  of  1500 
grams  of  lean  meat  sufficed  to  maintain  the  dog  experimented 
upon  approximately  in  equilibrium  as  regards  the  income  and 
outgo   of  both  nitrogen   and   carbon.      When,   however,   a  con- 


i5° 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


siderable  proportion  of  this  meat  was  replaced  by  fat,  starch,  or 
sugar,  not  only  was  the  nitrogen  equilibrium  maintained  but  the 
same  was  true  of  the  carbon,  as  appears  from  the  following  averages 
computed  from  Atwater  &  Langworthy's  "  Digest  of  Metabolism 
Experiments."  *  The  results  of  Pettenkofer  &  Voit's  first  series 
with  1500  grams  of  lean  meat  as  given  by  them  are  also  included 
in  the  table: 


Food  per  Day. 

Gain  or  Loss 
by  Body. 

Meat, 
Grms. 

Fat, 
Grms. 

Starch, 
Grms. 

Grape- 
sugar, 
Grms. 

Nitrogen, 
Grms. 

Carbon, 
Grms. 

Proteids  only: 

1500 
1500 

500 
500 

500 
500 

100 

200 

5.3 

200 

200 

0 
+  0.6 

+  0.3 
-0.6 

-1.8 
-1.3 

+   3.3 

Average  of  all  (22  experiments) 

Proteids  and  fat: 

100  grms.  fat  (1  experiment)  .  . 
200      "        "  (5  experiments) . 

Proteids  and  carbohydrates  : 

Starch  (8  experiments) 

Grape-sugar  (3  experiments)  . 

+   8.7 

+  27.1 
+  67.3 

+  9.0 
+  7.2 

While  it  is  true,  as  was  stated  on  page  109,  that  there  is  reason 
to  suppose  the  carbon  balance  as  computed  by  Pettenkofer  & 
Voit  to  be  somewhat  in  error,  this  in  no  way  affects  the  general 
showing  of  the  above  averages.  The  introduction  into  the  diet  of 
100-200  grams  of  fat  or  carbohydrates  made  it  possible  to  dispense 
with  two  thirds  of  the  proteids  previously  required  to  maintain  the 
animal,  the  remaining  500  grams  of  meat  being  nearly  or  quite  suffi- 
cient to  maintain  nitrogen  equilibrium.  The  fat  or  carbohydrates 
added  were  obviously  used  by  the  organism  as  sources  of  energy  in 
place  of  the  proteids  (or  their  non-nitrogenous  residue)  oxidized 
for  this  purpose  on  a  purely  proteid  diet,  since  the  stored  fat  of  the 
body  was  not  only  conserved  but  even  shows  a  gain. 

Rubner's  investigations  upon  the  source  of  animal  heat  t  afford 


*  U.  S.  Dept.  Agr.,  Office  of  Expt.  Stations,  Bull.  45. 
Biol.,  7,  450-480;  9,  6-13  and  450-467. 
t  Zeit.  f.  Biol.,  30,  125-132. 


Compare  Zeit.  f. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY. 


a  similar  illustration  of  this  effect  of  non-nitrogenous  nutrients. 
Assuming  average  figures  for  the  nitrogen  and  carbon  content  of  the 
food  materials  used,  he  obtained  the  following  results: 


Food  per  Day. 

Gain  or  Loss  by  Body. 

Meat, 
Grms. 

Fat, 

Grms. 

Nitrogen, 
Grms. 

Carbon, 
Grms. 

Proteids  alone  (1  experiment) 

"         and  fat  (2  experiments). .  .  . 

350 
80 

30 

+  1.66 
-0.08 

-2.69 
+4.46 

The  possibility  of  such  a  substitution  of  non-nitrogenous  nutri- 
ents for  the  food  proteids  as  is  illustrated  in  the  foregoing  experi- 
ments seems,  indeed,  almost  a  necessary  corollary  of  the  facts  con- 
cerning proteid  metabolism  considered  on  previous  pages.  We 
have  seen  that,  beginning  with  the  fasting  metabolism,  the  effect 
of  successive  additions  of  proteids  to  the  food  is  to  stimulate  the 
proteid  metabolism.  Only  a  relatively  small  proportion  of  the 
added  proteids  is  employed  by  the  organism  for  constructive  pur- 
poses, the  larger  part  of  it  undergoing  very  promptly  nitrogen 
cleavage  and  thus  constituting,  to  all  intents  and  purposes,  an  ad- 
dition to  the  supply  of  non-nitrogenous  material  available  for 
metabolism.  It  appears  quite  natural,  then,  that  the  portion  of 
the  proteid  supply  which  thus  serves  substantially  as  fuel  to  the 
organism  should  be  replaceable  in  the  food  by  non-nitrogenous 
materials  which  are  capable  of  serving  the  same  purpose. 

Fats  and  Carbohydrates. — The  apparent  identity  of  the  func- 
tions of  the  fats  and  carbohydrates  as  sources  of  energy  which  has 
been  shown  in  the  preceding  paragraphs  necessarily  implies  the 
possibility  of  their  mutual  replacement  in  the  food.  Rubner  *  has 
completed  the  chain  of  evidence  by  showing  experimentally  that  fat 
and  dextrose  may  thus  replace  each  other.  A  dog  received  for 
twelve  days  a  ration  of  300  grams  of  lean  meat  and  42  or  50  grams 
of  fat,  with  the  exception  of  three  days,  on  which  varying  amounts 
of  dextrose  were  substituted  for  the  fat.  On  six  days  the  respi- 
ratory products  were  determined.  Averaging  the  results  for  all 
the  days  on  which  the  food  was  the  same,  and  assuming  the  lean 

*  Zeit.  f.  Biol.,  19,  370. 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


meat  used  to  have  contained  3.4  per  cent,  of  nitrogen  and  12.51 
per  cent,  of  carbon,  and  the  fat  76.5  per  cent,  of  carbon,  we  have : 


Food  per  Day. 

Gain  or  Loss  by  Animal, 

Meat, 
Grms. 

Fat, 
Grms. 

Dextrose, 
Grms. 

Nitrogen, 
Grms. 

Carbon, 
Grms. 

J300 

7  300 

(300 
^300 
(300 

42 

50 

63^7 

79.7 

115.5 

+  1.81 
+  0.10 
+  1.78 
+  2.28 
+  1.98 

+  1.27 

Meat  and  dextrose.  . . . 

+  9.31 
-7.44 
-8.15 
+  6.21 

The  averages  of  Pettenkofer  &  Voit's  results  as  tabulated  on 
p.  150  may  likewise  be  regarded  in  this  light. 

Relative  Values. — The  close  similarity  in  the  functions  of  the 
several  non-nitrogenous  nutrients  is  too  obvious  to  have  escaped 
early  notice,  and  the  investigations  of  the  Munich  school  of  physi- 
ologists served  both  to  emphasize  the  similarity  and  to  follow  it 
into  details.  To  Rubner,  a  pupil  of  Voit,  is  generally  ascribed 
the  credit  of  having  first  placed  in  a  clear  light  the  quantitative 
relations  of  the  subject,  although  v.  Hosslin*  and  Danilewskyf 
enunciated  similar  ideas  at  about  the  same  time,  which,  however, 
were  not  based  on  their  own  experiments. 

As  the  result  of  his  investigations  upon  the  replacement  values 
of  the  nutrients,!  Rubner  announced  the  law  of  "isodynamic  re- 
placement." This  law  is,  in  brief,  that  the  several  nutrients  can 
replace  each  other  in  amounts  inversely  proportional  to  their  physi- 
ological heat  values,  that  is,  to  the  amounts  of  heat  which  they 
would  liberate  if  oxidized  to  the  same  final  products  which 
result  from  their  metabolism  in  the  body.  In  other  words,  aside 
from  the  minimum  of  proteids  the  nutrients  are  of  value  to  the 
organism  in  proportion  to  the  amount  of  energy  which  their  meta- 
bolism liberates— they  are  "the  fuel  of  the  body."  One  gram  of 
fat,  for  example,  when  oxidized  to  carbon  dioxide  and  water,  liber- 
ates about  9.5  Cals.  of  energy,  while  one  gram  of  starch  similarly 


*  Arch.  path.  Anat.  u.  Physiol.,  89.  333. 
t  Die  Kraftvorriue  der  Nahrungsstoffe 
JZeit.  f.  Biol.,  19,  313. 


THE  RELATIONS   OF  METABOLISM   TO  FOOD  SUPPLY.       153 

oxidized  liberates  but  about  4.2  Cals.  The  relative  values  of  fat 
and  starch,  then,  are  as  9.5:4.2  or  as  2.26:1.  Similarly,  one 
gram  of  proteids  oxidized  to  carbon  dioxide,  water,  and  the  nitrog- 
enous metabolic  products  of  feces  and  urine  liberates  (in  the  dog) 
about  4.4  Cals.  of  energy.  So  far,  therefore,  as  they  are  used  as 
a  source  of  energy  simply  and  not  for  constructive  purposes,  then- 
value,  compared  with  starch,  would  be  as  4.4  : 4.2  or  as  1.05  : 1. 

A  rival  theory  of  "isoglycosic  values,"  the  basis  of  which  has 
already  been  indicated  in  Chapter  II,  has  been  advanced  by  Chau- 
veau  *  and  his  school  in  Paris.  According  to  this  school,  dextrose 
(or  glycogen)  constitutes  the  material  which  is  consumed  in  the 
vital  activities  of  the  organism.  The  various  nutrients,  then,  will 
be  of  value  to  the  organism  in  proportion  to  the  amount  of  gly- 
cogen or  dextrose  which  they  can  supply,  and  the  chemical  equa- 
tions already  given  on  pp.  38  and  51  are  claimed  to  show  sub- 
stantially what  that  amount  is.  The  carbohydrates,  according  to 
this  theory,  yield  practically  their  entire  store  of  energy  to  the 
organism,  while  if  the  equations  mentioned  are  interpreted  liter- 
ally the  sugar  produced  from  one  gram  of  proteids  would,  accord- 
ing to  Chauveau's  equation,  contain  but  about  1.83  Cals.  of  poten- 
tial energy  in  place  of  the  4.4  Cals.  available  from  the  proteids 
according  to  Rubner.  If  the  proteids  are  assumed  to  be  split 
up  in  accordance  with  Gautier's  equation  the  resulting  dextrose 
would  contain  about  80  per  cent,  of  their  potential  energy,  and 
this  figure  is  used  in  computing  their  isoglycosic  value.  Similarly, 
the  sugar  derived  from  one  gram  of  fat  would  contain  about  6.07 
Cals.  out  of  the  9.5  Cals.  contained  in  the  original  fat.  In  other 
words,  while  Chauveau  does  not  question  that  the  actual  food  of 
the  living  cells  is  of  value  in  proportion  as  it  supplies  energy,  he 
holds  that  in  the  complex  organism  of  the  higher  animals  a  con- 
siderable share  of  the  original  potential  energy  of  fats  and  proteids 
is  lost  during  their  conversion  into  material  (carbohydrates)  which 
the  cells  can  use. 

The  conception  of  the  mutual  replacement  of  the  nutrients  on 
the  basis  of  the  amounts  of  energy  they  are  capable  of  liberating 
for  the  use  of  the  organism  has  proved  a  fruitful  one  and  been  the 
basis  of  much  subsequent  research.     A  full  discussion  of  it  and 

*  La  Vie  et  l'Energie  chez  1' Animate. 


154  PRINCIPLES   OF  ANIMAL    NUTRITION. 

of  the  modifications  which  later  investigation  has  made  necessary 
in  Rubner's  original  conclusions,  is  possible  only  in  connection 
with  a  general  study  of  the  energy  relations  of  the  food,  the  animal, 
and  the  environment  such  as  forms  the  subject  of  Part  II.  For 
the  present  we  may  content  ourselves  with  accepting  the  general 
idea  that  the  relative  values  of  the  nutrients  depend  in  very  large 
measure  upon  their  ability  to  furnish  energy  for  the  vital  activi- 
ties, deferring  until  later  the  consideration  of  quantitative  rela- 
tions. 

The  Non-nitrogenous  Ingredients  of  Feeding-stuffs. — 
The  discussions  of  the  foregoing  paragraphs  have  had  reference  to 
the  effects  produced  by  pure  or  approximately  pure  nutrients  upon 
the  metabolism  of  carnivora.  By  reason  of  the  simplicity  of  con- 
ditions which  is  possible  in  such  experiments  they  are  indispensa- 
ble in  a  study  of  the  fundamental  laws  of  nutrition.  We  must 
presume  also  that  the  general  principles  established  by  such 
experiments  are  applicable  to  all  warm-blooded  animals,  since 
we  know  of  no  radical  differences  in  their  vital  processes. 

In  making  such  an  application  to  the  nutrition  of  our  domestic 
herbivorous  animals,  however,  much  caution  is  necessary  to  avoid 
unwarranted  assumptions  and  conclusions.  Two  points  need  espe- 
cially to  be  borne  in  mind : 

First,  the  food  of  these  animals  is,  from  a  chemical  point  of  view, 
very  heterogeneous.  In  addition  to  true  proteids,  there  are  present, 
especially  in  coarse  fodders,  various  non-proteid  nitrogenous  sub- 
stances, while  the  non-nitrogenous  nutrients,  besides  hexose  carbo- 
hydrates and  true  fats,  include,  on  the  one  hand,  pentosans  and 
pentoses,  lignin,  and  all  the  variety  of  unknown  substances  com- 
prised under  the  conventional  terms  "nitrogen-free  extract"  and 
"  crude  fiber, "  and  on  the  other  the  waxes,  resins,  coloring  matters, 
etc.,  contained  in  the  "crude  fat." 

Second,  the  process  of  digestion  in  herbivora,  and  especially  in 
the  ruminants,  as  was  pointed  out  in  Chapter  I.  differs  materially 
from  that  in  carnivora  as  regards  the  part  played  by  fermentative 
processes,  particularly  in  the  solution  of  the  carbohydrates  and 
related  bodies  which  are  so  abundant  in  vegetable  materials. 

It  has  been  more  or  less  customary  to  regard  the  digested  por- 
tions of  the  crude  fiber  and  nitrogen-free  extract  of  feeding-stuffs 


THE  RELATIONS   OF  METABOLISM   TO  FOOD-SUPPLY.       155. 

as  consisting  essentially  of  carbohydrates.  The  basis  for  this 
assumption  is  the  demonstration  by  Henneberg  that  the  ultimate 
composition  of  that  portion  of  these  two  groups  of  substances 
which  is  not  recovered  in  the  feces  is  substantially  that  of  starch 
or  cellulose,  while  Kellner  *  has  more  recently  demonstrated  their 
equality  in  energy  value.  This  fact  of  itself,  however,  does  not 
justify  the  inference  of  equal  nutritive  value,  as  may  be  readily 
seen  in  the  case  of  starch.  It  is  obviously  not  a  matter  of  indiffer- 
ence whether  a  given  amount  of  this  substance  is  resorbed  from  the 
digestive  canal  of  a  steer  in  the  form  of  sugar  or  whether,  as  in  some 
of  Kiihn's experiments,  65  percent,  of  it  is  converted  into  methane, 
carbon  dioxide,  and  organic  acids,  yet  the  elementary  composition 
of  the  "  digested  "  portion  would  be  the  same  in  either  case. 

The  fact  is  that  while  the  resorbed  food  of  herbivora  contains 
proteids,  carbohydrates,  and  fats,  whose  functions  in  nutrition  must 
be  assumed  to  be  the  same  as  in  the  carnivora,  it  is  very  far  from 
consisting  entirely  of  them,  but  contains  also  a  variety  of  other 
substances  of  whose  exact  nature  and  proportions  we  are  compara- 
tively ignorant.  We  know,  of  course,  that  the  digested  non-nitrog- 
enous ingredients  of  feeding-stuffs,  taken  as  a  whole,  do  serve  as 
sources  of  energy.  When  an  ox  or  a  sheep  is  fed  exclusively  on 
ordinary  coarse  fodders  such  as  hay,  straw,  or  corn  stover,  the  small 
supply  of  proteids  that  he  receives  is  likely  to  be  little  if  any  in  ex- 
cess of  the  minimum  demand,  and  the  requirements  of  the  body  for 
energy  must  be  satisfied  very  largely  by  the  non-nitrogenous  mate- 
rials. Moreover,  the  supply  of  such  substances  as  starch,  sugars, 
and  true  fats  in  such  a  case  is  so  small  relatively  that  it  appears 
difficult  to  suppose  that  these  alone  are  sufficient  for  the  needs  of 
the  organism,  and  one  is  forced  to  the  conclusion  that  the  ill-known 
ingredients  of  the  "crude  fiber"  and  "nitrogen-free  extract",  are 
also  utilized. 

The  separation  and  identification  of  these  various  substances 
and  the  study  of  their  physiological  effects  presents  a  problem  at 
once  attractive  and  laborious  and  one  whose  complete  solution  we 
cannot  hope  soon  to  reach.  Some  few  data  as  to  certain  classes, 
however,  are  available. 

*  Compare  Part  II,  Chapter  X. 


156  PRINCIPLES  OF  ANIMAL   NUTRITION. 

Pentose  Carbohydrates. — It  has  already  been  shown  in  Chapter 
II  (p.  24)  that  such  of  the  pentose  carbohydrates  as  have  been 
experimented  upon  are  at  least  partially  oxidized  in  the  body,  and 
that  this  appears  to  be  especially  the  case  with  herbivora,  the  urine 
of  these  animals  seldom  containing  pentoses. 

It  is  of  course  conceivable  that  a  substance  may  be  oxidized 
in  the  body  without  producing  any  useful  effect  except  in  so 
far  as  the  resulting  heat  may  be  of  value  to  the  organism,  but  it 
seems  more  consonant  with  our  general  conceptions  of  the  nature 
of  metabolism  to  suppose  that  the  potential  energy  of  any  substance 
which  is  capable  of  entering  into  the  metabolism  of  the  cells  may  be 
utilized  as  a  source  of  energy  for  their  functions.  In  the  case  of  the 
pentoses,  moreover,  we  have  the  additional  fact,  seemingly  well 
established,  that  pentoses  may  give  rise,  directly  or  indirectly,  to  a 
production  of  glycogen.  (Compare  p.  26.)  If  we  suppose  the 
latter  body  to  be  formed  directly  from  the  pentoses,  then  their  nutri- 
tive value  is  established,  since  that  of  glycogen  is  unquestionable. 
If,  on  the  other  hand,  we  suppose  that  the  pentoses  enable  glycogen 
to  be  produced  by  protecting  other  materials  from  oxidation,  then 
their  nutritive  value  is  likewise  established,  since  they  serve  as  a 
source  of  energy  to  the  organism. 

Recent  respiration  experiments  by  Cremer  *  seem  to  fully  con- 
firm this  conclusion.  In  addition  to  an  only  partially  successful 
trial  with  a  dog,  four  experiments  were  made  in  which  the  urinary 
nitrogen  and  the  respiratory  carbon  of  rabbits  were  determined  on 
a  diet  of  varying  quantities  of  rhamnose  as  compared  with  a  preced- 
ing and  succeeding  day  of  fasting.  No  examination  of  the  feces  was 
made,  except  to  determine  the  amount  of  rhamnose  contained  in 
them.  Small  amounts  of  this  substance  were  also  found  in  the 
urine.  Neglecting  the  carbon  and  nitrogen  of  the  feces  and  esti- 
mating the  urinary  carbon  from  the  nitrogen  by  the  use  of  Rubner's 
factor,t  0.7462,  the  following  results  have  been  computed,  the  two 
or  three  fasting  days  in  each  experiment  being  averaged.  The 
amount  of  rhamnose  stated  is  exclusive  of  that  found  in  feces  and 
urine. 

*  Zeit  f.  Biol.,  42,  451. 
1  Ibid.,  19,  318. 


THE  RELATIONS  OF  METABOLISM    TO  FOOD-SUPPLY. 


57 


Lost  from  Body. 

Nitrogen. 
Grms. 

Carbon. 
Grms. 

2.401 
2.050 
1.612 
1.629 
1.061 
0.855 
2.028 
2.133 

13.831 

Experiments       |  n.58#grms.  rhamnose 

12.020 
16  542 

Experiment  ii :  \  17  09  |^;  rharrmose ;;;;;;;;; 

10.835 

10.742 

Experiment  III:    }  lg  Q6  |rms;  rhamnose  ;  \  [  \  [  [  |  |  | 

5.338 

13.383 

Experiment  IV:     ]  24 .  30  frms.' rhamnose  i'.:::".:: 

4.482 

The  conditions  in  the  first  experiment  were  not  regarded  as 
satisfactory.  In  the  other  three  the  loss  of  fat  from  the  body  was 
notably  diminished  by  the  administration  of  rhamnose,  precisely  as 
in  the  experiments  of  Pettenkofer  &  Voit  and  of  Rubner  (pp.  147 
and  148)  with  the  hexose  carbohydrates.  The  quantitative  results 
vary  considerably  in  the  individual  experiments,  but  in  the  second 
and  fourth  correspond  quite  closely  to  the  law  of  isodynamic 
replacement. 

Kellner  *  computes  from  the  results  of  respiration  experiments 
in  which  extracted  rye  straw  was  added  to  a  basal  ration  that  the 
furfuroids  (presumably  pentosans)  of  this  material  must  have  con- 
tributed to  the  production  of  fat  to  as  great  an  extent  as  starch  or 
cellulose.  (Compare  p.  183.)  A  fortiori,  therefore,  they  must  be 
capable  of  protecting  the  body  fat  from  oxidation. 

Organic  Acids. — Mention  was  made  in  Chapter  II  of  the  fact 
that  the  organic  acids,  which  are  found  to  some  extent  in  the  food 
and  which  are  produced  in  large  amounts  by  the  fermentation  of 
the  carbohydrates  in  the  digestive  apparatus  of  herbivora,  are  oxi- 
dized in  the  body.  From  this  latter  fact  we  should  anticipate  that 
they  might  serve  as  sources  of  energy  to  the  organism,  and  this 
anticipation  apparently  has  been  confirmed  by  several  investi- 
gators. 

Zuntz  &  v.  Mehring  t  determined  the  amount  of  oxygen  con- 
sumed by  fasting  rabbits  before,  during,  and  after  the  injection 


*  Landw.  Vers.  Stat.,  53,  457. 
f  Arch,  ges  Physiol.,  32,  173. 


158 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


into  the  circulation  of  sodium  lactate.     The  results  per  kilogram 
and  quarter  hour  were  as  follows: 


Before  Injection. 
Quarter  hours. 

Injec- 
tion. 

After  Injection. 
Quarter  hours. 

Fourth. 

Third. 

Second. 

First. 

First. 

Second. 

Third. 

Fourth. 

Apr.  19... 
a     20... 
"     22a 
"     226 
"     28... 

May    2... 
4... 

184.7 

155.3 
142.1 
155.4 
159.2 
176.6 
156.1 

c.c. 
184.5 
142.2 
132.6 
157.4 
150.7 
185.0 
167.6 

c.c. 
190.1 
164.1 
143.5 

i58'5 
158.2 
159.9 

c.c. 

183.3 
155.6 
138.5 
157.1 
155.0 
173.6 
152.4 

c.c. 

203.4 
168.3 
147.2 
164.8 
178.1 
171.8 
166.2 

c.c. 
185.4 
156.6 
153.6 
155.3 
163.2 
161.8 
156.0 

c.c. 
199.0 
158.5 
153.3 
160.7 
158.8 
173.4 
164.2 

c.c' 
188.6 
164.4 
155.4 
147.1 
172.9 
163.3 
159.0 

c.c. 

182.2 
160.0 
157.4 
154.1 
153.9 
180.2 
160.1 

Totals.... 
Averages . 

1129.6 
161.4 

1120.0 
160.0 

974.3 
162.4 

1115.5 
159.4 

1199.8 
171.4 

1131.9 
161.7 

1167.9 
166.8 

1150.7 
164.4 

1147.9 
164.0 

160.8    , 

164.2 

It  being  well  established  that  lactic  acid  is  readily  oxidized  in 
the  body  (compare  p.  27),  it  is  evident  that  in  these  experiments  it 
must  have  protected  the  body  fat  from  being  metabolized,  since 
otherwise  the  consumption  of  oxygen  would  have  increased.  Simi- 
lar, although  not  decisive,  results  were  obtained  with  sodium  buty- 
rate.  On  the  other  hand,  sodium  lactate  administered  by  the 
mouth  caused  more  or  less  increase  in  the  oxygen  cousumption. 
Wolfers  *  has  reported  confirmatory  results  with  sodium  lactate. 
Munk  f  injected  sodium  butyrate  into  the  veins  of  fasting  rabbits 
curarized  to  eliminate  the  effects  of  muscular  activity  and  secure 
uniform  metabolism,  and  determined  the  respiratory  exchange  by 
the  Zuntz  method  (p.  72).  The  oxidation  of  sodium  butyrate 
according  to  the  equation. 

C4H7Na02  +  502  =  3C02  +  3H20  +  NaHC03 

corresponds  to  a  respiratory  quotient  of  0.6,  which  is  less  than  that 


*  Arch.  ges.  Physiol.,  32,  222. 
t  Ibid.,  46,  322. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY. 


59 


of  the  fasting  animal.  The  material  lowering  of  the  quotient  which 
was  observed  was  therefore  interpreted  as  showing  that  the  sodium 
butyrate  was  oxidized,  and  this  conclusion  was  confirmed  by  the 
strongly  alkaline  character  of  the  urine  and  the  absence  from  it 
of  butyric  acid.  The  amount  of  sodium  butyrate  injected  during 
l\  to  H  hours  was  sufficient  in  the  several  experiments  to  supply 
from  60  to  100  per  cent,  of  the  respiratory  demand  of  the  fasting 
animal.  If  this  had  been  oxidized  uselessly — that  is,  if  the  energy 
liberated  had  not  been  of  use  to  the  organism — then  the  consump- 
tion of  oxygen  and  elimination  of  carbon  dioxide  should  have  in- 
creased correspondingly.  This,  however,  was  far  from  being  the 
case,  as  the  following  fifteen-minute  averages  for  the  periods 
before,  during,  and  after  the  injection  show: 


Animal  I,  weight  1.92  kgs.: 

Before  injection 

During        "       

After  "       

Animal  II,  weight  1.9  kgs.: 

Before  injection 

During        "        

After  "       

Animal  III,  weight  1.82  kgs 

Before  injection , 

During        "       

After  " 

Animal  IV,  weight  1 .47  kgs, 

Before  injection , 

During        "       , 

After  "        


Acid 

Injected, 

Grms. 


0.133 


0.199 


0.206 


0.186 


Oxygen 
Consumed 


260.9 
280.0 
253.3 


290.9 
325.2 
299.4 


305. 
330. 
306. 


278.9 
297.6 
278.1 


Carbon 
Dioxide 
Excreted, 


196.1 
190.5 
181.2 


228.3 
214.6 
230.9 


243.4 
238.0 
235.3 


201.0 
197.9 
205.2 


Respira- 
tory 
Quotient. 


0.75 
0.68 
0.71 


0.78 
0.66 
0.78 


0.79 
0.72 
0.77 


0.72 
0.68 
0.73 


In  place  of  an  increase  of  60  to  100  per  cent,  in  the  respiratory 
exchange  under  the  influence  of  the  sodium  butyrate,  there  was  an 
increase  of  only  7  to  8  per  cent,  in  the  oxygen  and  none  at  all  in  the 
carbon  dioxide.  It  is  evident,  therefore,  that  the  loss  of  fat  from 
the  body  must  have  been  largely  diminished,  the  butyric  acid  serv- 
ing as  a  source  of  energy  in  its  place.     A  stimulating  effect  upon 


i6o 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


the  heart's  action  was  noticed,  and  Bokai  is  quoted  as  having  shown 
a  similar  action  on  the  peristaltic  movements  of  the  intestines,  and 
these  facts  perhaps  account  for  some  of  the  increase  of  the  oxygen, 
but  Munk  shows  another  reason  for  most  of  it.  To  produce  1  Cal. 
of  energy  by  the  oxidation  of  sodium  butyrate  he  computes  to  re- 
quire 0.324  gram  of  oxygen,  while  to  produce  1  Cal.  by  the  oxida- 
tion of  fat  requires,  according  to  Zuntz  &  Hagemann  (Chapter  VIII), 
0.302  gram  or  6.2  per  cent,  more  in  the  first  case.  It  would  thus 
appear  that  the  replacement  of  fat  by  sodium  butyrate  was  sub- 
stantially isodynamic. 

Mallevre  *  experimented  with  sodium  acetate,  whose  respira- 
tory quotient  is  0.5,  by  the  same  method  as  Munk,  the  amount 
injected  equaling  86-100  per  cent,  of  the  respiratory  demand.  The 
results  per  quarter  hour  were: 


Weight  and  Condition. 


Sodium 
Acetate 
per  Kg. 
Weight, 
Grms. 


Oxygen 

Con- 
sumed, 
c.c. 

Carbon 

Dioxide 

Excreted. 

c.c. 

176.1 

193.8 
197.7 
178.0 

183.6 
167.1 
152.6 
171.2 

195.3 
231.8 
211.2 

149.6 
168.2 
163.3 

214.7 
244.8 
217.1 

165.9 
169.5 
166.6 

183.4 
208.1 
209 . 5 
194.7 

160.5 
167.0 
164.7 
164.7 

Respi- 
ratory 
Quotient. 


Weight,  1.44  legs.  ! 
Just  after  eating..  } 


Weight,  1.5  kgs.. 

After    two  days' 

fasting 

Weight,  1.82  kgs. 

After    two    days' 

fasting 

Weight,  1.7  kgs.  . 

After    one    day's 

fasting 


Before  injection. 
During        " 
Residual  effect. . 
After  injection. . 

II. 

Before  injection. 
During        " 
After  "        . 

III. 

Before  injection 
During        " 
After  " 

IV. 
Before  injection 
During        " 
Residual  effect. . 
After  injection. . 


0.201 


0.231 


0.152 


1.04 
0.86 
0.76 
0.96 


0.77 
0.71 
0.77 


0.77 
0.69 
0.77 


0.87 
0.80 
0.79 

0.85 


The  decrease  in  the  respiratory  quotient,  as  well  as  the  results 
of  the  examination  of  the  urine,  showed  that  the  sodium  acetate 


*  Arch.  ges.  Physiol.,  49,  460. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPFLY. 


161 


was  oxidized  in  the  body.  The  increase  in  the  amount  of  oxygen 
consumed  is  much  more  marked  than  in  Munk's  experiments,  rang- 
ing from  10.4  to  14  per  cent.  Moreover,  as  Mallevre  points  out,  in 
the  oxidation  of  sodium  acetate  about  the  same  volume  of  oxygen 
is  required  to  produce  a  unit  of  heat  as  in  the  case  of  fat.  Appar- 
ently, then,  while  the  sodium  acetate,  like  the  sodium  butyrate  in 
Munk's  experiments,  must  have  largely  diminished  the  metabolism 
of  the  body  fat,  it  also  stimulated  the  total  metabolism  and  was 
substituted  for  the  fat  in  less  than  the  isodynamic  ratio.  As  in 
Munk's  experiments,  a  stimulation  of  the  heart  action  and  also  an 
increased  peristalsis  of  the  intestines  was  observed. 

It  would  seem,  then,  that  lactic  and  butyric  acids,  when 
introduced  into  the  circulation  of  the  fasting  animal,  protect 
the  body  fat  from  oxidation,  and  replace  other  nutrients  in 
isodynamic  proportions.  Acetic  acid,  on  the  contrary,  seems  in- 
ferior to  the  other  two  in  this  respect,  and  it  is  of  interest  to  recall 
that  according  to  Weiske  &  Flechsig  (p.  123)  it  apparently  has 
also  less  effect  in  diminishing  the  proteid  metabolism. 

Crude  Fiber. — As  was  stated  on  p.  117,  the  early  experiments 
by  v.  Knieriem  *  upon  the  nutritive  value  of  cellulose  comprised 
respiration  experiments  as  well  as  determinations  of  the  proteid 
metabolism.  Combining  the  results  for  nitrogen  already  given  with 
those  for  carbon,  we  have  the  following: 


Number 

of 

Days. 

Food  per  Day. 
(Two  Animals.) 

Gain  or  Loss  of 

Nitrogen, 
Grms. 

Carbon, 
Grms. 

I 

9 
10 
5 
4 
3 

Milk  and  horn  dust 

-0.599 
+  0.104 
-0.330 
-0.318 
-0.023 

-4.521 

II 

Ill 

IV 

Same  +  18.63  grms.  crude  fiber  *  .  .  . 
Milk  and  horn  dust 

-0.434 
-4.868 
-1.673 

Vf.... 

"      +  33     "                "           

+  5.653 

*  Water-free. 

f  Results  regarded  by  the  author  as  of  doubtful  value. 


In  addition  to  its  effect  in  diminishing  the  proteid  metabolism, 
the  crude  fiber  in  these  experiments  seems  to  have  been  fully  as 
efficient  as  the  cane-sugar  as  a  substitute  for  body  fat. 
*  Zeit.  f.  Biol.,  21,  67. 


1 62  PRINCIPLES   OF  ANIMAL   NUTRITION. 

As  we  have  seen,  there  has  been  considerable  study  of  the  effects 
of  crude  fiber  on  the  proteid  metabolism,  but  no  other  comparative 
experiments  appear  to  have  been  made  regarding  the  replacement 
values  of  cellulose  and  other  carbohydrates  in  a  maintenance  ration. 

The  somewhat  lower  value  which  seems  to  be  indicated  for  the 
organic  acids  by  the  experiments  cited  in  the  previous  paragraph 
has  been  made  the  basis  of  conclusions  as  to  the  inferior  nutritive 
value  of  cellulose,  and  Zuntz,*  in  some  comments  on  Mallevre's 
experiments,  remarks  that  the  apparent  equality  between  cellulose 
and  starch  observed  in  experiments  on  ruminants  is  to  be  explained 
by  the  fact  that  in  these  animals  the  starch  also  undergoes 
fermentation,  a  fact  which  the  researches  of  G.  Kuhn  at  Mockern 
have  since  established.  In  other  words,  he  would  say  that  in  case 
of  ruminants  the  starch  has  as  low  a  value  as  the  cellulose  rather 
than  that  the  cellulose  has  as  high  a  value  as  the  starch. 

Kellner  has  recently  obtained  results,  to  be  discussed  a  little 
later,  which  seem  to  prove  a  participation  by  the  digested  cellulose 
in  actual  fat  production  to  as  great  an  extent  as  by  starch,  and 
which  therefore  seem  to  put  the  nutritive  value  of  the  form  of  cellu- 
lose used  by  him  beyond  dispute. 

Utilization  of  Excess  of  Non-nitrogenous  Nutrients.  —  No 
elaborate  scientific  investigation  is  needed  to  teach  us  that  food 
supplied  in  exceess  of  the  immediate  demands  of  the  organism  re- 
sults in  a  greater  or  less  storage  of  material  in  the  body,  this  material 
consisting,  in  the  mature  animal,  largely  of  fat.  But  while  the  fact 
of  fat  formation  is  obvious,  the  exact  source  of  the  fat  has  been  the 
subject  of  as  much  controversy  as  almost  any  physiological  question. 
As  we  have  seen  in  the  previous  section,  opinions  are  still  far  from 
being  unanimous  as  to  the  production  of  fat  from  proteids,  while 
until  quite  recently  the  same  might  have  been  said  regarding 
the  carbohydrates  as  a  source  of  fat.  A  very  complete  critical  re- 
view of  the  literature  of  the  subject  of  fat  formation  in  the  animal 
body  was  published  by  Soskin  f  in  1894,  and  to  this  the  writer  is  in- 
debted for  a  considerable  number  of  the  statements  and  references 
on  the  succeeding  pages. 

As  was  stated  on  p.  29,  the  older  physiologists  looked  upon  the 

*  Arch   ges.  Physiol.,  49,  447. 
t  Jour,  f    Landw.,  42.  157. 


THE  RELATIONS   OF  METABOLISM   TO  FOOD-SUPPLY.       163 

fat  of  the  food  as  the  sole  source  of  the  body  fat.  The  contrary- 
view  was  first  propounded  by  Liebig  *  in  1843.  After  drawing  the 
distinction  between  "plastic  materials"  (proteids),  which  serve  to 
build  up  the  tissue,  and  "respiratory  materials"  (non-nitrogenous 
substances),  which  serve  as  sources  of  heat,  he  asserts  that  any 
excess  of  the  latter  over  the  immediate  needs  of  the  organism  is  con- 
verted into  fat.  This  proposition,  which  was  based  upon  observa- 
tion  and  general  knowledge  rather  than  upon  specific  experiments, 
led  to  an  active  controversy  with  the  adherents  of  the  older  view 
and  to  much  direct  experimental  work. 

Liebig,  while  not  denying  that  the  food  fat  was  a  source  of  body 
fat,  maintained  that  the  amount  contributed  by  it  was  insignificant 
and  regarded  the  carbohydrates  as  the  chief  source  of  animal  fat. 
The  controversy  turned  upon  the  question  of  the  possibility  of 
accounting  for  the  body  fat  by  the  food  fat,  both  parties  tacitly 
agreeing  that  any  excess  was  to  be  credited  to  the  carbohydrates. 
The  principal  champions  of  the  older  view  were  Boussingault,  Dumas, 
and  Payen.f  Boussingault,  in  particular,  brought  forward  the 
results  of  experiments  on  milch  cows,  according  to  which  the  fat 
of  the  food  fully  sufficed  to  account  for  that  in  the  milk.  They 
all,  however,  ultimately  came  to  acknowledge  the  substantial  accu- 
racy of  Liebig's  view.  Thus  Dumas  &  Milne-Edwards  %  confirmed 
the  results  of  Huber  &  Gundlach,§  cited  by  Liebig,  according  to 
which  bees  can  produce  wax  from  honey  or  sugar.  Boussingault  || 
published  the  results  of  new  experiments  on  milch  cows  as  sus- 
taining his  previous  view  of  the  question,  but  later  ^f  convinced 
himself  by  careful  and  laborious  experiments  on  the  fattening  of 
swine  and  geese  of  its  untenability  and  of  the  correctness  of  Liebig's 
position.  Thus  in  one  of  his  experiments  nine  pigs  gained  103.2  kgs. 
of  fat  in  ninety-eight  days,  while  the  food  contained  but  67.6  kgs., 
of  which  about  8  kgs.  was  excreted  undigested  in  the  feces. 
Persoz  **   likewise,   in   experiments   with   geese,   obtained  similar 

*  Ann.  Chem.  Pharm.,  45,  112;   48,  126;    54,  376. 

t  Annal.  de  Chim.  et  de  Physique.,  3d  ser.  8,  63. 

%  Ibid.,  14,  400. 

§  Naturgeschichte  der  Bienen,  Kassel,  1842. 

|]  Annal.  de  Chim.  et  de  Physique.,  3d  ser.,  12,  153 

f  hoc.  at.,  14,  419. 

**  Annal.  de  Chim.  et  de  Physique.,  14,  408. 


1 64  PRINCIPLES  OF  ANIMAL   NUTRITION. 

results  and  also  observed  a  production  of  fat  by  these  animals  when 
fed  on  food  from  which  all  fat  had  been  removed. 

Fat. — That  the  fat  of  the  food  may  serve  directly  as  a  source  of 
body  fat  has  been  shown  by  Hofmann,*  who  fasted  a  dog  for  thirty 
days,  thus  rendering  the  body  almost  fat-free,  and  then  fed  for  five 
days  large  amounts  of  fat  bacon  containing  as  little  lean  meat  as 
possible,  and  from  which  there  were  digested  daily  370.8  grams  of 
fat  and  49.4  grams  of  protein.  At  the  end  of  the  five  days  the 
body  of  the  animal  contained  1352.7  grams  of  fat.  Estimating  its 
fat  content  at  the  close  of  the  fasting  period  at  150  grams,  there  was 
produced  daily  about  240  grams  of  body  fat.  According  to  the 
highest  recorded  estimates  not  over  26  grams  of  this  could  possibly 
have  been  formed  from  the  protein  of  the  food.  Hofmann  also 
shows  from  the  result  of  one  of  Pettenkofer  &  Voit's  respiration 
experiments,  in  which  meat  and  fat  were  fed,  that  part  of  the  ob- 
served gain  of  fat  must  have  had  its  source  in  the  fat  of  the 
food. 

The  latter  investigators  also  showed  in  the  last  of  the  experi- 
ments cited  on  p.  144  that  a  large  ration  of  fat  alone  may  result  in  a 
considerable  storage  of  fat.  Most  of  the  experiments  by  the  same 
investigators  in  which  lean  meat  and  fat  were  fed  show  not  merely  a 
diminution  of  the  loss  of  body  fat  but  an  actual  increase  in  its 
amount.  (Compare  the  averages  on  page  150.)  The  fact  is  most 
strikingly  shown,  however,  in  a  series  in  which  increasing  amounts 
of  fat  were  added  to  a  uniform  ration  of  meat  which  was  itself 
sufficient  to  maintain  both  nitrogen  and  carbon  equilibrium.  The 
results  as  given  by  Pettenkofer  &  Voit  f  are  contained  in  the  table 
at  the  top  of  p.  165,  those  on  the  basal  ration  of  meat  being  the 
same  as  those  given  also  on  p.  109  for  the  first  series. 

It  is  of  course  possible  to  interpret  these  results  as  showing  that 
the  fat  of  the  food  was  oxidized  and  protected  an  equivalent  amount 
of  the  non-nitrogenous  residue  of  the  proteids  from  oxidation  and 
that  the  latter  were  the  real  source  of  the  fat  gained.  No  necessity 
for  such  an  interpretation  is  apparent,  however,  and  the  direct 
explanation  appears  the  simpler  and  more  natural. 

The  results  of  experiments  upon  the  deposition  of  foreign  fats 

*  Zeit.  f.  Biol.,  8,  153. 
t  Ibid.,  9,  30. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY. 


65 


Number 

of 
Trials. 

Food. 

Nitrogen 

of 
Excreta. 

Total  Carbon 

of 

Excreta. 

Gain  or  Loss  of 

Meat. 

Fat. 

Flesh. 
(N-hO.034.) 

Fat. 

3 
2 

2 

2 

1500 
1500 
1500 
1500 
1500 
1500 

0 

30 

60 

100 

100 

150 

51.0 
49.6 
51.0 
47.7 
49.3 
49.5 

184.5 
180.6 
203.6 
182.4 
174.4 
193.1 

0 

+  42.8 
-  0.6 
+  97.8 
+  49.4 
+  44.8 

+     4.3 

+   32.4 
+   39.4 
+   91.1 
+  109.5 
+  135.7 

in  the  body  which  were  considered  in  Chapter  II,  p.  30,  also  testify 
to  the  direct  formation  of  body  fat  from  food  fat. 

Carbohydrates. — Among  the  experiments  of  Pettenkofer  & 
Voit  which  have  been  cited  in  the  foregoing  pages  are  several  which 
show  a  production  of  fat  upon  a  ration  of  lean  meat  with  the  addi- 
tion of  starch  or  dextrose  or  of  starch  alone.  A  more  complete 
summary  of  these  experiments  *  is  given  below: 


\umber 

of 
Experi- 
ments. 

Food  per  Day. 

Gain  or  Loss  of 

Meat 
Grms. 

Fat. 
Grms. 

hy^t°e"S>^>f  • 
Grms.    j     Grms- 

Fat, 
Grms. 

Starch ■< 

Proteids  and  dextrose 

r 

Proteids  and  starch 1 

1 

1 

3 
3 
1 

8 
1 
2 
1 

'566 
400 
500 
800 
1500 
1800 

16.9 
21.2 
20.2 

'5\6 
5.3 

13.7 
4.5 

10.1 

450 
597 
700 
200 
400 
200 
450 
200 
450 

-45.0 
-58.8 
-38.8 

-  8.1 

-  3.1 
-11.3 
+  40.6 
+    6.3 
+  70.6 

+   56.2 
+     3.4 
+  106.4 
+    15.0 
+  109.9 
+    19.5 
+   71.5 
+   18.1 
+  126.5 

Pettenkofer  &  Voit's  Conclusions: — In  discussing  these  results 
Pettenkofer  &  Voit  assumed  that,  as  computed  by  Henneberg,f  100 
grams  of  proteids  can  give  rise  to  a  maximum  of  51.4  parts  of 
fat.  On  this  basis  they  found  that,  with  two  apparent  exceptions, 
the  fat  of  the  food,  together  with  that  which  could  be  derived  from 

*  Zeit  f.  Biol.,  9,  435. 

f  Landw.  Vers.  Stat ,  10,  455,  foot-note. 


1 66  PRINCIPLES   OF  ANIMAL   NUTRITION. 

the  amount  of  proteids  metabolized,  was  sufficient  to  account  for 
the  gain  of  fat.  They  therefore  concluded  that  the  carbohydrates 
simply  protected  these  materials  from  oxidation  and  regarded  the 
formation  of  fat  from  the  former  as  improbable,  being  confirmed 
in  this  belief  by  the  observation  that  the  amount  of  fat  produced 
was  proportional  to  the  proteids  rather  than  to  the  carbohydrates 
of  the  food.  The  apparent  exceptions  they  regarded  as  due  to  a 
retention  of  undigested  starch  in  the  alimentary  canal.  In  brief, 
Pettenkofer  &  Voit,  while  not  denying  that  carbohydrates  aid  in 
the  production  of  fat,  regarded  their  action  as  an  indirect  one. 

It  should  be  added  that,  contrary  to  the  general  impression,  Voit 
did  not  absolutely  deny  the  formation  of  fat  from  carbohydrates, 
but  regarded  it  as  improbable  and  unproved.  Moreover,  he  came 
later  to  admit  the  truth  of  the  opposite  view,  and  even  furnished 
from  his  laboratory  experimental  evidence  in  its  support. 

At  an  earlier  date  Voit  *  had  likewise  made  experiments  on  a 
milch  cow,  the  result  of  which  was  that  not  only  all  the  fat  of  the 
milk,  but  most  of  the  milk-sugar  as  well,  could  be  accounted  for  by 
the  proteids  and  fat  of  the  food.  Voit  also  examined  the  numerous 
experiments  of  Dumas,  Persoz,  Boussingault,  and  others  (p.  163) 
upon  the  origin  of  animal  fat  and  satisfied  himself  that,  although 
they  undoubtedly  showed,  as  their  authors  claimed,  a  formation 
of  fat  from  other  ingredients  of  the  food,  the  amount  produced 
could  at  least  in  the  great  majority  of  cases  be  accounted  for  by 
the  proteids  of  the  latter. 

It  is  important  to  observe  that  the  evidence  supporting  Voit's 
view  was  negative  evidence.  The  results  could  be  explained  on  the 
hypothesis  that  the  carbohydrates  did  not  contribute  to  fat  pro- 
duction, but  while  a  large  number  of  such  results  might  render  the 
hypothesis  very  probable,  they  could  not  demonstrate  its  truth.  On 
the  other  hand,  even  a  single  well-authenticated  case  in  which  the 
fat  and  proteids  of  the  food  did  not  suffice  to  account  for  the  amount 
of  fat  formed  in  the  body  would  suffice  to  establish  the  possibility 
of  its  formation  from  other  materials.  A  few  apparent  cases  of  this 
sort  among  earlier  experiments  Voit  was  able  to  explain  plausibly, 
but  there  was  one  important  exception,  viz.,  the  experiments  of 

*  Zeit.  f.  Biol..  5,  79-169. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       167 

Lawes  &  Gilbert  *  at  Rothamsted,  in  1850,  on  the  fattening  of 
swine. 

Lawes  &  Gilbert's  Investigations. — These  experiments  consti- 
tuted part  of  a  series  of  feeding  trials  with  fattening  sheep  and  pigs, 
undertaken  to  test  the  then  current  view  of  Boussingault,  according 
to  which  the  feeding  value  of  stock  foods  was  proportional  to  their 
content  of  nitrogen.  From  the  results  of  their  extensive  experi- 
ments, Lawes  &  Gilbert  concluded  that  in  fattening  animals  both 
the  amount  of  food  consumed  by  a  given  weight  of  animal  within  a 
given  time  and  the  increase  in  weight  obtained  are  measured  rather 
by  the  supply  of  non-nitrogenous  than  of  nitrogenous  constituents 
in  the  food.  This  fact  of  itself  strongly  suggests  a  production  of 
fat  from  carbohydrates. 

In  connection  with  these  feeding  trials  investigations  were  also 
made  into  the  composition  of  the  increase  in  live  weight  during 
fattening.f  By  a  comparison  of  the  weight  and  composition  of 
one  of  the  fattened  pigs  with  those  of  an  animal  supposed  to  be 
precisely  similar  at  the  beginning  of  the  fattening  the  percentage 
composition  of  the  increase  was  found  to  be  approximately : 

Water 28.61 

Ash 0.53 

Proteids 7.76 

Fat 63.10 

100.00 

During  the  ten  weeks  of  the  fattening  the  animal  gained  88 
pounds,  containing  according  to  the  above  figures  55.5  pounds  of 
fat,  while  the  total  food  consumed  contained  but  13.7  pounds.  In 
other  words,  over  three  fourths  of  the  fat  was  formed  from  other 
ingredients  of  the  food. 

After  the  publication  of  Voit's  first  paper,  Lawes  &  Gilbert  X 
presented  the  results  of  this  and  eight  other  experiments  in  their 

*  Report  British  Association  Adv.  Sci.,  1852;  Jour.  Roy.  Agr.  Soc,  14, 
459;  Rep.  British  Asso.  Adv.  Sci.,  1854;  Rothamsted  Memoirs,  Vol.  II. 

t  Jour.  Roy.  Agr.  Society,  21,  465;  Phil.  Trans.,  Part  II,  1859,  p.  493. 

t  Rep.  British  Asso.  Adv.  Sci.,  1866;  Phil.  Mag.,  Dec,  1866;  Rotham- 
sted Memoirs,  Vol.  IV. 


1 68 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


bearing  on  the  origin  of  the  fat.  Nos.  2  to  5  were  selections  from 
the  first  two  series  of  the  experiments  of  1850  (designated  as  I  and 
II  in  the  table  below)  and  Nos.  6  to  9  were  experiments  upon  the 
equivalency  of  starch  and  sugar  in  food  reported  in  1854  *  (desig- 
nated below  by  S).  The  following  table  shows  the  original  numbers 
of  the  several  experiments  and  the  character  of  the  food  consumed  r 


No 

Original  Designation. 

Series. 

Number. 

1 
2 
3 
4 
5 
6 
7 
8 
9 

i 

i 

i 

ii 

s 
s 
s 
s 

12 
1 
5 
5 
1 
2 
3 
4 

Bean  meal,  lentil  meal,  bran,  and  barley  meal  ad  lib. 

Bean  meal,  lentil  meal,  bran,  and  corn  meal  ad  lib. 

Bean  meal  and  lentil  meal  ad  lib. 

Corn  meal  ad  lib. 

Barley  meal  ad  lib. 

Lentil  meal  and  bran,  with  sugar  ad  lib. 

Lentil  meal  and  bran,  with  starch  ad  lib. 

Lentil  meal  and  bran,  with  sugar  and  starch. 

Lentil  meal,  bran,  sugar,  and  starch  ad  lib. 

From  the  results  of  the  first  experiment,  the  amount  of  fat  con- 
tained in  the  observed  increase  in  live  weight  in  each  case  was  com- 
puted, the  animals  being  assumed  to  have  had  at  the  beginning  of 
the  fattening  the  composition  of  the  lean  pig  analyzed  and  at  its 
close  that  of  the  fat  pig.  These  amounts  were  then  compared  with 
the  amounts  which  could  have  been  produced  from  the  fat  and  pro- 
teids  of  the  food.  In  order  to  make  the  case  as  unfavorable  as 
possible  for  the  carbohydrates  the  authors  assumed: 

First,  that  all  the  fat  of  the  food  was  digested  and  laid  up  in  the 
body. 

Second,  that  all  the  nitrogenous  matter  of  the  food  was  digested, 
and  that  it  all  consisted  of  true  proteids. 

Third,  that,  after  deducting  the  amount  of  proteids  gained  by 
the  body,  the  total  carbon  of  the  remainder,  minus  that  required  to 
form  urea,  was  available  for  fat  formation. 

The  results  of  the  comparison  were  as  follows,  calculated  per 
100  pounds  gain  in  live  weight. 


Rep.  Brit.  Asso.  Adv.  Sci.,  1854. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.       169 


os  co 

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170  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Even  on  the  most  extreme  assumptions  it  is  only  possible  to 
regard  the  fat  produced  as  derived  wholly  from  the  proteids  of  the 
food  in  three  cases  in  which  an  excessive  proportion  of  the  latter  was 
fed.  If  the  probable  digestibility  of  the  foods  used  be  considered, 
and  Henneberg's  factor  (51-4  percent.)  for  the  possible  production 
of  fat  from  proteids  be  used,  the  results  show  even  more  decidedly 
a  formation  of  fat  from  carbohydrates.  In  a  later  paper,*  in  reply 
to  criticisms,  the  authors  state  that  they  have  reviewed  and  recal- 
culated many  of  their  experiments  with  the  result  that,  while  the 
experiments  with  ruminants  (sheep  and  oxen)  failed  to  furnish  con- 
clusive evidence  of  the  formation  of  fat  from  carbohydrates,  a 
large  number  of  those  with  pigs  unquestionably  showed  such  for- 
mation. 

In  view  of  their  historical  interest  it  has  seemed  desirable  to 
give  the  results  of  Lawes  &  Gilbert's  experiments  in  some  detail, 
although  at  the  time  they  hardly  secured  the  recognition  which 
was  due  them  and  Voit's  views  became  the  generally  accepted 
theory  for  the  next  twenty-five  years.  Notwithstanding  the  latter 
fact,  however,  results  of  experiments  on  herbivorous  animals  speed- 
ily began  to  accumulate  which  were  difficult  to  reconcile  with  Voit's 
hypothesis. 

Experiments  on  Ruminants. — Experiments  on  milch  cows  were 
made  by  Voit  himself,  as  already  noted.  G.  Kiihn  &  Fleischer  f 
a  little  later  discussed  the  results  of  two  of  their  extensive  feeding 
experiments  on  milch  cows  in  their  bearing  on  this  point,  and  M. 
Fleischer  %  did  the  same  with  the  results  of  similar  experiments 
made  by  Wolff  and  himself.§  Their  results  are  tabulated  on  the 
opposite  page. 

Neither  Voit's  nor  Fleischer's  results  are  such  as  to  require  the 
assumption  of  a  formation  of  fat  from  carbohydrates.  Those  of 
Kiihn  &  Fleischer  show  a  small  excess  of  fat  in  the  milk  over  that 
producible  from  the  fat  and  proteids  of  the  food,  but  the  authors 


*  Jour.  Anat.  and  Physiol.,  9,  577;  Rothamsted  Memoirs,  Vol.  IV. 

t  Landw.  Vers.  Stat.,  10,  418;   12,  451. 

%  Virchow's  Archiv,  51,  30. 

§  Jour.  f.  Landw.,  19,  371,  and  20,  395. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD  SUPPLY.       171 


Fat  of 
Fodder, 
Grms. 


Fat  from 
Protein, 
Grms. 


Total, 
Grms. 


Fat  of 

the  Milk, 
Grms. 


Y   ., .  j  Experiment  a 

Kiihn  &  Fleischer:  j  Ex?T ment  \ j ; 
Fleischer:  j  Experiment  I... ... 


318.8 
276.0 
183.5 
183.5 
170.5 
166.5 


401.8 
308.5 
79.5 
69.5 
158.5 
170.0 


720.6 
584.5 
263.0 
253.0 
329.0 
336.5 


577.5 
337.3 
277.5 
292.0 
303.5 
290.5 


regard  the  differences  as  within  the  limits  of  error  in  such  experi- 
ments. 

Studies  of  the  results  of  fattening  experiments  with  ruminants 
give  similar  results.  On  the  basis  of  Lawes  &  Gilbert's  determi- 
nations of  the  composition  of  the  increase  of  live  weight  in  fattening, 
the  amount  of  fat  produced  in  such  an  experiment  may  be  approxi- 
mately computed  and  compared  with  the  amounts  of  proteids  and 
fat  in  the  food.  Such  a  comparison  by  the  writer  *  in  seventy-seven 
experiments  on  sheep  showed  that,  with  one  or  two  possible  excep- 
tions, the  fat  and  proteids  of  the  food  were  sufficient  to  account  for 
the  amount  of  fat  formed,  although  in  some  of  the  experiments 
little  margin  was  left. 

Experiments  on  Swine. — Experiments  with  swine,  on  the  other 
hand,  as  Wolff  f  has  shown,  have  almost  without  exception  given 
results  which  can  scarcely  be  explained  except  upon  the  hypothesis 
of  a  formation  of  fat  from  carbohydrates.  These  animals,  as  Lawes 
&  Gilbert  pointed  out  in  their  early  papers,  are  especially  adapted 
to  experiments  of  this  sort,  since  they  consume  a  relatively  large 
amount  of  easily  digestible  food,  have  a  small  proportion  of  offal  to 
carcass,  and  are  by  nature  inclined  to  lay  on  fat  readily.  It  was 
therefore  to  be  expected  that  experiments  upon  swine  would  show 
a  production  of  fat  from  carbohydrates,  if  such  took  place,  more 
decisively  than  those  upon  ruminants. 

Experiments  on  pigs  by  Weiske  &  Wildt,J  it  is  true,  on  the 
same  plan  as  those  by  I, awes  &  Gilbert,  yielded  results  consistent 
with  Voit's  theory,  showing  a  formation  of  5565  grams  of  fat  in  the 


*  Manual  of  Cattle  Feeding,  p.  177. 

t  Ernahrung  Landw.  Nutzth.,  pp.  354-356 

X  Zeitschrift  f.  Biol.,  10,  1. 


172  PRINCIPLES  OF  ANIMAL   NUTRITION. 

body  as  compared  with  a  possible  6724  grams  from  the  fat  and 
proteids  of  the  food.  The  feeds  used,  however,  were  not  well  suited 
to  young  animals  and  the  gain  was  abnormally  small  in  proportion 
to  the  food  consumed,  so  that  the  results  could  not  be  expected  to 
be  decisive.  Moreover,  the  presence  of  non-proteid  nitrogen  in  the 
food  is  not  considered  in  the  computation.  (See  the  next  paragraph.) 

Sources  of  Uncertainty. — Up  to  this  point  the  results  of  experi- 
ments on  herbivorous  and  omnivorous  animals  had  been  somewhat 
conflicting.  Before  taking  up  the  later  investigations  it  is  desir- 
able to  point  out  some  of  the  uncertainties  attaching  to  experiments 
such  as  those  above  enumerated.  These  relate,  first,  to  the  amount 
of  fat  actually  produced,  and  second,  to  the  possible  sources  of 
supply  in  the  food. 

The  basis  for  estimating  the  amount  of  fat  actually  produced  by 
a  fattening  animal  was  in  two  cases  a  comparison  with  the  amount  in 
a  supposedly  similar  animal  at  the  beginning  of  the  fattening,  the 
fattened  animal  being  actually  analyzed.  In  the  remainder  the 
increase  in  live  weight  was  assumed  to  have  the  composition  found 
by  Lawes  &  Gilbert.  It  need  scarcely  be  pointed  out  that  the 
results  of  such  comparisons  can  be  only  approximate  and  are  sub- 
ject to  a  considerable  range  of  error.  Only  the  most  decided 
results  one  way  or  the  other  can  be  accepted  as  at  all  conclusive. 
In  experiments  on  milch  cows  the  production  of  milk  fat  can  of 
course  be  determined,  but  the  variations  in  the  weight  of  such  an 
animal  often  render  any  conclusions  as  to  gain  or  loss  of  body  fat 
so  difficult  that  the  results  as  a  whole  are  less  satisfactory  than 
those  on  fattening. 

The  possible  sources  of  fat  in  the  food,  aside  from  the  carbohy- 
drates, are  the  ether  extract  and  the  proteids.  As  regards  the  first, 
it  is  certain  that  not  all  the  digestible  ether  extract  of  stock  foods 
is  true  fat.  With  the  proteids  the  case  is  still  worse.  In  particu- 
lar we  now  know  that  a  portion,  and  in  some  cases  a  considerable 
portion,  of  the  total  nitrogenous  matter  of  feeding-stuffs  consists  of 
non-proteid  material,  which  so  far  as  we  know  contributes  little  if 
anything  directly  to  fat  production.  This  is  a  very  important  source 
of  error.     Thus  the  writer  *  has  shown,  as  has  also  Soxhlet,f  that  if 

*  Manual  of  Cattle  Feeding,  p.  182. 

t  Compare  Soskin,  Jour.  f.  Landw.,  42,  203. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY.       173 

account  be  taken  of  this  fact  the  teachings  of  Weiske  &  Wildt's 
experiment  cited  above  are  exactly  reversed  and  show  a  formation 
of  fat  from  carbohydrates.  A  consideration  of  the  same  fact,  of 
course,  tends  to  make  the  results  of  all  similar  experiments,  includ- 
ing those  on  milch  cows,  more  favorable  to  the  carbohydrates. 

Still  further,  it  is  doubtful  whether  100  parts  of  proteids  can 
actually  yield  51.4  parts  of  fat.  The  latter  number  was  computed 
by  Henneberg  from  the  elementary  composition  of  proteids  and  of 
urea  to  be  the  maximum  amount  obtainable.  Zuntz,*  however, 
has  called  attention  to  the  fact  that  if  the  proteids  actually  split  up 
in  the  manner  which  Henneberg's  calculation  supposes,  the  products 
must  contain  all  the  potential  energy  of  the  original  material,  so 
that  none  can  be  given  off  during  their  cleavage.  This  is  a  process 
wholly  without  analogy  in  the  animal  body,  and,  to  say  the  least, 
very  improbable.  It  would  seem  then,  that  even  if  we  still  hold  to 
a  formation  of  fat  from  proteids,  we  must  considerably  reduce  our 
estimate  of  its  amount. 

Later  Fattening  Experiments. — All  these  considerations  tend  to 
strengthen  the  belief  that  fat  is  formed  from  carbohydrates,  and 
more  recent  experiments  have  demonstrated  that  such  is  the  fact. 
Henneberg,  Kern,  &  Wattenberg.f  in  experiments  undertaken  to 
determine  the  rate  of  gain  and  the  composition  of  .the  increase  of 
fattening  sheep,  and  conducted  substantially  like  those  of  Lawes 
&  Gilbert  on  swine,  were  the  first  to  furnish  proof  of  the  formation 
of  fat  from  carbohydrates  by  ruminants.  Wolff|  having  pointed 
out  that  their  results  demonstrated  that  fact,  Henneberg  discussed 
this  feature  of  the  experiments  in  a  later  publication. §  Regarding  all 
the  digested  ether  extract  of  the  food  as  pure  fat,  and  assuming  that 
all  the  digested  nitrogenous  matters  were  true  proteids  capable  of 
yielding  51 .4  per  cent,  of  fat,  he  obtained  the  results  given  on  p.  174. 
Forty-two  per  cent,  more  fat  was  produced  than  could  be  accounted 
for  by  the  fat  and  proteids  of  the  food,  even  on  the  extreme 
assumptions  made.  Furthermore,  not  only  did  some  of  the 
nitrogenous  substances  of  the  food  undoubtedly  consist  of  non-pro- 

*  Landw.  Jahrb.,  8,  96. 

t  Jour.  f.  Landw.,  26,  549. 

j  Landw.  Jahrb.,  8,  I.  Supp.,  269. 

§  Zeit   f   Biol  ,  17,  345. 


174 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Digested 

Proteids  stored  up 

Proteids  available  for  fat  production, 
Equivalent  fat  (51 .4  per  cent.) 

Total  from  fat  and  proteids 

Actually  produced  by  animal 


Proteids, 
Grins. 

Fat, 
Grms. 

10220 
936 

2100 

9284 

4772 

6872 
9730 

teids,  but  a  high  figure  was  assumed  for  their  digestibility,  and  in 
computing  the  gain  of  fat  by  the  animal  no  account  was  taken  of 
the  fat  of  the  wool  and  of  the  offal.  Henneberg's  final  conclusion 
is  that  no  possible  errors  arising  from  differences  in  the  animals 
compared  or  from  irregularities  in  the  consumption  of  food  can 
explain  away  the  above  result. 

Soxhlet  *  made  similar  experiments  with  swine  fattened  on  rice, 
that  is,  on  a  feeding-stuff  poor  in  proteids  and  fat  and  rich  in  carbo- 
hydrates, with  the  result  that  only  17  to  18  per  cent,  of  the  fat  pro- 
duced could  be  accounted  for  by  the  digestible  protein  and  fat  of 
the  food.  In  two  experiments  with  the  same  species  of  animal  by 
Tschirwinsky  f  but  43  per  cent,  and  28  per  cent,  respectively  of  the 
fat  production  could  be  thus  accounted  for.  Of  six  experiments 
on  geese  by  B.  Schulze,J  four,  in  which  a  comparatively  wide  nutri- 
tive ratio  was  used,  showed  that  at  least  from  5  to  20  per  cent,  of  the 
fat  must  have  been  produced  from  carbohydrates.  Chaniewski  § 
likewise  experimented  on  geese  and  obtatined  much  more  decisive 
results,  from  72  to  87  per  cent,  of  the  observed  fat  production  being 
necessarily  ascribed  to  the  carbohydrates. 

Recent  experiments  by  Jordan  ||  have  shown  that  the  dairy  cow 
may  likewise  produce  fat  from  carbohydrates.  In  the  first  experi- 
ment a  cow  weighing  867  pounds  was  fed  for  fifty-nine  days  with 
food  from  which  most  of  the  fat  had  been  extracted,  the  digestible 


*  Bied.  Centr.  Bl.  Ag.  Chem.,  10,  674. 

t  Landw.  Vers.  Stat.,  29,  317. 

%  Landw.  Jahrb.,  11,  57. 

§  Zeit.  f.  Biol.,  20,  179. 

||  N.  Y.  State  Experiment  Station,  Bulls.  132  and  197. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD  SUPPLY. 


15 


protein  of  the  ration  being  varied  from  184  grams  to  841  grams  per 
day.  During  this  time  she  gained  33  pounds  in  weight,  and  her 
whole  appearance  was  such  as  to  negative  the  assumption  of  any- 
considerable  loss  of  body  fat.  In  the  second  experiment  one  cow 
was  fed  a  ration  poor  in  fat,  one  a  normal  ration,  and  one  a  ration 
unusually  rich  in  fat,  the  protein  supply  being  again  varied  through 
a  considerable  range.  As  in  the  previous  case  the  gain  in  weight 
and  the  general  condition  of  the  cows  forbade  the  assumption  that 
body  fat  was  drawn  upon  to  any  material  extent.  In  all  instances 
except  the  last  a  considerable  formation  of  fat  from  carbohydrates 
was  shown. 

The  following  table  gives  the  more  important  data  of  the  above 
experiments : 


Experimenter. 


Total 
Proteid* 

Meta- 
bolism, 

Grms. 


Equiva- 
lent Fat, 
Grms. 


Fat  of 
Food, 
Grms. 


Total 
from  Fat 

and 
Proteids, 

Grms. 


Fat 

Actually 

produced. 

Grms. 


Henneberg,     Kern, 
Wattenberg 

Soxhlet 

Tschirwinsky 

Schulze 

Chaniewski 

( 59  days  ) 
Jordan:  -{74     "     }•  . 
4     " 


Sheep 
Swine 

Swine 


Geese  -l 


Geese 


Cows 


284 
463 
169 
,934 
361f 
054 
049 
785 
785 
555 
555 
110 
203 
100 
109 
661 
209 


4,772 
1,779 
3,685 
3,050 
1,213 
383 1 
381 J 
286} 
286} 
194J 
194| 
55 
105 
51 
7,766 
17,816 
1,131 


2,100 

300 

340 

656 

203 

222 

221 

205 

205 

203 

203 

20 

32 

9 

1,490 

2,211 

1,504 


6,872 

2,079 

4,025 

3,706 

1,416 

605 

602 

491 

491 

397 

397 

75 

137 

60 

9,256 

20,027 

2,635 


9,730 

10,082 

22,180 

8,577 

5,429 

387 

539 

515 

612 

492 

471 

269 

640 

445 

17,585 

37,637 

3,289 


In  view  of  the  extreme  assumptions  made  in  these  computations 
as  to  the  possible  contribution  by  the  proteids  and  fat  of  the  food 


*  Digested  protein  of  food  less  gain  of  protein  by  the  animal, 
t  In  original  2572  grms. 

X  Computed  on  a  different  basis  from  the  other  experiments 
loc.  cit ,  p.  84. 


Compare 


i76 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


to  fat  production,  and  of  the  very  large  differences  between  this 
amount  and  the  fat  computed  to  have  been  actually  formed,  the 
possible  errors  of  the  method  are  relatively  insignificant,  and  these 
investigations,  together  with  the  earlier  ones,  must  be  regarded  as 
establishing  the  fact  of  a  formation  of  fat  from  carbohydrates. 

The  earliest  experiment  to  be  published  in  full  demonstrating 
the  production  of  fat  from  carbohydrates  in  the  body  of  the  dog, 
was  by  Munk.*  The  animal  was  deprived  of  food  long  enough  to 
render  it  certain  that  but  traces  of  fat  remained  in  the  body.  It 
was  then  fed  for  twenty-four  days  on  a  diet  consisting  of  small 
amounts  of  meat,  with  some  gelatine,  and  large  quantities  of 
starch  and  sugar.  In  the  body  of  the  animal  at  the  close  of  the 
experiment  1070  grams  of  fat  were  found,  of  which  Munk  estimates 
that  at  least  960  grams  must  have  been  produced  during  the  experi- 
ment, while  the  proteids  fed  could  have  produced  as  a  maximum 
only  415.3  grams  and  the  meat  itself  contained  but  75  grams  of  fat. 
Even  if  a  formation  of  fat  from  gelatine  be  admitted,  a  considerable 
excess  of  fat  remains  unaccounted  for  except  by  the  carbohvdrates 
of  the  food. 

Respiration  Experiments. — There  are  not  wanting,  however,  for 
final  demonstration,  experiments  with  the  respiration  apparatus,  in 
which  the  total  income  and  outgo  of  nitrogen  and  carbon  has  been 
determined. 

Meissl,  Strohmer,  &  Lorenz,t  in  very  carefully  conducted  respi- 
ration experiments  upon  swine,  using  a  wide,  a  medium,  and  a 
narrow  nutritive  ratio,  obtained  the  following  results: 


Food, 
Grms. 


Proteid 

Metabolism. 

Grms. 


Equivalent 
Fat, 
Grms. 


Fat  of 
Food, 
Grms. 


Total 

from  Fat 

and  Proteids, 

Grms. 


Fat 

Actually 

Produced, 

Grms. 


Rice. 


Barley 

Mesh"  meal,  rice,  and 
whey 


65.4 
64.1 
88.0 

381.6 


33.6 
33.0 
45.2 

196.1 


7.9 
16.4 
15.2 


41.5 
49.4 
60.4 

244.7 


353 
413 
208 


256.3 


Almost  simultaneously  C.  Voit  \  gave  a  preliminary  account  of 

*  Virchow's  Archiv,  101,  91. 

t  Zeit.  f.  Biol.,  22,  63. 

%  Sitzungsber  bayr.  Acad  d.  Wiss.;   Math.  Phys.  Classe,  1885,  p.  288. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY.        17  7 

respiration  experiments  made  in  his  laboratory  by  Lehmann  & 
E.  Voit  with  geese  and  by  Rubner  with  a  dog  which  demonstrated 
a  production  of  fat  from  carbohydrates.  Rubner's  experiment  was 
shortly  afterward  published  in  full.*  It  was  a  respiration  experi- 
ment covering  four  days  immediately  following  a  fortnight's  heavy 
feeding  with  meat.  On  the  first  two  days  of  the  experiment  the 
animal  fasted  and  on  the  second  two  received  only  starch  and  cane- 
sugar.     The  results  for  the  last  two  days  were: 

Proteid  metabolism 15.94  grams. 

Equivalent  fat,,  according  to  Rubner..       7.65       " 

Fat  of  food 9.40      " 

Maximum  from  fat  and  proteids 17 .  05      " 

Fat  actually  produced 1 1 7 .  25      " 

Even  after  making  all  possible  deductions  for  the  fact  that  some 
carbon  may  have  been  retained  in  the  body  in  the  form  of  glycogen 
instead  of  fat,  and  also  for  a  possible  residue  of  undigested  starch  in 
the  alimentary  canal  at  the  close  of  the  experiments,  Rubner  still 
computes  that  at  least  40.7  grams  of  fat  must  have  had  its  origin 
in  carbohydrates. 

Lehmann  &  E.  Voit's  experiments  have  only  recently  ap- 
peared^ In  their  introduction  they  report  also  the  results  of  ex- 
periments on  fattening  geese  made  by  C.  Voit  several  years  previous 
to  1883,  which  likewise  show  a  production  of  fat  from  carbohydrates. 

G.  Kuhn  and  his  associates, J  at  the  Mockern  Experiment  Station, 
have  demonstrated,  by  means  of  respiration  experiments  in  which 
starch  was  added  to  rations  but  slightly  exceeding  the  maintenance 
requirement,  a  formation  of  fat  from  carbohydrates  by  ruminants 
(oxen).  In  view  of  the  possibility  (see  p.  27)  that  part  of  the  car- 
bon of  the  urine  may  be  derived  from  the  non-nitrogenous  matter 
of  the  food,  and  in  order  to  be  on  the  safe  side,  the  authors  assume 
as  possible  that  all  the  carbon  of  the  proteids  metabolized  may 
have  been  stored  up  in  the  body  in  the  form  of  fat.  On  this  extreme 
and  improbable  assumption  their  results  were  as  shown  on  the 
following  page : 

*  Zeit.  f.  Biol.,  22,  272. 

t  Ibid.,  42,  619. 

t  Reported  by  Kellner;  Landw.  Vers.  Stat.,  44,  257. 


1 78 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Animal. 

Period. 

Proteid 

Metabolism, 

Grms. 

Equivalent 
Fat, 
Grms. 

Fat  of 
Food, 
Grms. 

Maximum 

from  Fat 

and  Proteids, 

Grms. 

Fat 

Actually 

Produced, 

Grms. 

I 

2a 

373.6 

259 

86 

345 

423 

I 

2b 

382.0 

265 

81 

346 

332 

II 

2 

297.4 

206 

77 

283 

434 

III 

2 

104.4 

72 

60 

132 

281 

IV 

2 

126.9 

88 

60 

148 

160 

III 

3 

506.9 

351 

69 

420 

375 

IV 

3 

548.8 

380 

74 

454 

388 

III 

4 

980 

679 

84 

763 

526 

V 

2a 

232 

161 

42 

203 

396 

V 

2b 

268 

186 

42 

228 

407 

V 

3 

149 

103 

39 

142 

703 

VI 

2a 

218 

151 

40 

191 

304 

VI 

26 

232 

161 

35 

196 

381 

VI 

3 

186 

129 

43 

172 

507 

111  most  of  these  experiments  the  rations  were  purposely  made 
poor  in  proteids  and  fat,  and  in  all  such  cases,  with  one  exception,  a 
formation  of  fat  from  carbohydrates  is  clearly  demonstrated.  In 
three  cases  in  which  large  amounts  of  proteids  were  fed,  as  well  as  in 
some  similar  experiments  not  included  in  the  above  table,  it  was 
possible  to  account  for  the  fat  production  otherwise,  but  such  nega- 
tive results  in  no  degree  weaken  the  positive  teaching  of  the  remain- 
ing trials. 

The  more  recent  investigations  of  Kellner  et  al*  at  the  same 
Station,  in  which  starch  was  added  to  a  basal  ration,  although  under- 
taken primarily  for  other  purposes,  likewise  show  the  formation  of 
an  amount  of  fat  inconsistent  with  the  hypothesis  of  its  production 
from  the  fat  and  proteids  of  the  ration  only. 

The  failure  of  Pettenkofer  &  Voit  to  obtain  affirmative  results 
in  their  earlier  experiments  appears  to  be  largely  explicable,  in  the 
light  of  more  recent  knowledge,  from  the  conditions  of  the  experi- 
ments themselves.  Pfiuger  f  has  recalculated  their  experiments  on 
the  same  basis  as  those  upon  the  formation  of  fat  from  proteids 
(see  p.  109),  and  has  pointed  out  that  in  the  majority  of  cases  the 
total  food  was,  according  to  his  computations,  scarcely  more  than 
sufficient  for  the  maintenance  of  the  organism,  thus  leaving  no 
excess  of  any  kind  for  fat  production.     Moreover,  out  of  those  ex- 

*  Landw.  Vers.  Stat  ..,  53,  1. 
t  Arch.  ges.  Physiol ,  52,  239. 


THE  RELATIONS  OF  METABOLISM   TO  FOOD-SUPPLY.        179 

periments  in  which  the  conditions  were  favorable  for  a  production 
of  fat  from  carbohydrates,  some  actually  do  show  that  result,  al- 
though they  were  classed  by  Voit  as  "exceptional  cases,"  while  its 
failure  to  appear  in  others  is  explained,  according  to  Pfliiger,  by  the 
increased  metabolism  due  to  maltreatment  of  the  animal  and  the 
overloading  of  its  digestive  organs  with  starch. 

Whether  we  admit  all  of  Pfluger's  criticism  or  not,  it  is  now  uni- 
versally conceded  that  the  carbohydrates  are  an  important  source 
of  fat.  If  we  are  to  go  further  and  deny  with  Pfliiger  the  production 
of  fat  from  proteids,  we  are  brought  back,  by  a  curious  reversal  of 
views,  substantially  to  Liebig's  classification  of  the  nutrients  into 
"  plastic "  and  "  respiratory, "  but,  as  already  pointed  out,  it  ap- 
pears altogether  probable  that  the  proteids  also  contribute  to  fat 
production.  However  this  may  be,  it  is  clear  that  in  the  case 
of  herbivorous  animals,  which  ordinarily  consume  relatively  little 
proteids  and  fat  and  large  amounts  of  carbohydrates,  the  latter  are 
the  most  important  factors  in  fattening,  and  the  results  of  Lawes 
&  Gilbert  (p.  167),  according  to  which  the  gain  of  fattening  ani- 
mals is  largely  determined  by  the  supply  of  non-nitrogenous  matters 
in  the  food,  are  seen  to  be  in  full  accord  with  the  most  careful  physi- 
ological investigation. 

Evidence  from  Respiratory  Quotient. — The  formation  of  fat  from 
carbohydrates  is  a  process  of  reduction.  If  we  suppose  all  the  car- 
bon of  100  parts  of  dextrose,  together  with  the  necessary  hydrogen 
and  oxygen,  to  be  united  to  form  fat  of  the  average  composition 
stated  on  p.  61,  we  have  the  following: 


Dextrose. 

Equivalent, 

Fat. 

Residue. 

Equivalent, 
Water. 

Excess    of 
Oxygen. 

40.00 

6.67 

53.33 

40.00 
6.28 
6.01 

0.39 
47.32 

0.39 
3.12 

44.20 

100.00 

52.29 

47.71 

3.51 

44.20 

The  excess  of  oxygen  we  may  further  suppose  to  unite  with  the 
carbon  of  41.44  additional  parts  of  dextrose,  producing  60.78  parts 
of  carbon  dioxide  and  24.86  parts  of  water.  The  process  would  be 
an  intra-molecular  combustion   analogous  to  a  fermentation,  pro- 


i  So  PRINCIPLES  OF  ANIMAL   NUTRITION. 

during  carbon  dioxide  without  the  intervention  of  oxygen  from  out 
side.  The  latter  fact,  of  course,  is  equally  true  whatever  substance 
combines  with  the  excess  of  oxygen  of  the  carbohydrate.  The 
tendency,  therefore,  will  be  to  increase  the  respiratory  quotient  and, 
if  large  amounts  of  carbohydrates  are  thus  transformed,  to  even 
raise  it  above  unity. 

Numerous  such  instances  are  on  record.  Thus  Regnault  & 
Reiset  *  report  a  quotient  of  1.024  in  case  of  a  hen,  and  Reiset  f  ob- 
tained quotients  of  1.004  and  1.054  with  a  ewe  and  a  boar.  Han- 
riot  &  Richet,J  in  studies  on  the  respiration  of  man,  found  that 
the  ingestion  of  carbohydrates  caused  the  respiratory  quotient  to 
rise  markedly  and  sometimes  to  exceed  unity.  Later  Hanriot  § 
studied  the  transformations  of  glucose  in  the  organism  of  man  and 
obtained  similar  but  more  marked  results,  the  quotient  reaching  as 
high  a  value  as  1.28. 

Magnus-Levy  |  has  likewise  observed  quotients  greater  than 
unity  in  the  case  of  a  dog  fed  large  quantities  of  carbohydrates,  and 
Bleibtreu,!"  in  experiments  on  fattening  geese  in  a  form  of  Regnault 
respiration  apparatus,  also  verified  this  fact,  as  have  Kaufmann  ** 
and  Laulanie  ft  m  experiments  upon  dogs  with  sugar.  The  exten- 
sive respiration  experiments  of  Zuntz  &  HagemannJ];  on  the  horse 
also  afford  numerous  instances  of  respiratory  quotients  greater 
than  unity. 

The  evidence  of  the  respiratory  quotient,  then,  is  entirely  in 
accord  with  the  conclusions  reached  by  other  methods  as  to  the 
formation  of  fat  from  carbohydrates. 

Non-nitrogenous  Nutrients  of  Feeding-stuffs. — It  has 
become  customary  to  regard  the  digestible  non-nitrogenous  ingre- 
dients of  feeding-stuffs,  aside  from  the  ether  extract,  as  consisting 
essentially  of  carbohydrates.     As  has  several  times  been  urged  on 

*  Ann  de  Chim.  et  de  Phys.  [3],  26,  45. 

t  Ibid.  [3],  69,  145. 

t  Comptes  rend.,  106,  419  and  496. 

§  Archives  de  Physiol.,  1893,  p.  248. 

|]  Arch.  ges.  Physiol.,  55,  1. 

IT  Ibid.,  56,464;  85,  366. 
**  Archives  de  Physiol.,  1896,  341. 
tf  Ibid,  1896,  791. 
%t  Landw.  Jahrb.,  27,  Supp  III. 


THE  RELATIONS   OF  METABOLISM    TO  FOOD-SUPPLY.        181 

preceding  pages,  however,  this  is  far  from  being  the  case  as  regards 
the  materials  actually  resorbed  from  the  digestive  tract  of  our 
common  domestic  animals,  particularly  the  ruminants.  A  demon- 
stration of  the  production  of  fat  from  carbohydrates,  therefore, 
does  not  necessarily  show  that  the  chemically  diverse  materials 
resorbed  from  coarse  fodders,  e.g.,  are  available  for  fat  produc- 
tion. 

As  a  matter  of  fact,  however,  what  a  large  proportion  of  the 
experiments  just  cited  actually  show,  under  a  strict  interpretation, 
is  that  fat  was  produced  from  the  non-nitrogenous  nutrients  of  the 
rations  other  than  fat.  In  many  of  the  experiments,  it  is  true,  nota- 
bly those  with  swine  and  with  geese,  the  ration  consisted  of  concen- 
trated feeding-stuffs  whose  "  nitrogen-free  extract "  consisted  to  a 
large  extent  of  hexose  carbohydrates.  Similarly,  in  G.  Kuhn's  ex- 
periments the  fat  production  was  secured  by  the  addition  of  starch 
to  rations  slightly  above  the  maintenance  requirement.  In  these 
cases,  therefore,  at  least  the  larger  part  of  the  fat  production  in 
excess  of  that  possible  from  proteids  and  food  fat  must  be  ascribed 
to  the  hexose  carbohydrates.  In  experiments  like  those  of  Henne- 
berg,  Kern  &  Wattenberg,  and  of  Jordan,  on  the  other  hand,  a 
not  inconsiderable  proportion  of  the  non-nitrogenous  nutrients 
was  necessarily  derived  from  coarse  fodders  and  was,  therefore, 
largely  of  undetermined  nature.  In  such  cases  it  is  obviously  im- 
possible to  say  whether  the  fat  production  was  at  the  expense  of 
the  hexose  carbohydrates  only  or  whether  the  other  non-nitrog- 
enous ingredients  participated  in  it. 

Other  considerations,  however,  seem  to  render  a  participation 
of  these  substances  in  fat  production,  directly  or  indirectly,  at  least 
highly  probable  if  not  certain. 

Crude  Fiber. — The  experiments  of  v.  Knieriem  (p.  161),  as  we 
have  seen,  seem  to  show  that  digested  cellulose  may  be  as  efficient  as 
other  carbohydrates  in  protecting  the  body  fat, — that  is,  as  part 
of  a  maintenance  ration.  The  numerous  experiments  cited  on 
pp.  117-123  likewise  indicate  that  it  has  an  effect  similar  to  that  of 
other  carbohydrates  in  diminishing  the  proteid  metabolism.  Kell- 
ner  *  has  also  investigated  its  value  in  a  fattening  ration,  using  for 
this  purpose  the  material  resulting  from  the  treatment  of  rye  straw 
*  Landw.  Vers.  Stat.,  53,  278. 


182 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


with  an  alkaline  solution  under  pressure  and  containing  76.78  pei 
cent,  of  "  crude  fiber."  This  material  was  added  to  a  basal  ration 
somewhat  more  than  sufficient  for  maintenance.  The  results  as 
regards  the  proteid  metabolism  have  already  been  considered 
(p.  121);  the  following  table  shows  the  effects  also  upon  the  fat 
production : 


Apparently  Digested. 

Gain. 

Crude  ]  Crude 
Fat,     |  Fiber, 
Grms.   j  Grms. 

N.-free 
Extract, 

Grms. 

Protein, 
Grms. 

Protein, 
Grms. 

Fat. 
Grms. 

OxH: 
Period  5 
"      4 

Extracted  straw . . 
Basal  ration 

Difference.. 

116 
101 

3129 
1083 

3351 
2912 

654 

749 

157 
43 

735 
191 

Period  3 

15 

92 
101 

2047 

1057 
1083 

439 

4773 
2912 

-95 

629 
749 

114 

78 
43 

544 
565 

"      4 

Basal  ration 

Difference 

Extracted  straw.. . 
Basal  ration 

Difference 

Starch 

191 

Ox  J: 
Period  5 

"      4 

-9 

110 
107 

-26 

3101 
1114 

1861 

3344 

2895 

-120 

747 
836 

35 

98 
33 

374 

693 
223 

Period  3 

3 

85 
107 

1987 

1105 
1114 

449 

4396 
2895 

-89 

764 
836 

65 

91 
33 

470 

472 

a      4 

Basal  ration 

Difference 

223 

-22 

-9 

1501 

-72 

58 

249 

The  varying  quantities  of  nutrients  digested  stand  in  the  way 
of  a  direct  comparison  of  the  results.  If,  however,  we  reckon  1 
gram  of  digested  fat  equivalent  to  2.25  grams  of  digested  crude 
fiber  or  nitrogen-free  extract  or  protein  (isodynamic  quantities 
according  to  the  usual  method  of  computation),  and  if  we  further 
convert  the  gain  of  proteids  into  its  equivalent  amount  of  fat,  on 
the  same  principle,  by  multiplication  by  5.7  and  division  by  9.4,  we 
have  the  results  shown  in  the  table  on  the  opposite  page. 

While  no  great  quantitative  accuracy  attaches  to  such  a  com- 
putation, it  is  sufficient  to  show  that  the  effect  produced  in  this  case 


THE  RELATIONS  OF  METABOLISM    TO  FOOD-SUPPLY.        183 


Total 

Carbohydrate 

Equivalent 

of  Nutrients, 

Grms. 

Total  Fat 

Equivalent 

of  Gain, 

Grms. 

Gain  per 

Kilogram 

Nutrients, 

Grms. 

OxH: 

2425 
1695 

2334 
1370 

613 
395 

509 
284 

252   8 

Starch,                        "»     3-4 

Ox  J: 

Extracted  straw,  period  5-4 

Starch,                       "       3-4 

233.0 

218.1 
207  3 

by  the  addition  to  the  basal  ration  of  digestible  matter  five  sixths 
of  which  was  derived  from  crude  fiber,  was  not  inferior  to  that 
produced  by  the  addition  of  an  equal  amount  of  pure  starch. 

It  would  seem  that  these  results  may  fairly  be  taken  as  showing 
that  the  products  of  the  digestion  of  cellulose  by  ruminants  are 
substantially  of  equal  value  with  those  of  the  digestion  of  starch. 
This,  however,  by  no  means  warrants  the  conclusion  that  starch  and 
cellulose  are  of  equal  value  in  ordinary  feeding-stuffs.  The  mate- 
rial used  in  these  experiments  had  been  so  altered  mechanically 
and  freed  from  incrusting  materials  by  the  treatment  to  which  it 
had  been  subjected  that  88.3  per  cent,  of  its  organic  matter  and 
95.8  per  cent,  of  its  crude  fiber  was  digested.  The  same  animals 
digested  but  52.5  per  cent,  of  the  crude  fiber  of  wheat  straw,  and 
the  digestible  organic  matter  of  the  latter  proved  far  less  efficient 
than  that  of  either  starch  or  extracted  straw.  A  full  discussion  of 
these  facts  may  be  more  profitably  undertaken  in  connection  with  a 
consideration  of  the  energy  relations  of  feeding-stuffs  in  Part  II; 
for  the  present  it  may  suffice  to  point  out  that  the  difference  just 
noted  appears  to  depend  on  physical  rather  than  chemical  causes. 

Pentose  Carbohydrates. — We  have  already  (p.  156)  seen  reason 
to  believe  that  the  pentose  carbohydrates  may  serve  as  a  source  of 
energy  to  the  organism  and  protect  other  materials  from  oxidation. 
This,  of  course,  is  equivalent  to  an  indirect  production  of  fat.  In 
the  same  connection,  however,  the  experiments  of  Kellner,  just 
mentioned,  were  referred  to  as  indicating  a  direct  participation  by 
these  bodies  in  fat  production.  About  one  third  of  the  digested 
matter  of  the  extracted  rye  straw  was  found  to  consist  of  bodies 


1 84 


PRINCIPLES   OF   ANIMAL    NUTRITION. 


yielding  furfural,  presumably  pentosans,  as  appears  from  the  follow- 
ing modified  form  of  the  last  table : 


Total  Carbohydrate  Equivalent 
of  Nutrients. 

Total  Fat 
Equivalent 

Pentosans, 
Grms. 

Other  Substances, 
Grms. 

of  Gain, 
Grms. 

OxH: 

Extracted  straw,  period  5-4 

Starch,                        "       3^ 

Ox  J: 

Extracted  straw,  period  5-4 

Starch,                         "       3-4 

809 
-34 

834 

-89 

1616 
1729 

1500 
1459 

613 
395 

509 

284 

If  we  regard  the  furfuroids  as  not  contributing  to  the  fat  pro- 
duction, then  we  must  assign  to  the  other  nutrients  of  the  extracted 
straw  a  value  from  66  to  74  per  cent,  greater  than  that  of  the 
digested  matter  of  the  starch,  a  result  which  is  hardly  conceivable. 
Apparently  we  must  admit  that  the  furfuroids  in  this  case  pro- 
duced approximately  the  same  effect  as  the  other  non-nitrogenous 
nutrients  and  were  at  least  indirectly  if  not  directly  a  source  of  fat. 


CHAPTER  VI. 

THE   INFLUENCE    OF   MUSCULAR  EXERTION   UPON 
METABOLISM. 

It  is  a  matter  of  common  experience  that  muscular  exertion 
results  in  a  very  marked  increase  in  the  vital  activities  of  the  body. 
The  rate  of  circulation  and  respiration  is  greatly  quickened  and  the 
increased  metabolism  in  the  organism  is  shown  by  the  loss  of  weight 
and  by  the  increased  demand  for  food  to  make  good  the  destruction 
of  tissue.  Indeed,  no  other  factor  even  approaches  muscular  exer- 
tion in  the  extent  to  which  it  increases  the  metabolic  activities  of 
the  body. 

We  have  now  to. consider  in  some  detail  the  nature  of  muscular 
exertion  and  the  precise  character  of  its  effects  upon  metabolism. 

§  i.  General  Features  of  Muscular  Activity. 

Muscular  Contraction. 

The  work  of  the  muscles  is  accomplished  by  contracting,  and  a 
brief  consideration  of  some  of  the  more  prominent  general  features 
of  muscular  contraction  will  conduce  to  an  intelligent  study  of  the 
main  subject  of  the  chapter.  It  will  be  possible  here  to  consider 
this  phase  of  the  subject  only  in  its  most  general  outline,  and  the 
reader  is  referred  to  works  on  physiology  for  details. 

When  a  suitable  stimulus,  which  in  the  living  animal  is  usually 
a  nerve  stimulus,  is  applied  to  a  muscle  it  contracts;  that  is,  it 
tends  to  grow  shorter  and  thicker.  This  change  is  brought  about 
by  a  shortening  and  thickening  of  the  individual  fibers  of  which 
the  muscle  is  built  up.  A  single  stimulus,  such,  for  example, 
as  that  caused  by  the  making  or  breaking  of  an  electric  circuit, 
gives  rise  to  what  is  known  as  a  simple  muscular  contraction.  If 
such  a  stimulus  is  repeated  with  sufficient  frequency  it  produces  a 

1 85 


1 86  PRINCIPLES  OF  ANIMAL   NUTRITION. 

series  of  simple  contractions  which  fuse  together,  resulting  in  a  state 
of  contraction  which  continues,  subject  to  the  effects  of  fatigue,  as 
long  as  the  stimulus  acts.  This  form  of  muscular  contraction 
has  received  the  name  of  "tetanus."  In  the  living  animal  the 
ordinary  contractions  of  the  muscles  brought  about  through  the 
nervous  system,  even  those  that  seem  but  momentary,  are  essen- 
tially tetanic  in  their  character. 

Chemical  Changes  during  Contraction. — Under  the  influence 
of  a  stimulus  sufficient  to  produce  a  muscular  contraction  there 
occurs  a  sudden  and  large  increase  in  the  chemical  changes  which 
are  continually  going  on  even  in  the  quiescent  muscle.  More  mate- 
rial is  metabolized  in  the  muscle  during  contraction  and  energy  is 
thus  liberated  for  the  performance  of  work. 

Our  knowledge  of  the  nature  of  these  chemical  changes  in  the 
contracting  muscle  is  comparatively  meager,  but  three  main  features 
appear  well  established: 

First,  during  contraction  the  neutral  or  slightly  alkaline  reac- 
tion of  the  quiescent  muscle  c'  anges  to  an  acid  reaction,  probably 
through  the  formation  of  sarcolactic  acid. 

Second,  there  is  a  large  increase  in  the  amount  of  oxygen  taken 
up  by  the  muscle  from  the  blood  and  a  still  greater  increase  in  the 
amount  of  carbon  dioxide  given  off  by  it.* 

Third,  under  normal  circumstances,  judging  from  the'  amount 
of  the  urinary  nitrogen,  there  appears  to  be  no  considerable  increase 
in  the  nitrogenous  products  of  metabolism. 

From  the  increase  in  oxygen  consumed  and  carbon  dioxide  given 
off  we  might  be  led  at  first  thought  to  suppose  that  the  increased 
activity  in  the  muscle  during  contraction  was  of  the  nature  of  a 
simple  oxidation.  Certain  other  facts,  however,  seem  to  show  that 
this  view  of  the  matter  is  inadequate. 

Oxidations  Incomplete. — That  the  increased  metabolism  in 
the  contracting  muscle  is  not  a  simple  oxidation  of  some  material 
t  carbon  dioxide  and  water  is  indicated  by  the  fact  of  the  produc- 
tion of  lactic  or  other  acid  in  the  muscle.  Plainly,  if  the  energy  for 
muscular  contraction  is  produced  by  oxidation  the  oxidation  is  at 
least  incomplete. 

*  Some  good  authorities  doubt  whether  the  carbon  dioxide  resulting 
from  muscular  exertion  actually  leaves  the  muscle  in  that  form.  Compare 
Bchaffer,  Text-book  of  Physiology,  1898,  Vol.  I,  p.  911. 


INFLUENCE  OF  MUSCULAR  EXERTION  UPON  METABOLISM.    187 

Respiratory  Quotient. — By  analogy  with  investigations  upon 
respiration  we  may  designate  the  ratio  between  the  oxygen  con- 
sumed and  the  carbon  dioxide  given  off  by  the  muscle  as  the  respi- 
ratory quotient  of  the  muscle.  Numerous  investigations  upon  this 
point  have  shown  that  during  contraction  much  more  carbon  diox- 
ide is  given  off  than  corresponds  to  the  oxygen  consumed,  or,  in 
other  words,  the  respiratory  quotient  of  the  active  muscle  is  con- 
siderably greater  than  unity. 

As  early  as  1862  Sczelkow  *  determined  the  gaseous  exchange 
between  the  blood  and  the  muscles  of  the  posterior  extremities  of 
a  dog,  tetanus  being  produced  by  an  electric  current.  He  found 
that  during  rest  more  oxygen  disappeared  from  the  blood  than 
corresponded  to  the  carbon  dioxide  taken  up  by  it,  while  during 
tetanus,  on  the  contrary,  the  carbon  dioxide  considerably  exceeded 
the  oxygen.  His  results,  calculated  for  the  posterior  extremities 
alone,  were  as  follows: 


Per  Minute. 

Experiment. 

Carbon 

Dioxide 

c.c. 

Oxygen 
c.c. 

Respiratory 
Quotient. 

j  Rest 

1.60 
10.37 

2.62 
12.38 

1.73 
10.62 

3.53 

12.19 

2.33 

12.95 

4.10 
3.92 

4.25 
10.52 

3.21 
7.55 

4.71 
9.38 

5.82 
18.71 

0.41 

1 

2.65 

(  Rest 

0.62 

2 

1.18 

(  Rest 

0.54 

3 

1  Tetanus 

(  Rest 

1.41 
0.75 

4 

1.30 

j  Rest 

0.40 

5 

0.80 

In  the  above  experiments,  with  a  single  exception,  the  quantity 
of  oxygen  consumed  by  the  active  muscles  was  more  than  that 
taken  up  in  a  state  of  rest,  but  the  increase  in  the  amount  of  carbon 
dioxide  given  off  was  still  greater,  so  that  the  respiratory  quotient 
was  largely  increased,  exceeding  unity  in  every  instance  but  one. 

*  Sitzungsber.  Wiener  Akad.  d.  Wiss.,  Math-Naturwiss.  Klasse,  45,  II,  171. 


1 88  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Chauveau  &  Kaufmann  *  have  more  recently  obtained  simi- 
lar results.  Their  experiments  were  made  upon  the  Levator  labii 
superioris  of  the  horse,  both  in  a  state  of  rest  and  in  a  state  of  activ- 
ity consequent  upon  the  consumption  of  food.  From  the  amount 
and  composition  of  blood  entering  and  leaving  this  muscle  the 
following  results  were  obtained  for  the  oxygen  consumed  and  carbon 
dioxide  given  off  per  kilogram  of  muscle  in  one  minute.  On  the 
average  of  the  three  experiments,  in  round  numbers,  twenty-one 
times  as  much  oxygen  was  consumed  during  work  as  during  rest  and 
twenty-nine  times  as  much  carbon  dioxide  was  given  off. 


Oxygen  Consumed. 

Carbon  Dioxide    Given  Off. 

Experiment. 

Rest, 
Grms. 

Work. 
Grms. 

Work  -+- 
Rest. 

Rest. 
Grms. 

Work. 
Grms. 

Work-f- 
Hest. 

2 

.00479 
.01167 
.00419 

.07148 
.20190 
. 14899 

14.9 
17.3 
35.6 

.00365 
.01168 
.00518 

. 12534 
.35488 
.25709 

34  3 

3 

4 

30.4 
49  6 

Average 

.00688 

. 14079 

20.5 

.00864 

.24577 

2S.5 

These  facts  show  plainly  that  the  increased  metabolism  of  the 
active  muscle  cannot  consist  wholly  of  a  direct  oxidation,  since  the 
carbon  dioxide  given  off  from  the  muscle  contains  more  oxygen 
than  direct  experiment  shows  to  have  been  taken  up  by  the  muscle 
during  the  same  time. 

Oxygen  not  Essential. — A  further  and  still  more  striking 
proof  of  the  above  assertion  is  found  in  the  fact  that  the  living 
muscle  can  execute  a  considerable  number  of  contractions  in  the 
entire  absence  of  oxygen. 

Setschenow  is  quoted  by  Ludwig  &  Schmidt  f  as  having  found 
that  muscles  would  contract  freely  when  supplied  with  oxygen-free 
blood,  while  L.  Hermann  J  has  shown  that  an  excised  muscle  may 
continue  to  contract  in  a  vacuum.  The  well-known  investigations 
of  Pfliiger  §  show  that  frogs  may  continue  to  live  and  execute  more 
or  less  extensive  motions  in  an  atmosphere  of  pure   nitrogen  for 

*  Comptes  rend.,  104,  1126,  1352,  1409. 

t  Verhandl.  Sachs  Akad.  d.  Wiss.,  Math-Phys.  Klasse,  20,  12. 

%  Unters.  u.  Stoffw.  der  Muskeln. 

§  Arch  ges.  Physiol.,  10,  313. 


INFLUENCE   OF  MUSCULAR  EXERTION   UPON  METABOLISM.    189 

several  hours,  giving  out  considerable  amounts  of  carbon  dioxide, 
and  Bunge  *  has  made  similar  observations  upon  the  movements 
of  certain  intestinal  worms  (Ascaris)  in  one  per  cent,  salt  solution 
made  as  nearly  oxygen-free  as  possible. 

Weinland  t  has  shown  that  in  the  latter  case  the  energy  is 
derived  chiefly  from  the  cleavage  of  glycogen  with  the  production 
of  carbonic  and  valerianic  acids. 

Summary. — The  three  classes  of  facts  just  adduced  justify  the 
conclusion  that  the  chemical  changes  by  which  energy  is  liberated 
in  a  muscular  contraction  are  not  simply  oxidations,  but  are  of  the 
nature  of  a  cleavage  of  some  complex  substance  or  substances  with 
evolution  of  carbon  dioxide.  There  is,  in  other  words,  a  sudden 
"  explosive  "  decomposition  of  substances  elaborated  in  the  muscle 
during  rest.  Of  the  nature  of  the  material  thus  broken  down  we 
have  little  definite  knowledge.  We  can  say,  however,  that  if  it  is 
nitrogenous  matter  its  nitrogen  is  ordinarily  retained  in  the  muscle 
in  some  form  and  that  in  effect  the  metabolized  material  is  non- 
nitrogenous.  The  increase  in  the  consumption  of  oxygen  during 
work  appears  to  be  to  a  certain  extent  a  secondary  process,  accom- 
plishing the  further  oxidation  of  the  primary  products  of  metabolism. 
At  the  same  time,  the  fact  that  the  amount  of  oxygen  consumed 
responds  very  promptly  to  work  and  also  to  rts  cessation  shows  that 
these  primary  products,  whatever  they  may  be,  are  very  speedily 
oxidized,  either  in  the  muscle  or  elsewhere  in  the  organism. 

Thermal  Changes  during  Contraction. — A  considerable  por- 
tion of  the  energy  set  free  during  muscular  exertion  always  takes 
ultimately  the  form  of  heat,  When  the  muscle  acts  without  shorten- 
ing, as  when  supporting  a  weight  (isometric  contraction)  —  that  is, 
when  no  external  work  is  done — all  the  metabolized  energy  takes 
the  form  of  heat.  If,  oiWhe  other  hand,  the  weight  be  lifted  (iso- 
tonic contraction)  —  if  external  work  is  done  —  a  portion  of  the 
energy  takes  the  form  of  motion.  The  interesting  question  of  the 
relation  between  the  external  work  performed  and  the  total  amount 
of  energy  metabolized  will  be  considered  later.  For  the  present  it 
is  sufficient  to  state  that  muscular  action  always  produces  heat 
and  that  a  very  considerable  share  of  the  metabolized  energy 
ultimately  takes  this  form.    • 

*  Zeit  physiol.  Chem  .  8,  48.  f  Zeit.  f.  Biol.,  42,  55. 


19°  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Muscular  Tonus. — The  chemical  and  thermal  changes  just 
enumerated  as  characterizing  the  muscle  during  contraction  are 
taking  place  in  it  to  a  less  extent  at  all  times.  Even  at  rest  the 
muscle  respires  and  produces  heat,  as  is  well  illustrated  by  Sczel- 
kow's  and  Chauveau  &  Kaufmann's  experiments  quoted  above. 

The  living  muscles  of  the  body  are  elastic  and  may  be  said  to 
be  always  slightly  on  the  stretch,  as  is  shown  by  the  fact  that  when 
cut  they  gape  open  and  that  they  shorten  when  their  attachments  to 
the  bones  are  severed.  This  slight  degree  of  contraction  of  the  resting 
muscles  has  been  called  muscular  tonus,  and  it  is  at  least  a  plausible 
conclusion  that  the  chemical  changes  taking  place  in  a  quiescent 
muscle  furnish  the  energy  to  maintain  this  tonus.  According  to 
Chauveau  *  we  may  regard  the  essence  of  muscular  contraction  as  a 
sudden  increase  in  the  elasticity  of  the  muscle.  He  holds  that  all 
the  energy  liberated  by  muscular  metabolism  is  converted  first  into 
the  elastic  force  of  the  muscle  and  only  secondarily  into  heat .  Ac- 
cording to  this  view  the  slight  degree  of  elasticity  of  the  quiescent 
muscle  is  produced  by  the  constant  metabolism  going  on  within  it. 
In  active  muscular  contraction  this  process  is  greatly  exaggerated 
and  the  katabolic  processes  exceed  the  anabolic,  thus  giving  rise  to 
a  great  increase  in  muscular  elasticity  which  in  turn  may  be  con- 
verted into  work.  In  repose  following  work,  we  may  assume  that 
the  substances  broken  down  during  contraction  are  built  up  again, 
while  in  prolonged  repose  the  two  processes  must  substantially 
balance  each  other. 

Muscular  tonus  is  most  noticeable  during  the  waking  hours, 
under  the  influence  of  external  stimuli  to  the  central  nervous  sys- 
tem, and  consequently  the  rate  of  metabolism  and  the  heat  produc- 
tion tend  to  be  greater  than  during  sleep.  To  this  is  to  be  added,  as 
a  further  cause  of  greater  metabolic  activity  during  the  waking  hours, 
those  continual  slight  movements  of  the  body  which  usually  take 
place  even  in  what  is  commonly  spoken  of  as  a  state  of  rest  and 
which  may  be  designated  as  incidental  movements. 

That  the  total  amount  of  metabolism  required  for  the  mainte- 
nance of  muscular  tonus  is  considerable  seems  to  be  indicated  by 
the  observations  of   Rohrig    &    Zuntz,|  and  of   Colasanti,t  who 

*  Le  Travail  Musculaire  et  l'Energie  qu'il  Represente.     Paris,  1891. 
t  Arch.  ges.  Physiol.,  4,  57;  12,  522.  %  Ibid.,  16,  157. 


INFLUENCE  OF  MUSCULAR  EXERTION  UPON  METABOLISM-   191 

found  that  when  the  motor  nerves  of  the  rabbit  are  paralyzed  by 
curari  the  rate  of  metabolism,  as  measured  by  the  respiratory  ex- 
change, falls  to  about  one  half  the  amount  during  rest  and  does 
not  react  to  changes  of  external  temperature.  Pfliiger  *  computes 
from  his  experiments  a  similar  reduction  of  about  35  per  cent. 
Under  these  conditions  the  heat  production  of  an  animal  is  insuffi- 
cient to  maintain  its  normal  temperature,  and  unless  the  loss  of 
heat  from  the  body  is  hindered  by  coverings  or  otherwise  it  soon 
perishes.  Frank  &  F.  Voit,f  on  the  contrary,  found  that  curarized 
dogs  excreted  no  less  carbon  dioxide  than  in  the  normal  state,  pro- 
vided the  body  temperature  was  kept  normal. 

Secondary  Effects  of  Muscular  Exertion. 

The  greater  activity  of  the  muscular  metabolism  during  the 
performance  of  work  gives  rise  to  important  secondary  effects,  par- 
ticularly upon  the  circulation  and  respiration.  It  is  a  familiar  fact 
that  in  active  exercise  the  action  of  the  heart  is  largely  increased 
and  the  breathing  becomes  deeper  and  more  rapid,  and  that  ordi- 
narily the  limit  of  muscular  exertion  is  set,  not  by  the  power  of  the 
muscles  themselves,  but  by  the  ability  of  the  heart  and  lungs  to 
keep  pace  with  the  demands  upon  them. 

Circulation. — The  circulating  blood  is  the  medium  by  which 
oxygen  is  conveyed  to  the  muscles  and  carbon  dioxide  and  other 
products  of  their  metabolism  removed.  The  latter  function  is  of 
special  importance,  since  an  accumulation  in  the  muscle  of  the 
products  of  its  own  metabolism  speedily  reduces  and  ultimately 
suspends  its  power  to  contract.  In  active  muscular  exercise,  there- 
fore, an  increase  in  the  rate  of  circulation  is  essential  to  the  con- 
tinued activity  of  the  muscles.  This  increase  appears  to  be  brought 
about  by  the  accumulation  in  the  blood  of  the  products  of  metab- 
olism, which  act  as  a  stimulus  to  the  vaso-motor  center.  The 
result  is  a  dilation  of  the  peripheral  blood-vessels,  which  is  aided  by 
the  mechanical  effects  of  muscular  contraction.  To  offset  this  and 
prevent  a  fall  of  arterial  blood  pressure,  the  visceral  capillaries  are 
probably  constricted,  while  the  rapidity  and  strength  of  the  heart- 
beats are  largely  increased.     The  rapidity  of  the  circulation  as  a 

*  Arch.  ges.  Physiol.,  18,  247.  t  Zeit.  f.  Biol.,  42,  349. 


I92  PRINCIPLES.  OF  ANIMAL   NUTRITION. 

whole  is  thus  greatly  augmented,  while  at  the  same  time  a  larger 
percentage  of  the  total  blood  passes  through  the  muscles.  For 
example,  in  the  experiments  of  Chauveau  &  Kaufmann,  cited 
above,  the  ratio  between  the  circulation  in  the  resting  as  compared 
with  the  active  muscle  varied  from  1 :  3.35  to  1 :  6.60.  Zuntz  & 
Hagemann,*  in  their  investigations  upon  the  work  of  the  heart, 
found  the  average  amount  of  blood  passing  through  the  heart  of  a 
horse  per  minute  to  be  during  rest  29.16  liters  and  during  work 
53.03  liters.  By  this  increase  in  the  rate  of  circulation  through 
the  muscles  the  carbon  dioxide  and  other  injurious  products  of 
muscular  metabolism  are  rapidly  removed  and  an  abundant  supply 
of  oxygen  is  ensured.  In  fact  it  is  usually  true  that  during  work 
which  is  not  excessive  the  venous  blood  contains  less  carbon  diox- 
ide and  more  oxygen  than  during  rest. 

Since  the  heart  is  a  muscular  organ,  it  is  obvious  that  this  in- 
crease in  the  circulatory  activity  must  add  materially  to  its  metab- 
olism. In  the  performance  of  work,  therefore,  there  is  an  expend- 
iture of  matter  and  energy,  not  only  for  the  work  of  the  skeletal 
muscles  but  likewise  for  the  additional  work  of  the  heart.  Zuntz 
&  Hagemann  in  their  experiments  upon  the  horse  just  mentioned 
compute  that  during  moderate  work  the  metabolism  due  to  the 
work  of  the  heart  amounts  to  3.8  per  cent,  of  the  total  metabolism 
of  the  body. 

Respiration. — The  greater  activity  of  the  circulation  conse- 
quent upon  muscular  exertion  would  be  futile  were  not  provision 
made  for  more  efficient  aeration  of  the  blood  in  the  lungs  through  an 
increased  activity  of  the  respiration.  The  latter  appears  to  be 
brought  about,  like  the  increase  in  the  circulatory  activity,  by  the 
effect  of  the  greater  amount  of  metabolic  products  in  the  blood, 
acting  in  this  case  upon  the  respiratory  center.  It  has  been  shown 
that  an  accumulation  of  carbon  dioxide  in  the  blood  does  not  have 
this  effect,  but  that  a  lack  of  oxygen,  such  as  occurs,  for  example, 
in  asphyxiation,  provokes  powerful  movements  of  the  respiratory 
organs.  In  ordinary  work,  however,  whatever  may  be  the  case  in 
excessive  muscular  exertion,  the  effect  is  not  caused  by  a  lack  of 
oxygen,  for  the  blood,  as  already  noted,  is  usually  more  arterialized 

*  Landw.  Jahrb.,  27,  Supp.  Ill,  405. 


INFLUENCE   OF  MUSCULAR  EXERTION  UPON  METABOLISM-    193 

than  during  rest.  Apparently  the  stimulation  of  the  respiratory- 
center  is  brought  about  by  the  other  products  of  muscular  metab- 
olism, whatever  they  may  be,  which  find  their  way  into  the  blood. 
Under  the  influence  of  this  stimulus  the  respiratory  movements 
increase  in  frequency  or  depth  or  both,  thus  making  possible  a 
more  active  gaseous  exchange  between  the  blood  and  the  air  in  the 
lungs.  This  action  is  usually  so  efficient  that  the  expired  air  dur- 
ing work  contains  a  smaller  proportion  of  carbon  dioxide  than  it 
does  during  rest,  notwithstanding  the  fact  that  the  total  quantity 
eliminated  is  much  greater. 

Since  respiration,  like  circulation,  is  maintained  by  muscular 
action,  it  is  true  in  the  former  case  as  in  the  latter  that  a  greater 
activity  of  the  function  necessitates  a  greater  metabolism  for 
that  purpose.  Zuntz  &  Hagemann  *  have  recently  investigated  the 
work  of  respiration  in  the  horse,  the  augmented  respiratory  activ- 
ity being  brought  about  by  an  admixture  of  carbon  dioxide  to  the 
inspired  air,  this  resulting  in  a  marked  increase  in  the  depth  of  the 
respiratory  movements.  With  the  animal  upon  which  most  of  the 
experiments  were  made  they  found  an  increment  of  from  2.02  c.c.  to 
5.23  c.c.  of  oxygen  consumed  for  each  increment  of  one  liter  in  the 
volume  of  air  respired.  In  general,  although  with  some  exceptions, 
the  work  of  respiration  as  thus  measured  increased  with  the  in- 
creased depth  of  the  respiratory  movements.  The  results  upon 
other  horses  were  somewhat  variable.  It  was  observed,  however, 
that  in  the  performance  of  ordinary  work  by  the  horse  the  effect 
was  chiefly  upon  the  frequency  of  respiration  rather  than  its  depth. 
The  former  effect  the  authors  believe  to  involve  less  work  than  the 
latter  and  moreover  an  amount  largely  independent  of  the  total 
volume  of  air  respired. 

§  2.  Effects  upon  Metabolism. 

It  is  obvious  from  the  foregoing  paragraphs  that  the  production 
of  external  work  is  a  complex  phenomenon.  As  regards  its  effects 
upon  the  total  metabolism,  the  main  features  involved  seem  to  be: 

1.  An  explosive  decomposition  of  some  unknown  "contractile 
substance"  in  the  muscles. 

*  Landw.  Jahrb.,  27,  Supp.  Ill,  361. 


194  PRINCIPLES   OF  ANIMAL   NUTRITION. 

2.  The  oxidation  somewhere  in  the  organism  of  the  immediate 
products  of  this  decomposition  to  the  final  excretory  products. 

3.  Since  the  state  of  contraction  appears  to  be  only  an  exagger- 
ation of  the  muscular  condition  during  rest,  we  may  reasonably 
suppose  that  there  is  a  continual  re-formation  of  the  "contractile 
substance  "  going  on. 

4.  As  secondary  effects  there  is  a  marked  increase  in  the  activ- 
ity of  circulation  and  respiration,  thus  involving  supplementary 
muscular  exertion. 

It  is  plain  that  however  interesting  and  important  to  the  physi- 
ologist may  be  studies  of  the  changes  in  the  muscle  itself,  from 
the  point  of  view  of  the  statistics  of  nutrition  the  important  thing 
is  the  total  effect  upon  the  expenditure  of  matter  and  energy  by  the 
organism  under  varying  conditions  of  work.  The  energy  relations 
of  the  subject  will  be  discussed  subsequently  in  Part  II.  Here  we 
are  concerned  more  particularly  with  the  nature  of  the  material 
expended  in  the  production  of  work,  and  as  a  matter  of  convenience 
we  may,  as  in  the  two  preceding  chapters,  take  up  first  the  effect 
upon  the  proteid  metabolism  and  second  that  upon  the  metabolism 
of  the  non-nitrogenous  substances. 

Effects  upon  Proteid  Metabolism. 

Earlier  Investigations. — Since  the  muscles,  which  are  the 
instruments  by  means  of  which  work  is  produced,  are  composed 
essentially  of  proteid  material,  it  was  natural  to  regard  the  proteids 
as  the  source  of  muscular  power  and  to  assume  that  the  energy 
developed  during  work  was  supplied  by  an  increased  metabolism 
of  these  substances.  This  view  was  supported  by  the  authority  of 
Liebig,  who,  however,  does  not  appear  to  have  based  it  upon  any 
actual  experiments,  and  it  was  quite  generally,  although  not 
universally,  accepted. 

C.  Voit  *  appears  to  have  been  the  first  to  subject  this  idea  to 
investigation.  His  experiments  were  made  upon  a  dog  weighing 
about  32  kilograms.  The  work  performed,  by  running  in  a  tread- 
mill, was  considerable,  being  estimated  to  average  1.7  kgm.  per 
second  for  the  whole  twenty-four  hours.  Experiments  were  made 
*  Untersuchungen  liber  den  Einfluss  des  Kochsalzes,  des  Wassers,  und 
der  Muskelbewegungen  auf  den  Stoffwechsel.  1860.  Compare  the  summary 
by  E.  v.  Wolff  in  Die  Ernahrung  der  landw.  Nutzthiere,  pp.  386-388. 


INFLUENCE  OF  MUSCULAR  EXERTION  UPON  METABOLISM-   195 

both  during  fasting  and  with  a  daily  ration  of  1500  grams  of  lean 
meat.     The  results  obtained  were  as  follows: 


Number  of 
Experiment. 

Meat 

Eaten, 
Grms. 

Water 
Drunk 
Grms. 

Urine 

Excreted, 

Grms. 

Urea 

Excreted, 

Grms. 

I 

•I 
'1 

1500J 
1500  -j 

Rest 

Work 

Rest 

Work 

Rest 

Rest 

Work 

Rest 

Work 

Rest 

258 
872 
123 
527 
125 
182 
657 
140 
412 
63 

186 

518 

145 

186 

143 

1060 

1330 

1081 

1164 

1040 

14.3 
16.6 
11.9 
12.3 

II 

Ill 

10.9 
109.8 
117.2 

IV 

109.9 
114.1 
110.6 

The  average  increase  of  the  proteid  metabolism,  as  measured 
by  the  urea  excreted,  was  in  the  fasting  experiments  11.8  per  cent, 
and  in  the  experiments  with  food  4.95  per  cent.  The  absolute 
difference  in  grams,  however,  was  materially  less  in  the  fasting 
experiments,  although  approximately  the  same  amount  of  work 
was  performed  in  both  cases.  A  similar  experiment  upon  an 
older  and  quite  fat  dog  while  fasting  showed  an  increase  of  only 
6  per  cent,  in  the  proteid  metabolism. 

Subsequently  Pettenkofer  &  Voit  *  made  similar  experiments 
upon  a  man,  the  work  consisting  in  turning  a  heavy  wheel  provided 
with  a  brake.  The  work  was  performed  in  the  respiration  appara- 
tus. The  results  showed  a  large  increase  in  the  carbon  dioxide 
excreted,  but  scarcely  any  effect  was  noted  upon  the  excretion  of 
nitrogen,  as  will  be  seen  from  the  following  table: 


Nitrogen 

of  Urine, 

Grms. 

Carbon 

Dioxide 

Excreted, 

Grms. 

Water  Excreted. 

Oxygen 

Taken  Up. 

Grms. 

In 

Urine, 
Grms. 

Evapo- 
rated, 
Grms. 

of 
Experi- 
ments. 

Fasting  : 

Rest 

12.4 
12.3 

17.0 
17.3 

716 
1187 

928 
1209 

1006 
746 

1218 
1155 

821 
1777 

931 

1727 

762 
1072 

832 
981 

2 

Work 

1 

Average  diet : 
Rest 

3 

Work 

2 

'96  PRINCIPLES  OF  ANIMAL   NUTRITION. 

Pettenkofer  &  Voit  regard  the  slight  increase  in  the  proteid 
metabolism  which  they  observed  in  most  cases  as  a  secondary  effect 
of  muscular  exertion.  They  have  shown,  as  we  have  seen,  that 
when  the  cells  of  the  body  are  abundantly  supplied  with  non-nitroge- 
nous nutrients,  either  in  the  form  of  food  or  of  body  fat,  there  is  a 
tendency  to  diminish  the  proteid  metabolism.  In  work,  on  the  con- 
trary, large  amounts  of  non-nitrogenous  material  are  oxidized,  as 
their  respiration  experiments  show.  The  supply  of  these  nutrients 
to  the  cells  is  thus  diminished,  and  it  is  to  this  that  they  attribute 
the  increase  in  proteid  metabolism. 

Results  like  those  just  given  can  hardly  be  interpreted  other- 
wise than  as  showing  that  the  non-nitrogenous  constituents  of  the 
body  or  of  the  food,  rather  than  the  proteids,  are  the  source  of  the 
energy  expended  in  muscular  work,  but  the  first  attempt  to  com- 
pare the  amount  of  work  performed  with  the  energy  available  from 
the  proteids  metabolized  was  the  famous  experiment  of  Fick  & 
Wislicenus  *  in  1866.  These  observers  made  an  ascent  of  the 
Faulhorn  and  found  that  the  amount  of  proteids  metabolized 
during  and  after  the  ascent,  as  measured  by  the  urea  excreted,  was 
insufficient,  according  to  their  computations,  to  account  for  more 
than  one  third  of  the  energy  required  to  raise  their  bodies  to  the 
height  of  the  mountain,  making  no  allowance  for  the  work  of  the 
internal  organs,  nor  for  those  muscular  exertions  which  did  not 
contribute  directly  to  the  work  done. 

Fick  &  Wislicenus  found  no  considerable  increase  in  the  uri- 
nary nitrogen  in  their  experiment.  Subsequent  investigators, 
among  whom  may  be  mentioned  Parkes,f  Noyes,J  Haughton,§ 
Meissner,||  Schenk,^[  and  Engelmann,**  have  reported  appar- 
ently conflicting  results  regarding  the  influence  of  work  on  the 
proteid  metabolism.  In  some  cases  an  increase  was  observed, 
while  in  other  cases  no  material  effect  was  apparent.  The  increase 
when  observed  was  never  large  except  in  the  experiments  of  Engel- 

*  Vrtljschr.  Naturf. Gesell.  Zurich,  10,  317. 

t  Phil.  Mag.,  4th  ser.,  32,  182. 

J  Amer.  Jour.  Med.  Sci.,  Oct.,  1867. 

§  Brit.  Med.  Jour.,  15,  22. 

||  Virchow's  Jahresber.,  1868,  p.  72. 

H  Centralb.  Med.  Wiss.,  1874.  p.  377. 

**  Archiv  f.  (Anat.  u.)  Physiol.,  1871,  p.  14. 


INFLUENCE   OF  MUSCULAR  EXERTION  UPON  METABOLISM.   197 


mann,  and  was  entirely  insufficient  to  account  for  the  energy  ex- 
pended. Oppenheim  made  the  interesting  observation  that  work 
pushed  to  the  point  of  producing  dyspnoea  caused  a  marked  increase 
in  the  proteid  metabolism. 

Influence  of  Total  Amount  of  Food — Kellner's  Inves- 
tigations.-— Doubtless  the  conflicting  results  of  earlier  experiments 
are  due  in  part  to  defective  technique,  but  they  arise  in  part  also, 
as  it  would  seem,  from  another  cause  to  which  attention  was  first 
called  by  Kellner  in  1879-80.  Kellner's  experiments  were  made 
upon  the  horse.  They  differed  from  most  earlier  experiments,  first, 
in  that  the  comparison  was  made  between  different  amounts  of  work 
instead  of  between  work  and  rest,  and  second,  that  the  individual 
periods  instead  of  covering  only  a  few  days  were  extended  over  two 
or  three  weeks. 

Series  I. — Kellner's  first  series  *  was  made  primarily  for  the 
purpose  of  testing  the  influence  of  work  upon  the  digestibility  of  the 
food,  but  the  total  nitrogen  of  the  urine  was  also  determined.  The 
methods  employed  for  this  purpose  were  somewhat  imperfect,  there 
being  some  mechanical  loss  and  probably  also  a  loss  of  ammonia 
from  the  urine,  but  the  author  believes  the  results  of  the  several 
periods  to  be  fairly  comparable.  The  amount  of  work  performed 
was  measured  by  a  dynamometer.  The  numerical  results  of  the 
measurement  have  since  been  shown  to  be  too  high,  but  the  relative 
amount  in  the  several  periods  is  not  thought  to  be  materially 
affected  by  this  error.  The  results  of  the  several  periods  are  briefly 
summarized  in  the  following  table : 


Work, 
Kgm. 

Nitrogen. 

Live  Weight 

Digested, 
Grms. 

In  Urine, 
Grms. 

of  Period, 
Kgs. 

I 

II 

Ill 

IV 

625,000 
1,250,000 
1,875,000 
1,100,000 

625,000 

134.41 
128.32 
132.72 
126.40 
129.41 

99.0 
109.3 
116.8 
110.2 

98.3 

534.1 
529.5 
522.5 
508.8 
518.0 

V 

While  the  above  figures  show  a  considerable  nitrogen  deficit, 
the  urinary  nitrogen  increased  and  decreased  with  the  amount  of 
*  Landw.  Jahrb.,  8,  701. 


198  PRINCIPLES  OF  ANIMAL   NUTRITION. 

work  performed  in  a  manner  which  can  scarcely  be  explained  other- 
wise than  as  a  result  of  the  changes  in  the  latter.  The  ration  con- 
sumed was  amply  sufficient  for  the  light  work  of  the  first  and  fifth 
periods.  When,  however,  more  work  was  demanded  from  the 
animal,  the  live  weight  promptly  fell  off,  showing  that  the  total 
ration  was  insufficient.  This  insufficiency  of  the  total  ration  Kellner 
believes  to  be  the  cause  of  the  increase  in  the  proteid  metabolism. 

A  consideration  of  the  daily  results  confirms  this  view.  In 
passing  from  periods  of  lighter  to  those  of  heavier  work  the  increase 
followed  promptly  upon  the  change.  In  Period  III,  with  the  most 
severe  work,  the  proteid  metabolism  continued  to  increase  through- 
out the  period  and  apparently  had  not  reached  its  limit  at  the 
close.  Conversely,  when  the  work  was  diminished  in  Periods  IV 
and  V  it  decreased  as  promptly  as  it  had  increased.  Finally,  it 
should  be  noted  that  the  additional  amount  of  proteids  metab- 
olized was  entirely  insufficient  to  furnish  an  amount  of  energy 
equivalent  to  the  increase  in  the  work. 

In  four  succeeding  series  of  experiments  Kellner  *  has  investi- 
gated this  phenomenon  more  fully,  some  of  the  sources  of  error  noted 
above  having  been  avoided  in  the  later  researches.  The  results,  as 
will  appear,  still  show  a  deficit  of  nitrogen.  Kellner  estimates  that 
about  6  grams  of  nitrogen  per  day  were  required  for  the  growth  of 
hoofs,  hair,  epidermis,  etc.,  and  believes  that  there  was  some  loss  of 
urinary  nitrogen  mechanically  and  chemically. 

Series  II. — In  this  series  of  experiments  the  ration,  consisting 
of  7.5  kilograms  of  hay  and  4  kilograms  of  beans,  was  purposely 
made  rich  in  protein.  In  spite  of  this  liberal  supply  of  protein, 
however,  the  same  result  as  in  the  first  experiment  was  noted  to  an 
even  more  marked  extent.  As  in  the  first  series,  too,  the  increase 
in  the  excretion  of  nitrogen  promptly  disappeared  when  the  amount 
of  work  was  diminished. 

Series  III. — In  this  series  the  animal  was  brought  as  nearly  as 
possible  into  equilibrium  with  his  food  upon  rather  light  work.  The 
work  was  then  trebled,  while  at  the  same  time  an  addition  was 
made  to  the  non-nitrogenous  ingredients  of  the  ration  by  substitut- 
ing for  a  portion  of  the  beans  an  amount  of  oats  containing  the  same 
absolute  quantity  of  protein.  In  this  second  period  there  was  a 
slight  increase  in  the  digestibility  of  the  protein  and,  therefore,  a 
*  Landw.  Jahrb.,  9,  651. 


INFLUENCE   OF  MUSCULAR  EXERTION   UPON  METABOLISM.    199 


corresponding  increase  in  the  urinary  nitrogen  (compare  Chapter 
V),  but  this  was  small  compared  with  the  much  greater  amount  of 
work  performed.  Moreover,  it  did  not,  as  in  the  first  series  of 
experiments,  augment  from  day  to  day  during  the  period  of  severe 
work.  The  following  table  shows  the  principal  results  of  this 
series,  the  figures  for  urinary  nitrogen  and  for  live  weight  being 
given  for  the  first  and  second  halves  of  each  period: 


Work, 
Kgm. 

Nitrogen. 

Live 

Period. 

Digested, 
Grms. 

In  Urine, 
Grms. 

Weight, 
Kg. 

I 

810,000 

2,430,000 

810,000 

173.6 
178.8 

178.8 

j    158.9 
I    164.1 
]    174.0 
\    174.8 
j    166.4 
I    171.4 

560.3 

II 

556.8 
541.3 

Ill 

539.7 
542.5 

543.3 

Series  IV. — Upon  the  basis  of  the  foregoing  facts  Kellner  deter- 
mined the  maximum  amount  of  work  which  his  horse  could  perform 
on  a  fixed  medium  ration  without  causing  an  increase  in  the  proteid 
metabolism.  One  kilogram  of  starch  was  then  added  to  the  ration 
and  the  maximum  amount  of  work  that  could  be  performed  upon 
this  new  ration  without  causing  such  an  increase  was  determined. 
In  the  nature  of  the  case  this  determination  could  not  be  of  the 
highest  accuracy,  but  it  is  amply  sufficient  for  our  present  purpose. 
The  principal  results  are  given  in  the  following  table,  the  amount 
of  work  being  expressed  by  the  number  of  revolutions  of  the  dyna- 
mometer, since  relative  results  are  all  that  are  required: 


Work, 
Rev. 

Nitrogen. 

Live 

Digested, 
Grms. 

In  Urine, 
Grms. 

Weight, 
Kg. 

I 

1      Without      1 
starch 

)        With         ) 
S        starch        f 

300 
600 
600 
500 
400 

800 
600 

>       121.1 

J-       120.1       | 

107  2          540  0 

Ha 

110  2          538  3 

116 

115  6          533  1 

Ill 

109  4     J     532.5 

IV 

109  6     |     530  7 

I 

115  5          517  1 

II 

109  6          filfi  4 

PRINCIPLES   OF  ANIMAL   NUTRITION. 


Kellner  estimates  that  the  maximum  amount  of  work  which 
could  be  performed  on  the  ration  containing  starch  was  700  rev- 
olutions as  compared  with  a  maximum  of  500  revolutions  without 
starch.  Even  if  this  estimate  of  Kellner' s  be  regarded  as  high,  it  is 
evident  from  the  figures  given  that  the  addition  of  the  starch  enabled 
materially  more  work  to  be  performed  without  an  increase  in  the 
proteid  metabolism.  The  results  obtained  in  this  and  the  subse- 
quent series  have  been  made  the  basis  of  interesting  computations 
regarding  the  utilization  of  the  potential  energy  of  the  food  which 
will  be  considered  in  Part  II. 

Series  V. — This  series  was  precisely  similar  to  the  preceding  one, 
except  that  the  addition  of  non-nitrogenous  matter  to  the  ration  was 
made  in  the  form  of  oil  by  substituting  flaxseed  for  linseed  meal. 
The  protein  of  the  ration  remained  unchanged,  while  the  fat  was 
increased  by  203  grams.  The  results  were  entirely  similar  to  those 
with  starch,  as  the  following  table  shows: 


Work, 
Rev.  • 

Nitrogen. 

Live 

Period. 

Digested, 
Grms. 

In  Urine, 
Grms. 

Kg. 

la 

Without      [ 
)■      addition     < 
of  fat 

)        With         [ 
)■    addition      -I 
i        of  fat         ! 

500 
500 
550 
550 

700 
700 
650 
650 

]                           f 
>■      159.0      -J 

[      153.9      < 

148.9 
149.2 
147.5 
153.0 

148.1 
153.9 
145.6 
145.0 

496.5 

16 

493.2 

Ha 

485.8 

116 

479.4 

la 

476.0 

16 

469.0 

Ha 

466.4 

116 

460.8 

While  Kellner's  method  of  investigation  may  be  regarded  as 
somewhat  imperfect  and  necessarily  giving  bat  approximate  results, 
yet  it  suffices  to  bring  out  in  a  very  striking  manner  the  intimate 
relation  existing  between  the  supply  of  non-nitrogenous  nutrients 
in  the  food  of  a  working  animal  and  the  effect  of  the  work  upon  the 
proteid  metabolism.  In  conclusion,  it  should  be  noted  that  in 
all  Kellner's  experiments  there  was  a  fairly  abundant  supply  of 
protein.  Whether  the  same  result  would  be  obtained  on  a  ration 
containing  the  minimum  amount  of  proteids  required  by  the  organ- 
ism is  not  shown.  In  no  case  was  the  increase  in  the  proteid  metab- 
olism, when  observed,  sufficient  to  supply  energy  equivalent  to  the 
additional  work  done. 


INFLUENCE  OF  MUSCULAR  EXERTION  UPON  METABOLISM.    201 

Later  Investigations. — In  1882  North  *  made  experiments 
upon  himself  in  which  a  considerable  amount  of  work,  mainly  walk- 
ing from  30  to  47  miles  while  carrying  a  load  of  about  27  pounds, 
was  performed  on  one  day  of  each  experiment.  The  account  of  the 
experiments  does  not  give  sufficient  data  for  computing  the  total 
amount  of  work  performed,  but  it  was  evidently  very  considerable 
and  resulted  in  a  marked  increase  in  the  excretion  of  nitrogen.  It 
is  not  possible,  however,  to  determine  whether  the  total  food  was 
adequate  for  the  work  days,  but  it  was  no  greater  then  than  during 
the  periods  of  rest. 

Argutinsky,f  in  experiments  upon  himself,  observed  as  a  result 
of  rather  severe  work  a  very  marked  increase  in  urinary  nitrogen 
which  continued  at  least  three  days  after  the  cessation  of  the  work. 
Munk  X  subsequently  criticised  Argutinsky's  results  on  the  ground 
that  the  supply  of  non-nitrogenous  nutrients  in  his  diet  was  insuffi- 
cient. Krummacher  §  obtained  results  quite  similar  to  those  of 
Argutinsky,  but  his  experiments  are  open  to  the  same  criticism  as 
those  of  his  predecessor,  namely,  an  insufficient  supply  of  non- 
nitrogenous  nutrients,  as  he  himself  points  out  in  a  later  paper. 
Hirschfeldt  |  failed  to  observe  any  material  increase  in  the  nitrogen 
excretion  as  the  result  of  work  upon  a  diet  containing  a  considera- 
ble excess  of  food  over  the  amount  required  for  maintenance.  This 
was  true  both  upon  a  diet  containing  little  protein  and  one  abun- 
dantly supplied  with  this  nutrient. 

Pniiger,  like  Liebig,  regards  protein  as  the  sole  source  of  mus- 
cular energy.  As  yet  only  a  preliminary  sketch  of  his  investiga- 
tions has  been  published.^  He  fed  a  lean  dog  upon  prepared  lean 
meat,  that  is,  upon  a  nearly  pure  proteid  diet,  for  seven  months. 
The  animal  remained  apparently  in  perfect  health  and  was  able  to 
perform  a  large  amount  of  work.  Under  the  influence  of  the  work 
the  excretion  of  nitrogen  was  observed  to  increase  somewhat,  but 
not  sufficiently  to  account  for  the  energy  expended  in  the  work. 
This  phenomenon  Pfliiger  explains  by  supposing  that  during  work 

*  Proc.  Roy.  Soc,  36,  14. 

f  Arch.  ges.  Physiol.,  46,  552. 

t  Arch.  f.  (Anat.  u.)  Physiol.,  1890,  p.  552. 

§  Arch.  ges.  Physiol.,  47,  454. 

||  Virchow's  Archiv.,  121,  501. 

t  Arch.  ges.  Physiol.,  50,  98. 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


the  organism  economizes  in  its  demands  for  proteids  elsewhere  than 
in  the  muscles.  The  further  interesting  observation  was  made  that 
with  continuous  work  the  proteid  metabolism,  which  at  first  showed 
an  increase,  diminished  again  and  even  reached  its  original  value. 
With  a  ration  containing  but  little  protein  and  much  non-nitrogenous 
material,  a  small  increase  of  the  proteid  metabolism  was  observed 
as  the  result  of  work.  The  preliminary  account  of  the  experi- 
ments affords  no  adequate  data  for  computing  the  sufficiency  of 
the  total  food. 

Krummacher,*  in  his  second  investigation,  made  three  separate 
experiments.  In  the  first  of  these  the  total  food  was  estimated  to 
be  approximately  sufficient  for  maintenance  (38  Cals.  per  kilogram), 
while  in  the  other  two  it  was  much  in  excess  of  this.  The  following 
table  shows  the  total  amount  of  food  per  kilogram,  expressed  as 
Calories  of  metabolizable  energy,f  the  amount  of  work  performed, 
and  the  percentage  increase  of  the  proteid  metabolism: 


Energy  of  Food. 

Work 

Measured, 

Kgm. 

Increase 

Total. 
Cals. 

Per  Kg. 

Weight.  Cals. 

Metabolism, 
Per  Cent. 

2459 
5034 
5701 

38 
64 

72 

153,070 
324,540 
401,965 

21 

p»      ii.::::::::::: 

22 

in 

7 

The  work  done  consisted  in  turning  an  ergostat.  It  has  been 
shown  by  subsequent  investigators  that  not  over  30  per  cent,  of 
the  energy  of  the  body  material  metabolized  in  the  performance  of 
work  in  this  way  can  be  recovered  in  the  work  actually  done. 
Assuming  this  high  figure,  and  further  that  Krummacher's  esti- 
mate of  the  maintenance  requirements  is  accurate,  it  appears  that 
the  food  in  these  experiments  was  insufficient  to  supply  the  energy 
required  for  the  amount  of  work  actually  done. 

It  was  observed,  as  in  other  experiments  of  this  nature,  that  the 
increased  excretion  of  nitrogen  continued  for  a  day  or  two  after  the 
cessation  of  the  work.  Only  in  the  first  experiment,  however,  was 
even  the  total  proteid  metabolism  during  the  periods  of  work,  to- 
gether with  the  excess  above  the  rest  value  observed  on  succeeding 
*  Zeit.  f.  Biol.,  33,  108.  t  See  Chapter  X. 


INFLUENCE  OF  MUSCULAR  EXERTION  UPON  METABOLISM.    203 

days,  sufficient  to  supply  an  amount  of  energy  equal  to  that 
actually  measured  on  the  ergostat,  so  that  at  least  the  larger  share 
of  the  energy  must  have  been  derived  from  non  -  nitrogenous 
materials. 

Zuntz  &  Schumburg,*  in  investigations  upon  soldiers,  observed 
an  increase  of  the  proteid  metabolism  as  the  result  of  marching, 
carrying  a  considerable  weight.  The  increase,  however,  seemed  to 
bear  no  direct  relation  to  the  amount  of  work  performed,  but  rather 
to  the  conditions  under  which  it  was  done.  Thus  excessive  heat  or 
sultriness  of  the  atmosphere,  resulting  in  unusual  fatigue,  was  ac- 
companied by  an  increased  excretion  of  nitrogen.  The  increase 
continued  during  the  two  days  following  the  work. 

Frentzel  f  experimented  upon  dogs.  In  the  first  series  the  ani- 
mals were  fed  pure  fat,  while  in  the  second  series  no  food  was  given. 
The  work,  which  was  done  upon  a  tread  power,  was  considerable. 
In  the  first  series  there  was  an  increase  of  9.25  per  cent,  in  the  nitro- 
gen excretion  in  the  work  experiments,  while  in  the  second  series  a 
maximum  increase  of  44.26  per  cent.- was  computed,  which,  how- 
ever, is  believed  by  the  author  to  be  too  high.  A  method  of  com- 
putation which  he  considers  more  nearly  correct  makes  the  increase 
in  the  second  period  13.31  per  cent.  In  the  first  series  of  experi- 
ments the  food  consisted  of  150  grams  of  fat  per  day  except  upon 
one  of  the  work  days,  when  only  80  grams  were  consumed.  No  data 
are  given  regarding  the  sufficiency  of  this  ration,  but  according  to 
E.  Voit's  compilation  \  it  would  appear  hardly  adequate  for  the 
maintenance  of  a  dog  of  the  weight  used  (36  kilograms).  The  work, 
therefore,  even  in  the  first  series,  was  probably  done  upon  insuf- 
ficient food.  In  neither  case  was  the  increase  in  the  amount  of 
protein  metabolized  equivalent  in  energy  content  to  the  actual 
amount  of  external  work  done,  and  in  the  first  series  even  the  total 
proteid  metabolism  was  not,  while  if  we  allow  for  the  consumption 
of  energy  in  internal  work,  heat  production,  etc.,  it  was  not  suf- 
ficient in  either  series. 

Atwater  &  Sherman  §  have  reported  observations  upon  the 

*  Arch.  f.  (Anat.  u.)  Physiol.,  1895,  p.  378. 

t  Arch.  ges.  Physiol.,  68,  212. 

t  Zeit.  f.  Biol.,  41,  115 

§  U.  S.  Dept.  Agr.,  Office  of  Experiment  Stations,  Bull.  98. 


204  PRINCIPLES   OF  ANIMAL   NUTRITION. 

food  consumption,  digestion,  and  metabolism  of  three  bicyclers 
during  a  six-day  contest.  They  find  that,  in  spite  of  an  apparently 
liberal  diet  containing  large  amounts  of  protein,  all  three  riders 
lost  considerable  proteid  tissue  during  the  race.  The  conditions  of 
the  investigation  were  not  such  as  to  permit  of  a  determination 
of  the  sufficiency  of  the  food  consumed,  but  the  computations  by 
Carpenter  of  the  actual  amount  of  work  done  seem  to  render  it  very 
probable  that  the  body  fat  must  have  been  drawn  upon  to  a  con- 
siderable extent. 

Recapitulation. — The  investigations  above  cited  seem  to  show 
beyond  a  doubt  that  when  work  is  performed  upon  food  less  than 
sufficient  to  maintain  the  body  and  supply  the  amount  of  energy 
required  for  the  work  the  proteid  metabolism  is  somewhat  increased. 

Whether  the  converse  of  this  is  true,  namely,  that  when  the 
food  is  sufficient  such  an  increase  in  the  proteid  metabolism  does 
not  occur,  is  not  so  clear,  for  the  reason  that  in  most,  if  not  all,  of 
the  cases  we  have  no  adequate  data  as  to  the  sufficiency  of  the 
food.  It  is  plain,  however,  that  the  question  is  not  so  easily  inves- 
tigated as  might  appear  at  first  sight,  and  that  the  final  solution  of 
the  relations  of  work  to  proteid  metabolism  can  only  be  reached  by 
means  of  investigations  in  which  the  total  metabolism  both  of  matter 
and  energy  is  determined. 

Gain  of  Proteids  during  Work. — Caspari  and  Bornstein  have 
recently  made  further  investigations  into  the  possibility  of  a  gain 
of  protein  as  a  result  of  work  which  was  mentioned  above  in  con- 
nection with  Pfliiger's  experiments. 

Caspari  *  experimented  upon  a  dog  which  received  an  amount 
of  food  computed  to  have  been  fully  sufficient  for  its  maintenance 
and  to  supply  energy  for  the  work  done.  Furthermore,  a  consider- 
able portion  of  the  non-nitrogenous  nutrients  of  the  ration,  consist- 
ing largely  of  carbohydrates,  was  given  shortly  before  the  work  was 
done,  while  in  some  cases  additional  sugar  or  fat  was  given  at  that 
time.  In  the  first  experiment,  work  was  performed  upon  three 
successive  days.  Upon  the  second  of  these  there  was  a  consider- 
able increase  of  the  urinary  nitrogen,  but  upon  the  third  its  amount 
fell  below  that  of  the  rest  period.  The  average  for  the  three  days 
of  work  was  almost  exactly  equal  to  the  value  found  for  the  last 
*  Arch.  ges.  Physiol.,  83,  509. 


INFLUENCE   OF  MUSCULAR  EXERTION   UPON  METABOLISM.   205 


day  of  the  rest  period  and  less  than  the  average  for  the  four  pre- 
vious rest  days. 

In  the  second  experiment  the  work  was  continued  for  four  days, 
then  a  rest  day  intervened,  and  then  the  work  was  continued  for 
five  more  days.  At  the  outset  there  was  a  slight  increase  of  the 
proteid  metabolism,  but  in  the  second  period  of  five  days  it  showed 
a  marked  decrease  resulting  in  a  progressive  gain  of  nitrogen  by 
the  body,  as  is  shown  in  the  following  tabular  statement  of  the  daily 
average  results: 


Day. 

External 
Work 
Done, 
Cals. 

Nitrogen 
of  Food, 
Grms. 

Average 

Nitrogen 

of  Feces, 

Grms. 

Nitrogen 

of  Urine, 

Grms. 

Gain  or 

Loss  of 

Nitrogen, 

Grms. 

1-   5 

5-   7 

0 

0 

0 

597 

467 

597 

596 

0 

595 

590 

593 

588 

586 

25.11 
25.11 
25.11 
25.11 
25.11 
25.11 
25.11 
25.11 
25.11 
25.11 
25.11 
25.11 
25.11 

1.89 

1.89 
1.89 
1.78 
1.78 
1.78 
1.78 
1.78 
1.78 
1.78 
1.78 
1.78 
1.78 

23.68 

22.00 

21.98 

24.72  (?) 

23.32 

23.23 

21.83 

22.06 

20.82 

19.64 

20.39 

19.87 

19.79 

-0.46 
+  1.22 

7-  8 

+  1.24 

8-9 

9-10 

-1.39 

+  0.01 

10-11.... 

+  0.10 

11-12.... 

+  1.50 

12-13.... 

+  1.27 

13-14... . 

+  2  51 

14-15. . . 

+  3  69 

15-16 

+  2.94 

16-17 

+  3.46 

17-18 

+  3.54 

This  gain  Caspari  ascribes  to  an  actual  growth  of  the  muscles  as 
the  effect  of  exercise,  this  growth  according  to  him  taking  the  form 
of  a  hypertrophy  of  the  fibers.  No  determinations  of  the  gain  or 
loss  of  carbon  were  made. 

Bornstein,*  who  had  previously  investigated  the  possibility  of 
increasing  the  store  of  proteids  in  the  body  by  the  addition  of  pro- 
teids  to  the  food,  has  also  contributed  to  the  investigation  of  this 
phase  of  the  question.  His  experiments  were  made  upon  himself. 
For  seven  days  he  consumed  a  uniform  ration  containing  a  moder- 
ate amount  of  protein  and  sufficient  non-nitrogenous  nutrients, 
according  to  previous  experience,  to  maintain  his  body.  The  latter 
was  in  equilibrium  with  the  food  as  regards  nitrogen  from  the  first 
day.  Then  the  proteid  supply  was  increased  by  approximately 
50  per  cent,  by  the  ingestion  of  pure  proteids  and  light  work  (17,000 
*  Arch.  ges.  Physiol.,  83,  540. 


206  PRINCIPLES   OF  ANIMAL    NUTRITION. 

kgm.  per  day)  done  by  turning  an  ergostat.  As  a  result  of  the  in- 
creased supply  of  proteids  in  the  food  the  proteid  metabolism  in- 
creased promptly,  reaching  its  maximum  upon  the  fifth  day,  when 
it  very  slightly  exceeded  the  supply.  From  that  time,  however, 
it  decreased  gradually  during  the  remaining  thirteen  days  of  the 
experiment,  so  that  a  gain  of  proteids  by  the  body  resulted,  which 
was  still  in  active  progress  when  the  experiment  was  discontinued. 
Counting  from  the  time  when  the  proteid  metabolism  reached  its 
maximum  the  average  gain  of  nitrogen  per  day  was 

First  five  days 1 .  28  grams 

Last  five  days 2 .  06      " 

Average  of  all 1 .  475     " 

The  author  computes  that  22  per  cent,  of  the  proteids  added  to 
the  food  was  stored  up  in  the  body.  In  a  previous  similar  experi- 
ment without  work  it  was  found  that  only  16  per  cent,  was  thus 
stored. 

Two  respiration  experiments  with  the  Zuntz  apparatus  were 
made  during  the  work.  The  difference  between  their  results  and 
those  of  similar  experiments  during  rest  was  used  as  the  basis  for 
computing  the  actual  amount  of  energy  metabolized  in  the  body  for 
the  performance  of  work.  This  was  found  to  be  equal  to  0.0100875 
Cal.  per  kgm.  external  work,  which  is  equivalent  to  171.5  Cals.  for 
the  whole  daily  work  of  17,000  kgm.  Assuming  the  original  ration 
to  have  been  a  maintenance  ration,  Bornstein  computes  that  the 
portion  of  the  added  proteids  which  was  actually  metabolized  was 
insufficient  to  supply  the  energy  necessary  for  the  work  done  and 
that  some  of  the  fat  of  the  body  was  drawn  upon.  The  loss  in 
live  weight  was  found  to  agree  with  this  assumption. 

The  above  investigations  seem  to  show,  not  only  that  work  may 
be  done  without  increasing  the  proteid  metabolism  but  that  it  may 
actually  result  in  diminishing  it,  a  fact  which  appears  in  harmony 
with  the  common  observation  that  the  tendency  of  exercise  is  to 
build  up  the  muscular  tissue. 

Summary. — While  the  results  which  have  been  cited  are  not  in 
all  respects  conclusive,  and  while  further  investigation  is  required 
to  fully  elucidate  the  relations  of  muscular  exertion  to  proteid  metab- 
olism, the  following  general  conclusions  seem  to  be  justified  by  the 
evidence  now  available: 


INFLUENCE   OF  MUSCULAR   EXERTION   UPON  METABOLISM.    207 

1 .  The  non-nitrogenous  ingredients  of  the  food  or  of  the  tissues 
are  the  chief  source  of  muscular  energy.  In  by  far  the  greater 
number,  if  not  all,  of  the  experiments  upon  this  subject  the  amount 
of  protein  metabolized,  as  measured  by  the  nitrogen  excretion,  was 
insufficient  to  furnish  energy  equivalent  to  the  work  done,  the  de- 
ficiency being  in  many  cases  very  great.  This  statement,  it  will  be 
observed,  does  not  assert  that  the  proteids  are  not  concerned  in  the 
production  of  this  energy.  We  may  regard  it  as  very  probable  that 
the  non-nitrogenous  matter  metabolized  has  first  entered  into  the 
structure  of  the  muscular  protoplasm,  which,  as  we  know,  consists  • 
largely  of  proteids,  but  in  a  contraction  it  is  largely,  if  not  wholly, 
the  non-nitrogenous  groups  contained  in  the  protoplasm  which  are 
metabolized  rather  than  the  nitrogenous  groups. 

2.  With  insufficient  food  there  may  be  a  considerable  increase 
in  the  proteid  metabolism  as  a  result  of  muscular  exertion,  espe- 
cially when  pushed  to  exhaustion. 

3.  This  increase  is  far  from  sufficient  to  supply  energy  for  the 
work  actually  done,  is  not  usually  proportional  to  it,  and  seems 
dependent  to  a  considerable  degree  upon  accompanying  conditions. 

4.  With  sufficient  food  the  increase  of  the  total  proteid  metab- 
olism consequent  upon  muscular  exertion  is  at  the  most  slight  and 
possibly  equal  to  zero. 

5.  In  some  cases  a  storage  of  proteids  has  been  observed  to 
result  from  the  performance  of  work. 

Functions  of  Proteids. — If  the  above  conclusions  are  admitted, 
it  is  possible  to  suppose  that  in  a  muscular  contraction  under  favor- 
able conditions — that  is,  when  there  is  an  abundant  supply  of  non- 
nitrogenous  material — there  is  no  increased  metabolism  of  the 
proteids.  This  view  of  the  subject  would  regard  the  question  as 
being  simply  one  of  the  relative  supply  of  nutrients,  the  energy 
being  evolved  from  non-nitrogenous  nutrients  when  these  are  in 
abundance,  while  in  default  of  them  the  proteids  are  drawn  upon. 

Another  view  of  the  subject,  however,  is  possible,  and  perhaps 
more  probable.  It  would  appear  that  muscular  exertion  tends  to 
produce  two  opposite  effects  upon  the  proteid  metabolism:  first, 
to  break  down  additional  protein,  as  is  shown  when  work  is  done 
upon  insufficient  food;  and  second,  to  build  up  proteid  tissue  when 


208  PRINCIPLES  OF  ANIMAL   NUTRITION. 

the  food  is  sufficient,  as  is  illustrated  in  the  experiments  of  Caspari 
and  Bornstein. 

As  a  basis  for  a  tentative  hypothesis,  it  seems  allowable  to  sup- 
pose that  both  these  processes — that  of  anabolism  and  katabolism 
of  proteids — are  continually  taking  place  in  the  muscle  and  that 
both  are  exaggerated  by  exercise.  In  other  words,  we  majr  imagine 
that  the  performance  of  work  by  a  normally  developed  muscle 
requires  an  increased  proteid  katabolism,  which  is  balanced,  at  least 
in  the  course  of  the  twenty-four  hours,  by  a  corresponding  increase 
in  the  proteid  anabolism.  With  a  liberal  supply  of  food  proteids, 
then,  a  part  of  the  latter  would,  during  rest,  simply  undergo  nitro- 
gen cleavage  and  be  used  virtually  as  "fuel,"  but  when  work  was 
done  they  (or  part  of  them)  would  be  used  to  replace  the  proteids 
katabolized  in  the  muscles.  Upon  this  hypothesis,  the  proteids 
might  play  a  not  unimportant  part  in  the  production  of  muscular 
work  without  any  evidence  of  it  appearing  in  an  increased  nitrogen 
excretion.  It  is  to  be  remarked,  however,  that  even  on  this  suppo- 
sition the  proteids  could  not  be  regarded  as  furnishing  all,  or  even, 
in  many  cases,  a  large  share,  of  the  energy  liberated.  On  insuffi- 
cient food,  the  hypothesis  would  assume  that  the  energy  supply  is 
deficient  and  that  proteids  which  would  otherwise  be  used  for 
muscular  anabolism  are  diverted  to  use  as  "fuel,"  probably  under- 
going a  preliminary  nitrogen  cleavage  and  furnishing  their  non- 
nitrogenous  residue  to  the  muscles  as  a  source  of  energy. 

The  above  tentative  hypothesis  implies  that  if  work  were  per- 
formed upon  a  ration  containing  only  the  minimum  amount  of 
proteids  required  during  rest,  it  would  cause  an  increase  of  the 
proteid  metabolism,  no  matter  how  much  non-nitrogenous  mate- 
rial was  supplied,  because  there  would  be  no  proteids  available 
which  could  be  diverted  to  repair  the  waste  assumed  to  be  occa- 
sioned by  muscular  activity.  Up  to  the  present  time,  however, 
we  possess  no  experimental  investigation  of  this  phase  of  the  ques- 
tion. 

However  this  may  be,  we  know  that  the  performance  of  work 
requires  a  well-developed  muscular  system.  To  produce  and  de- 
velop such  a  system,  a  liberal  supply  of  protein  is  essential,  while 
we  may  reasonably  suppose  that  to  maintain  it  involves  a  larger 
proteid  supply  in  the  food  than  is  required  to  maintain  the  proteid 


INFLUENCE   OF  MUSCULAR  EXERTION  UPON  METABOLISM.   209 

tissue  on  a  lower  level.  This  fact  alone  would  indicate  the  need  of 
a  reasonably  liberal  supply  of  protein  in  the  food  of  working  animals. 
If  the  hypothesis  above  outlined  be  approximately  correct,  it  is 
necessary  that  the  food  also  contain  protein  which  during  rest  may 
be  simply  a  source  of  heat,  but  which  during  work  may  be  diverted 
to  repair  the  increased  waste  of  nitrogenous  tissues  caused  by  ex- 
ertion. This  accords  with  the  well-established  fact  that  the  dieta- 
ries selected  by  athletes  and  others  who  undertake  severe  physical 
exertion  are  almost  invariably  rich  in  protein.*  It  is  of  course 
difficult  to  say  how  far  the  large  amount  of  proteids  in  the  dietaries 
of  athletes  represents  a  real  physiological  demand  and  how  far  it  is 
a  matter  of  tradition  or  of  taste,  but  it  hardly  seems  likely  that  so 
universal  an  opinion  should  be  lacking  in  some  considerable  basis 
of  fact. 

Effects  upon  the  Carbon  Metabolism. 

In  the  foregoing  paragraphs  we  have  seen  that  as  a  rule  the 
total  proteid  metabolism  is  not  much  affected  by  muscular  exertion. 
While  proteids  undoubtedly  have  important  functions  in  connection 
with  the  production  of  work,  it  is  nevertheless  true  that  normally 
the  energy  liberated  in  muscular  contraction  is  derived  chiefly  or 
wholly  from  the  breaking  down  of  non-nitrogenous  material. 
Moreover,  even  in  those  cases  in  which  a  considerable  increase  of 
the  proteid  metabolism  has  been  observed,  its  amount  has  been 
entirely  insufficient  to  account  for  the  extra  evolution  of  energy. 
It  therefore  becomes  of  especial  importance  to  consider  the  effects 
of  work  upon  the  carbon  balance. 

The  Gaseous  Exchange. — Since  the  influence  of  muscular  ex- 
ertion upon  the  proteid  metabolism  is  at  most  small,  it  is  possible  to 
compare  the  carbon  metabolism  during  work  and  rest  without 
material  error  upon  the  basis  of  the  gaseous  exchange  simply,  and 
as  a  matter  of  fact  a  large  share  of  our  knowledge  of  the  subject 
rests  upon  determinations  of  the  respiratory  exchange. 

Is  Largely  Increased. — The  fact  that  muscular  work  largely 
increases  the  evolution  of  carbon  dioxide  and  water  and  the  con- 
sumption of  oxygen  by  the  organism  is  too  familiar  from  ordinary 

*  For  a  summary  of  American  experiments  bearing  upon  this  point  see 
Atwater  &  Benedict,  Boston  Medical  and  Surgical  Journal,  144,  601  and  629 


210  PRINCIPLES   OF  ANIMAL   NUTRITION. 

experience  and  too  well  established  scientifically  to  require  more 
than  illustration.  The  fact  of  such  an  increase  was  shown  in  the 
researches  of  Lavoisier.  Scharling,*  who  as  early  as  1843  con- 
structed an  apparatus  somewhat  like  the  Pettenkofer  respiration 
apparatus  (see  p.  70),  states  in  his  account  of  his  experiments 
that  moderate  work  increases  the  excretion  of  carbon  dioxide  and 
that  it  is  also  greater  shortly  after  a  meal.  Of  other  early  researches 
upon  this  point  may  be  mentioned  those  of  Hirn  t  in  1857,  and 
especially  those  of  Smith  \  in  1859.  The  investigations  of  Petten- 
kofer &  Voit  §  in  1866  appear  to  have  been  the  first  to  be  executed 
in  accordance  with  modern  methods.  Their  results  have  already 
been  cited  in  their  bearing  upon  the  influence  of  work  on  proteid 
metabolism,  but  may  be  repeated  here: 


Nitrogen 

of  Urine, 

Grms. 

Carbon 

Dioxide 

Excreted, 

Grms. 

Water  Excreted. 

Oxygen 

Taken 

Up. 

Grms. 

Number 
of  Experi- 
ments. 

Tn 
Urine. 
Grms. 

Evapo- 
rated. 
Grms. 

Fasting  : 

Rest 

12.4 
12.3 

17.0 
17.3 

716 
1187 

928 
1209 

1006 
746 

1218 
1155 

821 
1777 

931 
1727 

762 
1072 

832 
981 

2 

Work 

1 

Average  diet: 
Rest 

3 

Work 

2 

Subsequent  investigators  such  as  Speck,  ||  Hanriot  &  Richet,T 
Katzenstein,**  Loewy,jf  and  many  others  have  fully  confirmed  the 
results  of  the  early  experimenters.  The  increase  in  the  oxygen 
taken  up  was  not  actually  demonstrated  in  all  of  these  experiments, 
but  it  was  in  some  and  may  be  reasonably  inferred  in  the  remainder- 

*  Ann   Chem   Pharm  ,  45,  214 

t  Comptes  rend  Soc.  de  Physique  de  Colmar,  1857;  Revue  Scientifique, 
ler  Semestre.  1887. 

J  Phil.  Trans..  1859,  p  681. 

§  Zeit  f  Biol.,  2.  478. 

|]  Schriften  der  Gesell.  der  ges  Naturwiss  zu  Marburg,  1871;  Arch,  klin 
Med  ,  45,  461. 

1  Comptes  rend.,  104,  435  and  1865;  105,  76;  Ann.  de  China,  et  de  Phys., 
(6),  22,  485. 

**  Arch  ges  Physiol.,  49,  330. 

ft  Ibid.,  49,  405. 


INFLUENCE  OF  MUSCULAR  EXERTION  UPON  METABOLISM.   211 


Effects  are  Immediate. — Experiments  like  those  of  Petten- 
kofer  &  Voit,  extending  over  twenty-four  hours,  give  simply  the 
total  effect  of  the  performance  of  work  upon  the  carbon  balance. 
By  the  use  of  the  Zuntz  type  of  apparatus,  however,  it  is  possible 
to  follow  the  gaseous  exchange  in  its  details  through  successive 
short  periods  as  well  as  to  determine  the  amount  of  oxygen  con- 
sumed. The  data  thus  obtained  give  a  clear  picture  of  the  imme- 
diate effects  of  work  upon  metabolism  and  have  led  to  the  extensive 
use  of  this  type  of  apparatus  in  experiments  of  this  nature.  The 
results  of  these  experiments  agree  with  common  experience  in 
showing  that  these  effects  appear  very  promptly  and  soon  reach 
their  maximum,  disappearing  as  promptly  after  the  work  ceases. 
In  other  words,  the  increase  in  the  carbon  metabolism  is  very 
closely  confined  to  the  time  during  which  the  work  is  actually 
performed. 

The  Respiratory  Quotient. — The  ratio  between  carbon  dioxide 
produced  and  oxygen  consumed,  commonly  known  as  the  respira- 
tory quotient,  as  has  been  pointed  out  in  Chapter  III,  enables  us 
to  form  a  fairly  clear  idea  as  to  the  general  nature  of  the  total  mate- 
rial metabolized,  and  hence  much  study  has  been  bestowed  upon 
the  relation  between  these  two  quantities. 

Is  Variable. — We  have  already  seen  that  the  respiratory  quo- 
tient may  vary  considerably  during  repose,  being  largely  deter- 
mined by  the  nature  of  the  food.  The  same  thing  is  true  of  the 
respiratory  quotient  during  work. 

Zuntz,*  in  experiments  on  a  fasting  dog,  obtained  the  follow- 
ing values  for  this  quotient: 


Number  of 
Experi- 
ments. 

Average 

Respiratory 

Quotient. 

2 

6 
8 
5 
10 

0  69 

0  71 

0  73 

Locomotion  up  hill 

0  77 

Horizontal  draft 

0.77 

In  Zuntz  &  Hagemann's  f  experiments  upon  the  horse  the  respi- 
ratory quotient  in  the  single  work  periods  ranged  from  0.729  upon  a 

*  Arch.  ges.  Physiol.,  68,  191. 

t  Landw.  Jahrb.,  27,  Supp.  Ill,  296-331. 


2  12  PRINCIPLES   OF  ANIMAL   NUTRITION. 

ration  of  green  alfalfa  to  0.996  upon  hay,  straw,  and  oats.    The 
averages  obtained  for  different  forms  of  work  were  as  follows : 

Walking,  nearly  horizontal 0 .  865 

"         up  a  slight  incline 0.847 

"  "  "  steeper  incline 0.900 

Draft,  nearly  horizontal 0.890 

Walking  with  load,  horizontal 0 .  840 

"  "        "      up  incline 0.893 

Trot,  nearly  horizontal 0.882 

"      with  load,  nearly  horizontal 0 .  873 

"      horizontal  draft 0.927 

The  total  range  of  the  respiratory  quotient  in  these  experiments 
was  0.84  to  0.93.  It  is  thus  seen  to  be  higher  with  herbivorous 
animals,  subsisting  largely  upon  carbohydrates,  than  with  the  dog. 

Change  Caused  by  Work. — Chauveau  states  as  the  result  of  his 
investigations  upon  the  origin  of  muscular  power  that  the  per- 
formance of  work  always  increases  the  respiratory  quotient. 

His  first  experiments  *  were  made  upon  a  man  who  had  fasted 
for  sixteen  hours.  The  work  consisted  in  the  alternate  ascent  and 
descent  of  a  staircase,  the  work  of  ascending  being  equal  to  about 
29,000  kgms.  in  the  seventy  minutes  of  the  experiment.  Samples 
of  the  expired  air  were  taken  by  the  Tissot  apparatus  f  for  five 
minutes  at  a  time  at  intervals  during  the  work  and  the  respiratory 
quotient  determined  by  a  comparison  of  its  composition  with  that 
of  the  normal  atmosphere.  The  following  were  the  results  for  the 
respiratory  quotient: 

Immediately  before  work 0 .  75 

First  to  fifth  minute 0.84 

Tenth  to  fifteenth  minute 0.87 

Fortieth  to  forty-fifth  minute 0 .  95 

Sixty-fifth  to  seventieth  minute 0. 74 

*Comptes  rend.,  122,  1163. 

t  Archives  de  Physiol.,  1896,  p.  563.     The  apparatus  is  of  the  Zuntz  type. 


INFLUENCE  OF  MUSCULAR   EXERTION  UPON  METABOLISM-   213 

A  second  experiment,*  begun  after  fifteen  hours'  fasting,  was 
divided  into  two  periods.  The  first  was  similar  to  the  previous 
experiment,  but  lasted  for  thirty  minutes  only,  the  work  of  ascent 
equaling  in  that  time  about  30,000  kgms.  The  subject  then  rested 
for  a  time  during  which  he  consumed  105  grams  of  butter.  Two 
hours  after  the  ingestion  of  the  butter  the  experiment  was  repeated, 
samples  of  the  expired  air  being  taken  for  three  minutes  at  a  time. 
The  results  as  regards  the  respiratory  quotient  were  as  follows: 

Fasting. 

Three  minutes  before  beginning  work 0 .  706 

Twelfth  to  fifteenth  minute 0.804 

Twenty-seventh  to  thirtieth  minute 0.812 

Rest 0 .  812 

Two  Hours  after  Ingestion  of  Butter. 

Three  minutes  before  beginning  work 0 .  666 

Twelfth  to  fifteenth  minute. 0.783 

Twenty-seventh  to  thirtieth  minute 0 .  809 

In  conjunction  with  Laulanie  f  he  has  also  experimented  on 
dogs  and  rabbits,  the  muscular  contractions  being  caused  by  electric 
shocks.  The  method  of  determining  the  respiratory  exchange,  as 
described  by  Laulanie,  consisted  in  using  a  Pettenkofer  type  of 
apparatus  with  a  small  but  constant  known  rate  of  ventilation. 
The  outgoing  air  passed  through  a  small  gasometer,  but  the  current 
could  be  shunted  and  the  sample  of  air  contained  in  the  gasometer 
analyzed.  No  details  of  the  experiments  or  of  the  methods  of  cal- 
culation are  given.  The  first  table  on  the  following  page  contains 
Laulanie's  summary  of  the  results.  § 

An  even  greater  increase  in  the  respiratory  quotient  has  been 
observed  by  other  investigators.      Thus  Hanriot  &  Richet  |  found 

*  Comptes  rend.,  122,  1169 

t  Ibid.,  122,  1244,  1303:  Archives  de  Physiol .  1896,  p.  572. 

%  Archives  de  physiol  .  1896,  pp.  619  and  636. 

§  Energetique  Musculaire,  p.  70. 

||  Comptes  rend.,  104,  435  and  1865;  105,  76. 


2I4 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Animal. 

Food. 

No.  of 

Expts. 

Respiratory  Quotient. 

Before 
Work. 

During 
Work. 

After 
Work. 

Rabbit  . . 

Ad  libitum 

7 
5 
2 

0.880 

0.776 
1.016 

0.970 
0.849 
1.027 

0  799 

Dog  ...  . 

Fasting  from  1  to  7  days 

0.733 

Dog  ...  . 

Abundantly  fed  with  milk  porridge. 

1.033 

in  the  increments  of  carbon  dioxide  and  oxygen  over  the  rest  values 
quotients  much  greater  than  unity  and  reaching  in  one  case  3.5  (?). 
Speck  *  likewise  found  an  increase  in  the  respiratory  -quotient  as 
the  result  of  work.  Although  he  observed  numerous  exceptions, 
he  regards  it  as  the  rule  that  it  increases  with  the  severity  of  the 
work. 

On  the  other  hand,  Katzenstein,f  in  experiments  on  men,  found 
in  some  cases  no  considerable  increase  in  the  respiratory  quotient 
during  work.  He  gives  the  following  average  results,  of  which 
those  in  the  first  table  do  not  relate  to  exactly  the  same  subjects 
in  the  three  cases  : 

Turning  Ergostat. 

Repose 0. 754 

Light  work 0.824 

Heavy  work 0.823 


Walking. 

Subject 
No.  1. 

Subject 
No.  2. 

Subject 
No.  3.* 

Subject 
No.  4. 

0.801 
0.805 
0.799 

0.73 
0.77 
0.79 

0.77 
0.82 
0.865 

0.75 

0.895 

0.86 

In  all  cases,  the  determinations  of  the  respiratory  exchange 
covered  only   a  few  minutes  soon   after  the   work  began,   and 


*  Arch.  klin.  Med.,  45,  461. 
t  Arch.  ges.  Physiol.,  49,  330. 
%  A  very  corpulent  individual. 


INFLUENCE   OF  MUSCULAR  EXERTION   UPON  METABOLISM.    215 

no  mention  is  made  of  the  nature  of  the  diet  except  in  one  case 
(fasting).  The  individual  results  were  rather  variable,  but  most 
weight  is  given  to  those  on  Subject  No.  1,  with  whom  most  of  the 
experiments  were  made.  Katzenstein  believes  Speck's  results  to 
be  due  in  part  to  a  change  in  the  rate  of  respiration,  causing  the 
excretion  of  carbon  dioxide  to  exceed  its  actual  production  (p.  73), 
and  in  part  to  a  deficiency  of  oxygen  in  the  tissue  of  the  contracting 
muscles. 

Loewy,*  like  Katzenstein,  found  that  work  pushed  to  the  point 
of  producing  a  considerable  degree  of  fatigue  raised  the  respiratory 
quotient,  while  moderate  work  did  not.  Rapid  turning  of  the 
wheel  of  the  ergostat,  preventing  full  breathing,  or  compression 
of  the  upper  arm  by  means  of  a  rubber  band,  produced  the  same 
effect,  which  he  attributes  to  a  lack  of  oxygen.  The  most  marked 
results  seem  to  be  those  for  the  first  few  minutes  of  work,  although 
in  one  case  work  continued  for  ten  to  twenty  minutes  and  producing 
fatigue  raised  the  respiratory  quotient. 

Probably  the  most  extensive  and  carefully  conducted  investiga- 
tions of  this  nature  are  those  of  Zuntz  and  his  associates  upon  the 
dog,  and  particularly  on  the  horse.  Some  data  from  the  latter 
investigations  have  already  been  given  on  p.  212.  The  following- 
table  adds  to  the  averages  there  quoted  those  for  the  corre- 
sponding rest  periods.  In  these  experiments  there  was  a  distinct 
lowering  of  the  respiratory  quotient  instead  of  an  increase.      In 


Kind  of  Work. 

Periods. 

Respiratory  Quotient. 

Repose. 

Work. 

Walking  nearly  horizontal 

0,  b,  e,  /,  i,  0 

a,  b,  e,  0 
a,  b,  e,  f,  i,  n 

b,  e,  f,  i 
e,  i,  0 

e,  i,  0,  0 
a,  e,  f,  0 

e,  i,  0 

9,  f,  i 

0.943 
0.940 
0.953 
0.956 
0.915 
0.915 
0.943 
0.915 
0.943 

0.865 
0.847 

0.900 

0.890 

Walking  with  load,  nearly  horizontal. .  . 

0.840 
0.893 

0.882 

"           with  load 

"     horizontal,  with  draft 

0.873 
0.927 

*  Arch.  ges.  Physiol.,  49,  405. 


i6 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


all  cases  the  animal  was  liberally  fed,  usually  with  oats,  hay,  and 
cut  straw. 

Variation  during  Work. — In  their  experiments  cited  above, 
Chauveau  &  Laulanie  find  that  the  rise  of  the  respiratory  quotient 
which  they  regard  as  the  invariable  result  of  muscular  exertion 
occurs  promptly  upon  the  beginning  of  the  work,  and  the  same  thing 
is  shown  by  the  earlier  results  of  Chauveau.  As  the  work  is  con- 
tinued, however,  the  quotient  shows  a  tendency  to  fall  again,  some- 
time even  going  below  its  original  rest  value,  while  in  a  period  of 
rest  following  work  a  still  further  decrease  is  observed.  The 
results  of  their  experiments  *  are  contained  in  the  table  on  the 
opposite  page. 

Zuntz  &  Hagemann  f  also  report  a  number  of  experiments  on 
the  horse  in  which  the  respiratory  exchange  was  determined  in  suc- 
cessive periods  of  work.  The  following  are  their  results  for  the 
respiratory  quotient: 


No.  of 

Successive  Values  of  Respiratory  Quotient. 

Aggregate  Length 
of  Work  Periods, 

Experiment. 

1 

2 

3 

Min. 

37   

.917 
.913 
.929 
.925 
.920 
.865 
.928 
.910 
.974 
.863 
.911 
.949 
.936 
.931 

.865 
.806 
.889 
.948 
.931 
.868 
.921 
.926 
.905 
.820 
.922 
.934 
.909 
.904 

80 

38     

897 
875 
911 

837 

87i 
878 
.883 

121£ 

41              

102 

42               

100 

43 

92$ 

45   

54 

46 

34 

47 

48 

58 

65 

63 

73 

66 

71 

67 

124 

78 

78 

96 

75 

The  results  cited  in  the  foregoing  paragraphs  would  appear  to 
justify  the  general  conclusion  that  in  the  case  of  fasting  animals  or 


*  Comptes  rend.,  122,  1244. 
t  hoc.  cit.,  pp.  290-292. 


INFLUENCE  OF  MUSCULAR  EXERTION  UPON  METABOLISM.   217 


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218  PRINCIPLES  OF  ANIMAL   NUTRITION. 

of  those  insufficiently  fed  the  respiratory  quotient  is  increased  by 
the  performance  of  work,  while  with  well-fed  animals,  especially 
those  receiving  an  abundance  of  carbohydrates,  this  effect  is  not 
apparent.  As  the  work  is  continued,  there  appears  in  many  cases 
to  be  a  tendency  toward  a  diminution  of  the  quotient,  while  in  rest 
following  work  a  still  further  decrease  may  occur. 

Nature  of  Non-nitrogenous  Material  Metabolized. — As  already 
pointed  out,  a  comparative  study  of  the  final  products  of  metab- 
olism during  rest  and  work  does  not  itself  afford  direct  evidence 
as  to  the  nature  of  the  material  actually  metabolized  in  a  muscu- 
lar contraction,  but  simply  shows  the  total  effect  of  the  contraction 
itself  and  of  the  secondary  activities  resulting  from  it  upon  the 
make-up  of  the  schematic  body.  When  we  attempt  to  go  further 
than  this,  other  methods  of  investigation  are  requisite,  although 
experiments  like  those  already  cited  may  afford  important  con- 
firmatory evidence. 

Conclusions  from  Respiratory  Quotient. — The  significance 
of  the  respiratory  quotient  in  experiments  upon  work  has  already 
been  illustrated  in  Chapter  III  (p.  76).  Neglecting  any  slight  error 
due  to  small  changes  in  the  proteid  metabolism,  the  variations  in 
the  respiratory  quotient  as  outlined  in  the  foregoing  paragraphs 
enable  us  to  trace  the  corresponding  changes  in  the  nature  of  the 
carbon  metabolism. 

The  metabolism  of  a  fasting  animal  at  rest  is,  as  was  shown  in 
Chapter  IV,  largely  a  metabolism  of  fat.  Corresponding  to  this, 
the  respiratory  quotient  of  such  an  animal  approaches  the  value 
0.7  for  pure  fat,  although  never  quite  reaching  it,  since  some  pro- 
tein is  always  metabolized.  Numerous  instances  of  this  fact  are 
seen  in  the  experiments  already  cited.  When  such  an  animal  per- 
forms work,  the  respiratory  quotient  has  been  found  to  increase 
materially,  thus  showing  that,  in  addition  to  the  fat,  carbohydrate 
material  is  being  metabolized.  This  is  entirely  in  accord  with  the 
well-established  fact  that  muscular  exertion  causes  the  glycogen, 
both  of  the  muscles  and  of  the  liver,  to  decrease  and  even  disappear 
entirely.  With  an  animal  at  rest  and  liberally  supplied  with  car- 
bohydrate food,  on  the  other  hand,  the  respiratory  quotient  ap- 
proaches or  even  reaches  unity,  showing  that  the  metabolism  is 
essentially  carbohydrate  in  character.     When  work  is  required  of 


INFLUENCE  OF  MUSCULAR  EXERTION   UPON  METABOLISM.   219 

such  a  subject,  little  change  is  noted  at  first  in  the  respiratory  quo- 
tient. The  cells  of  the  body  being  richly  supplied  with  carbohy- 
drates apparently  utilize  these  as  the  most  readily  available  source 
of  energy.  In  either  case,  however,  continued  work  makes  large 
demands  upon  the  non-nitrogenous  materials  available,  the  store 
of  carbohydrates  in  the  body  is  rapidly  depleted,  and  the  fat  of  the 
body  is  drawn  upon  to  an  increasing  extent  as  a  source  of  energy, 
the  necessary  result  being  a  diminution  in  the  respiratory  quotient. 
In  the  experiments  of  Chauveau  &  Laulanie  only  the  respira- 
tory quotients  corresponding  to  the  total  metabolism  are  given,  and 
consequently  the  changes  in  the  character  of  the  metabolism  indi- 
cated above  can  only  be  traced  qualitatively.  In  Zuntz  &  Hage- 
mann's  investigations  the  increments  of  the  carbon  dioxide  and 
oxygen  over  the  rest  values  are  given,  and  from  them  the  propor- 
tion of  oxygen  applied  respectively  to  the  oxidation  of  fat  and  of 
carbohydrates  is  computed.  The  following  average  results  for  the 
various  forms  of  work  show  clearly  that  the  ratio  of  fat  to  carbo- 
hydrates metabolized  may  vary  through  a  very  wide  range. 


Kind  of  Work. 

Periods. 

Oxygen  per  Minute 
applied  to  the  Oxida- 
tion of 

Fat, 
c.c. 

Carbohy- 
drates, c.c. 

a,  b,  e,  f,  i,  0 

a,  b,  e,  0 
a,  b,  e,  f,  i,  n 

b.  e,  f,  i 
e,  %,  0 

e,  i,  0,  0 
a,  e,  f,  0 

e,  i,  0 

9,f,i 

4.3638 
10.433 
8.665 
8.882 
5.962 
8.525 
7.852 
12.718 
14.007 

2  9962 

7  465 

15  215 

12  992 

Walking  with  load,  nearly  horizontal . . 

3.317 

14  892 

14  201 

"     with  load,  nearly  horizontal 

16.023 
45  050 

The  Intermediary  Metabolism. — As  stated,  the  conclusions 
drawn  from  the  respiratory  quotient  relate,  strictly  speaking,  to 
the  total  effect  of  muscular  exertion  upon  the  store  of  matter  in  the 
body.  The  results  of  such  experiments  show  that,  as  a  consequence 
of  a  given  amount  of  work,  a  certain  quantity  of  fat  and  of  carbo- 
hydrates has  been  oxidized  somewhere  in  the  organism. 


220  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Many  eminent  physiologists,  however,  notably  Zuntz  and  his 
pupils,  go  further  and  regard  both  the  fat  and  the  carbohydrates  of 
food  or  body  tissue  as  immediate  sources  of  muscular  energy  and  as 
of  value  for  this  purpose  in  proportion  to  their  content  of  potential 
energy — that  is,  to  their  heats  of  combustion.  In  other  words,  they 
hold  that  either  fat  or  carbohydrates  may  be  in  effect  directly 
metabolized  by  the  muscular  tissue  and  that  each  under  like  condi- 
tions yields  substantially  the  same  proportion  of  its  potential  energy 
in  the  form  of  mechanical  work. 

On  the  other  hand,  Chauveau  *  and  Seegen  f  and  their  followers, 
as  has  already  been  indicated,  regard  the  carbohydrates  as  the  im- 
mediate source  of  energy  for  all  the  vital  activities  and  hold  that  fat 
(or  proteids)  must  first  be  converted  into  dextrose  by  the  liver  before 
it  can  be  utilized.  It  is  particularly  with  regard  to  muscular  exer- 
tion that  this  theory  has  been  elaborated,  the  conclusions  as  to  other 
forms  of  vital  activity  being  to  a  considerable  extent  based  upon 
analogy  with  the  former. 

Functions  of  the  Liver. — According  to  this  theory  the  material 
which  is  actually  metabolized  in  a  muscular  contraction  is  a  carbo- 
hydrate, viz.,  either  the  dextrose  carried  to  the  muscle  by  the  blood 
or  the  glycogen  which  is  stored  up  in  it.  Muscular  activity  is  thus 
brought  into  intimate  relations  with  the  sugar-forming  function  of 
the  liver,  and  a  chief  office  of  that  organ  is  considered  to  be  the 
preparation  of  the  necessary  carbohydrate  material  from  the  various 
ingredients  of  the  food.  The  main  facts  which  have  been  estab- 
lished may  be  summarized  as  follows  (compare  Chapter  II,  §§  1 
and  2) : 

1.  Dextrose  is  being  constantly  formed  by  the  liver,  which  not 
only  modifies  the  carbohydrates  of  the  food  but  likewise  appears  to 
produce  dextrose  from  proteids  and  particularly,  according  to  this 
school  of  physiologists,  from  fat. 

2.  Dextrose  is  as  constantly  being  abstracted  from  the  blood  by 
the  tissues,  particularly  the  muscular  tissues,  as  is  shown  by  the 
constancy  of  the  proportion  of  dextrose  in  the  blood. 

3.  The  dextrose  content  of  the  blood  is,  according  to  Chauveau, 

*  La  Vie  et  l'Energie  chez  l'Animale. 
f  Die  Zuckerbildung  im  Thierkorper. 


INFLUENCE  OF  MUSCULAR  EXERTION  UPON  METABOLISM.    221 

maintained  during  fasting  until  the  very  last  stages  of  inanition. 
When  it  finally  disappears  there  is  a  rapid  fall  in  the  body  temper- 
ature and  death  speedily  follows. 

4.  Both  the  production  of  dextrose  by  the  liver  and  its  con- 
sumption in  the  tissues  appear  to  be  augmented  by  muscular  exer- 
tion. 

The  latter  fact  is  shown  by  the  well-known  experiments  of 
Chauveau  &  Kaufmann  *  upon  the  masseter  muscle  of  the  horse. 
Comparing  the  amount  of  blood  passing  through  the  muscle  and 
the  decrease  in  its  percentage  of  dextrose  in  rest  and  in  work  they 
found  that  the  consumption  of  dextrose  in  the  two  cases  was  in  the 
proportion  of  1 :  3.372.  Subsequent  experiments  f  upon  the  Leva- 
tor labii  superioris  of  the  horse,  the  results  of  which  as  to  the  gaseous 
exchange  have  already  been  cited  (p.  188),  gave  the  following 
figures  for  the  dextrose  abstracted  from  the  blood  per  kilogram  of 
muscle  in  one  minute: 


Rest,  Grms. 

Work,  Grms. 

Work  -+■  Rest. 

0.00598  (?) 
0.06358 
0.03976  (?) 

0.03644 

0.07026  (?) 

0.22303 

0.12852 

11.75 

"          3 

3.51 

"           4 

3.23 

0.14027 

3.85 

The  authors  also  call  attention  to  the  fact  that  in  these  two 
series  of  experiments  the  arterial  blood  supplied  to  the  active  muscle 
contained  a  higher  percentage  of  dextrose  than  that  supplying  the 
same  muscle  in  a  state  of  repose,  notwithstanding  the  consumption 
of  this  substance  by  the  muscle,  and  conclude  that  muscular  activ- 
ity stimulates  the  production  of  dextrose  by  the  liver.  The  observa- 
tion of  Kiilz,J  that  prolonged  muscular  exertion  may  cause  the  dis- 
appearance of  glycogen  from  the  liver,  may  perhaps  be  interpreted 
as  sustaining  this  conclusion. 


*  Comptes  rend.,  103,  974,  1057,  1153. 
f  Ibid.,  104,  1 126,  1352,  1409. 
t  Arch.  ges.  Physiol.,  24,  41. 


222  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Muscular  Glycogen. — Especial  interest  attaches  in  this  connec- 
tion to  the  behavior  of  the  glycogen  of  the  muscles.  Nasse  * 
appears  to  have  been  the  first  to  show  that  the  muscular  glycogen 
is  consumed  during  contraction.  This  result  has  been  abundantly 
confirmed  by  other  investigators,  notably  by  Weiss,|  while,  as  just 
stated,  Kiilz  has  shown  that  the  same  thing  is  true  of  the  glycogen 
of  the  liver. 

It  has  also  been  shown  that  glycogen  accumulates  in  muscles 
whose  activity  has  been  suspended  by  section  of  their  nerves  or  other- 
wise. An  early  statement  to  this  effect,  unaccompanied  by  experi- 
mental proof,  is  by  MacDonnel.J  Chandelon  §  investigated  the 
influence  upon  the  glycogen  content  of  the  hind  leg  of  a  rabbit  of, 
first,  ligature  of  the  arteries,  and  second,  section  of  the  motor  nerves. 
The  first  treatment  caused  a  large  loss  and  the  second  a  large  gain 
of  glycogen.  Morat  &  Dufourt  |  confirmed  these  results  and  also 
found  that  the  formation  of  muscular  glycogen  was  more  rapid  in  a 
fatigued  quiescent  muscle  than  in  a  normal  one,  while  Aldehoff  •" 
has  shown  that  in  a  fasting  animal  glycogen  persists  longer  in  the 
muscles  than  in  the  liver  and  reappears  first  in  the  former  when  food 
is  given. 

In  view  of  these  facts  it  can  hardly  be  doubted  that  the  muscu- 
lar glycogen  is  in  some  way  a  source  of  energy  to  the  muscles,  being 
destroyed  during  contraction  and  stored  up  again  during  rest. 

Chauveau's  Interpretation. — By  a  comparison  of  their  results  for 
dextrose  just  cited  on  p.  221  with  those  for  the  gaseous  exchange 
of  the  muscle  as  given  on  p.  188,  Chauveau  &  Kaufmann  show  that 
during  rest  there  was  a  storage  of  dextrose  and  of  oxygen  in  the 
muscle.  During  work,  on  the  contrary,  more  carbon  dioxide  was 
produced  by  the  muscle  than  corresponded  to  the  amount  of  dex- 
trose which  was  abstracted  from  the  blood,  and  this  carbon  dioxide 
contained  more  oxygen  than  was  supplied  to  the  muscle  by  the 


*  Arch.  ges.  Physiol ,  2,  97;  14,  482. 

t  Sitzungsber.  Wiener  Akad  der  Wiss.,  Math-Nat.  Klasse,  64,  II,  284. 

t  Proc.  Roy.  Irish  Acad.,  Ser.  I,  7,  271. 

,  Arch.  ges.  Physiol  ,  13,  626. 

||  Archives  do  Physiol  ,  1892,  327  and  457. 

«[  Zeit.  f.  Biol.,  25,  137. 


INFLUENCE  OF  MUSCULAR  EXERTION  UPON  METABOLISM.    223 

blood  during  the  same  time.     The  average  results,  computed  in 
milligrams  per  minute,  were: 


During  Rest, 
Mgrms. 


During  Work, 
Mgrms. 


Oxygen  from  blood 

"       in  C02  produced 

"       required  to  oxidize  dextrose  taken  up  from 

blood 

Carbon  of  C02  produced 

"        "  dextrose  taken  up 


0.11803 
0.08424 

0.58305 
0.03160 
0.21862 


2.48490 
3.15052 

2.35055 
1.18128 
0.88118 


During  rest  the  muscle  was  storing  up  both  carbohydrate  (gly- 
cogen) and  oxygen,  thus  supplying  itself  with  a  reserve  of  potential 
energy.  During  activity  this  reserve,  as  well  as  the  supply  brought 
by  the  blood,  was  drawn  upon  for  the  performance  of  work. 

The  fluctuations  of  the  respiratory  quotient  resulting  from  mus- 
cular exertion  are  explained  by  Chauveau  in  outline  as  follows : 

At  first  there  is  a  rapid  oxidation  of  the  stored  glycogen  of  the 
muscles  and  of  the  dextrose  of  the  blood,  resulting  in  a  respiratory 
quotient  approaching  unity.  As  the  work  progresses  the  store  of 
carbohydrate  material  in  the  organism  becomes  relatively  exhausted, 
unless  there  is  a  large  supply  of  it  in  the  food,  and  to  meet  the 
demands  of  the  muscles  an  increased  production  of  dextrose  from 
the  fat  of  the  food  or  of  the  body  takes  place  in  the  liver.  This 
change,  however,  according  to  the  equation  proposed  on  p.  38,  con- 
sumes 67  molecules  of  oxygen  for  each  18  molecules  of  carbon  diox- 
ide produced.  This  process,  superadded  to  the  combustion  of 
carbohydrates  in  the  muscles,  results  in  the  observed  lowering  of 
the  respiratory  quotient.  The  further  lowering  of  the  quotient 
during  a  succeeding  rest  period  results  from  the  great  diminution 
in  the  amount  of  carbohydrates  oxidized  in  the  muscles,  the  for- 
mation of  carbohydrates  from  fat  in  the  liver  still  continuing 
for  a  time  in  order  to  replenish  the  exhausted  store  of  muscular 
glycogen. 

Fat  as  a  Source  of  Muscular  Energy. — According  to  the  above 
theory,  fat  is  only  indirectly  a  source  of  muscular  energy,  in 
that  it  serves  for  the  production  of  dextrose  in  the  liver,  and  the 


224  PRINCIPLES  OF  ANIMAL   NUTRITION. 

same  thing  is  held  to  be  true  of  protein  so  far  as  it  contributes 
energy  for  muscular  exertion. 

As  we  have  seen  in  Chapter  II,  however,  the  formation  of  dex- 
trose from  fat  in  the  liver  is  by  no  means  universally  admitted,  and 
Chauveau's  ingenious  theory  as  to  the  immediate  source  of  muscu- 
lar energy  has  not  lacked  opponents.  If  it  is  true,  fat  has  a  much 
lower  value  for  that  purpose  than  corresponds  to  its  potential 
energy  as  measured  by  its  heat  of  combustion.  If  it  be  assumed 
to  be  converted  into  dextrose  in  accordance  with  the  equation  on 
p.  38,  it  is  easy  to  compute  that  about  36  per  cent,  of  its  potential 
energy  will  be  liberated  as  heat  in  the  process  and  that  consequently 
only  the  64  per  cent,  remaining  in  the  resulting  dextrose  will 
be  available  to  the  muscles.  Consequently  the  relative  values  of 
fat  and  dextrose  for  the  production  of  work  will  be  as  162  to  100 
and  not  as  253  to  100. 

While  the  evidence  of  the  respiratory  quotient  is  not  incon- 
sistent with  Chauveau's  theory,  it  is  also  not  inconsistent  with  the 
view  which  supposes  fat  to  be  directly  metabolized  for  the  produc- 
tion of  mechanical  work.  The  difference  lies,  not  in  the  amounts 
of  carbon  dioxide  and  oxygen  evolved  but  in  the  place  where  and 
the  form  in  which  the  energy  is  liberated,  and  the  question  can 
therefore  be  satisfactorily  discussed  only  on  the  side  of  its  energy 
relations. 

Postponing  that  discussion  for  the  present,  it  may  be  remarked 
here  that  while  it  appears  to  be  true,  as  already  stated,  that  the 
muscular  glycogen  and  the  dextrose  of  the  blood  are  a  source  of 
muscular  energy,  and  perhaps  the  most  readily  available  one,  it 
by  no  means  follows  that  they  are  the  only  source.  The  muscle 
contains  other  non-nitrogenous  reserve  materials  besides  glycogen, 
and  notably  a  not  inconsiderable  amount  of  fat  and  of  lecithin. 
Moreover,  recent  investigations  (see  pp.  63  to  05)  have  shown  that 
the  amount  of  the  muscular  fat  is  greater  than  was  formerly  sup- 
posed, and  that  some  of  it  cannot  be  extracted  with  ether  and 
behaves  almost  as  if  in  chemical  combination.  Indeed,  it  appears 
not  improbable  that  both  fat  and  carbohydrate  molecular  groupings, 
as  well  as  proteids,  enter  into  the  structure  of  living  protoplasm. 
Finally,  not  only  the  muscle  but  the  blood  which  nourishes  it 
contains  fat  as  well  as  carbohyhrates,  the  former  indeed  being  more 


INFLUENCE  OF  MUSCULAR  EXERTION   UPON  METABOLISM-    225 

abundant  than  the  latter.  There  would  seem  to  be  no  inherent 
difficulty,  then,  in  supposing  that  the  fat  of  the  muscle  and  of  the 
blood  serves  directly  as  a  source  of  energy,  although  the  writer  is  not 
aware  of  any  investigations  upon  the  influence  of  the  contraction 
of  a  muscle  upon  its  fat-content. 


PART  II. 
THE  INCOME  AND   EXPENDITURE   OF  ENERGY. 


CHAPTER  VII. 
FORCE  AND  ENERGY.       ' 

Force  is  defined  as  whatever  is  capable  of  changing  the  rate  of 
motion  of  a  mass  of  matter.  When  a  force  acts  upon  a  mass,  im- 
parting to  it  a  certain  velocity,  it  does  work,  the  amount  of  work 
being  measured  by  the  product  of  the  force  into  the  distance  through 
which  it  acts.  Energy  may  be  defined  as  the  capacity  to  do  work. 
Any  mass  of  matter  which  can  act  upon  another  mass  in  such  a 
way  as  to  change  its  rate  of  motion  is  said  to  possess  energy. 

Kinetic  and  Potential  Energy. — In  studying  energy  we 
distinguish  between  kinetic  energy,  or  the  energy  due  to  motion, 
and  potential  energy,  or  the  energy  due  to  position.  The  falling 
weight  of  a  pile-driver  at  the  instant  it  strikes  the  pile  possesses  a 
certain  amount  of  kinetic  energy  and  does  a  corresponding  amount 
of  work  on  the  pile.  When  it  is  raised  again  a  certain  amount  of 
work  is  done  on  it.  and  when  it  comes  to  rest  at  the  top  of  the  ma- 
chine a  corresponding  amount  of  energy  is  stored  up  in  it  as  poten- 
tial energy.  As  long  as  the  weight  is  supported  at  this  point  it 
does  no  work,  but  simply  possesses  the  possibility  of  doing  work. 
When  it  is  allowed  to  fall  again,  this  potential  energy  due  to  its 
position  is  converted  into  the  actual  or  kinetic  energy  of  motion, 
and  when  it  reaches  the  point  from  which  it  was  raised  and  strikes 
the  pile  it  does  work  upon  the  latter  exactly  equal  to  that  formerly 

226 


FORCE  AND  ENERGY.  227 

stored  up  in  the  weight  as  potential  energy,  which  again  was  equal 
to  the  energy  expended  in  raising  it. 

An  even  simpler  example  of  the  conversion  of  potential  into 
kinetic  energy  and  vice  versa  is  a  swinging  pendulum.  When  at 
rest  for  an  instant  at  the  end  of  a  vibration  it  possesses  a  certain 
amount  of  potential  energy,  corresponding  to  its  vertical  height 
above  the  lowest  point  of  its  arc.  When  it  reaches  this  lowest 
point,  so  far  as  the  mechanism  of  which  it  forms  part  is  concerned 
it  has  no  more  potential  energy  because  it  cannot  fall  any  farther. 
In  place  of  this,  however,  neglecting  mechanical  resistances,  it  con- 
tains an  exactly  equivalent  amount  of  kinetic  energy,  due  to  its 
motion.  During  the  second  half  of  the  swing  this  kinetic  energy 
is  expended  in  again  raising  the  pendulum,  and  when  it  has  all  been 
expended  the  pendulum  will  (in  the  absence  of  external  resistance) 
have  been  raised  to  exactly  the  same  height  as  before  above  its 
lowest  point.  In  other  words,  its  kinetic  energy  will  have  been  re- 
converted into  an  equivalent  amount  of  potential  energy  and  so 
the  alternate  conversion  and  re-conversion  goes  on  as  long  as  the 
pendulum  continues  to  swing. 

The  same  facts  which  have  been  illustrated  above  in  the  case  of 
the  motion  of  visible  masses  of  matter  are  likewise  true  of  molecular 
and  atomic  motions.  When  molecules  of  carbon  dioxide  and  water 
are  converted  into  starch  in  the  green  leaves  of  the  plant,  work  is 
done  upon  them  by  the  energy  of  the  sun's  rays.  Their  constituent 
atoms  are  forced  apart  and  compelled  to  assume  new  groupings.  In 
this  process  a  certain  amount  of  kinetic  energy  has  disappeared 
and  the  resulting  system  of  starch  molecules  and  oxygen  molecules 
contains  a  corresponding  amount  of  potential  energy.  Under 
suitable  conditions  the  reverse  process  may  also  take  place.  The 
atoms  may,  so  to  speak,  fall  together  and  resume  their  old  positions, 
producing  the  original  amounts  of  carbon  dioxide  and  water  and 
giving  off  in  the  process  the  exact  amount  of  kinetic  energy  which 
was  originally  absorbed.  This  energy  may  appear  in  the  form  of 
heat,  as  in  ordinary  combustion,  or  in  any  other  of  the  various 
forms  of  energy,  according  to  circumstances. 

The  last  example  is  but  an  illustration  of  the  general  fact  that 
in  every  chemical  reaction  there  occurs  a  transformation  of  energy 
which  most  commonly  takes  the  form  of  an  evolution  or  absorption 


228  PRINCIPLES  OF  ANIMAL   NUTRITION. 

of  heat.  That  branch  of  science  which  deals  with  the  connection 
between  chemical  and  thermal  processes  is  known  as  thermo-chem- 
istry.  Since  kinetic  energy  in  the  animal  is  derived  from  chemical 
processes,  and  since  it  largely  takes  the  form  of  heat,  we  may  regard 
the  study  of  the  transformations  of  energy  in  the  organism  as  con- 
stituting a  branch  of  thermo-chemistry  and  proceed  to  a  consider- 
ation of  the  fundamental  laws  upon  which  the  latter  subject  is 
based. 

The  Conservation  of  Energy. — In  any  system  of  bodies  not 
acted  on  by  external  forces  the  sum  of  the  potential  and  kinetic 
energy  is  constant.  In  other  words,  while  the  ratio  of  potential 
to  kinetic  energy  may  vary,  and  while  each  may  take  various  forms, 
as  mass-motion,  heat,  electric  stress,  etc.,  there  is  no  loss  of  energy 
in  these  conversions.  Energy,  like  matter,  is  indestructible.  This 
great  law  of  the  conservation  of  energy  was  first  clearly  enunciated 
by  Mayer,  and  forms  the  foundation  of  all  modern  conceptions  of 
physical  processes.  In  the  case  of  the  swinging  pendulum  used 
above  as  an  illustration  the  total  energy  of  the  system  composed 
of  the  earth  and  the  pendulum  is  constant,  a  portion  of  it  simply 
alternating  between  the  potential  and  kinetic  states.  So,  too,  in 
the  system  of  atoms  of  carbon,  hydrogen,  and  oxygen,  the  potential 
energy  contained  in  the  system  before  the  starch  is  burned  is  simply 
converted  into  the  kinetic  energy  of  heat,  while  the  total  energy 
of  the  system  remains  the  same. 

Initial  and  Final  States. — An  important  consequence  of  the 
law  of  the  conservation  of  energy,  which  was  first  deduced  and 
demonstrated  experimentally  by  Hess  in  1840,  is  known  as  the  law 
of  initial  and  final  states.  This  law  is  that  in  any  independent 
system  the  amount  of  energy  transformed  from  the  potential  to 
the  kinetic  form,  or  vice  versa,  during  any  change  in  the  system, 
depends  solely  upon  the  initial  and  final  states  of  the  system  and 
not  at  all  upon  the  rapidity  of  the  transformation  or' upon  the  kind 
or  number  of  the  intermediate  stages  through  which  it  passes. 
Although  this  law  is  true  in  the  general  form  here  stated,  it  was 
originally  propounded  as  related  to  chemical  reactions  and  forms 
the  basis  of  the  science  of  thermo-chemistry.  If  we  start  with 
starch  and  oxygen  and  end  with  the  corresponding  quantities  of 
carbon  dioxide  and  water,  the  amount  of  kinetic  energy  evolved  is 


FORCE  AND  ENERGY.  229 

the  same,  no  matter  whether  the  starch  be  burned  almost  instanta- 
neously in  pure  oxygen  or  whether  it  be  subjected  to  slow  oxidation 
in  the  tissues  of  a  plant  buried  in  the  soil;  whether  carbon  dioxide 
and  water  are  the  immediate  products  of  the  action  or  whether  the 
starch  be  previously  transformed  into  maltose,  glycogen,  dextrose, 
lactic  acid,  etc.,  etc.,  as  in  the  body  of  the  animal.  We  have  simply 
to  determine  the  potential  energy  of  the  system  in  its  initial  and  in 
its  final  state,  and  the  difference  is  equal  to  the  amount  of  kinetic 
energy  evolved  during  the  change.  The  truth  of  this  law,  as  ap- 
plied to  chemical  processes,  has  been  fully  demonstrated  by  the 
researches  of  Berthelot  and  Thomsen.  That  the  same  law  applies 
to  the  processes  taking  place  in  the  body  of  the  animal  is  exceed- 
ingly probable,  a  priori,  and  has  been  demonstrated  experimentally 
by  the  researches  of  Rubner  and  of  Atwater  and  his  associates. 

Heats  of  Combustion. — We  have  no  means  of  determining 
the  total  amount  of  potential  energy  contained  in  a  system,  but  can 
only  measure  that  portion  which  is  manifested  by  the  change  to 
the  kinetic  or  the  potential  form  during  some  change  in  the  system. 
In  other  words,  we  may  assume  the  potential  energy  of  the  system 
in  some  particular  state  as  zero  and  obtain  a  numerical  expression 
for  its  potential  energy  in  some  other  state  as  compared  with  this 
standard  state.  For  the  latter  we  shall  naturally  select  that  one  in 
which  no  further  conversion  of  potential  into  kinetic  energy  can, 
according  to  our  experience,  take  place. 

In  the  case  of  organic  substances,  such  as  those  entering  into 
the  metabolism  of  the  animal,  the  system  consists  of  the  substance 
itself  and  oxygen,  and  the  state  of  complete  oxidation  is  the  one  in 
which  experience  shows  that  no  further  evolution  of  kinetic  energy 
is  possible  by  chemical  means.  Thus,  to  recur  to  the  example  of 
starch,  if  one  gram  be  oxidized  in  accordance  with  the  equation 

C«H,  0O5  +  602 = 6C02 + 5H20, 

the  amount  of  heat  evolved  will  be  4183  cals.,*  this  being  the  amount 
of  energy  converted  from  the  potential  to  the  kinetic  form.  From 
the  system  represented  by  the  second  member  of  the  above  equa- 
tion we  can  get  no  further  evolution  of  heat.     We  therefore  repre- 

*  For  the  units  of  measurement  see  the  following  paragraph. 


230  PRINCIPLES   OF  ANIMAL   NUTRITION. 

sent  its  potential  energy  by  0  and  accordingly  that  of  the  system 
starch  +  oxygen  by  4183  cals.  for  each  gram  of  starch.  This  value 
is  called  the  heat  of  combustion  of  starch,  and  shows  how  much 
energy  can  be  liberated  from  this  substance  by  its  conversion  into 
C02  and  H20.  It  is  common  to  speak  of  this  as  the  potential  energy 
of  the  starch,  and  the  expression  has  the  advantage  of  brevity, 
but  it  should  not  be  forgotten  that  it  is  really  the  potential  energy 
of  the  system  C6H10O5  +  602  as  compared  with  the  system 
6C02  +  5H,0. 

In  like  manner  the  heat  of  combustion  of  any  organic  com- 
pound, or  of  any  mixture  of  compounds  such  as  a  feeding-stuff, 
represents  the  amount  of  energy  which  a  given  weight  of  it  evolves 
in  the  form  of  heat  when  completely  oxidized.  In  the  case  of 
nitrogenous  bodies  the  final  products  are  C02,  H20,  N2,  and  in 
case  of  proteids  S03. 

Heats  of  combustion  may  be  determined  at  constant  pressure 
or  at  constant  volume.  When  the  substance  is  burned  under  ordi- 
nary atmospheric  pressure  the  amount  of  heat  evolved  may  include, 
besides  that  due  to  the  difference  in  the  chemical  energy  of  the 
substance  before  and  after  burning,  a  mechanical  component  due 
to  the  fact  that  the  volume  of  the  products  is  not  the  same  as  that 
of  the  original  substances.  If  it  is  greater,  work  is  done  in 
overcoming  atmospheric  pressure  and  the  heat  production  is 
diminished  by  a  corresponding  amount.  In  the  contrary  case,  work 
is  done  by  the  atmosphere  upon  the  products  of  combustion  and 
heat  is  evolved.  When  the  substance  is  burned  in  a  confined 
volume  of  oxygen,  as  in  the  bomb-calorimeter,  the  possibility  of 
such  mechanical  action  is  eliminated  and  we  obtain  a  quantity  of 
heat  representing  solely  the  difference  in  chemical  energy.  The  heats 
of  combustion  at  constant  volume  are  therefore,  from  a  theoretical 
point  of  view,  the  more  correct.  On  the  other  hand,  however,  all 
ordinary  processes  of  combustion,  including  those  occurring  in  the 
animal  organism,  take  place  under  atmospheric  pressure,  which  is 
practically  constant,  and  therefore  the  actual  heat  value  of  a  sub- 
stance oxidized  in  the  body  is  measured  by  its  heat  of  combustion 
at  constant  pressure.  If  there  is  no  change  in  volume  during  the 
combustion,  then  the  two  heats  of  combustion  are,  of  course,  iden- 
tical.    This  is  the  case,  for  example,  with  the  carbohydrates,  which 


FORCE  AND  ENERGY.  231 

form  so  large  a  part  of  the  food  of  herbivorous  animals.  Further- 
more, the  difference  in  the  case  of  the  other  common  nutrients  is  so 
slight  that  the  heats  of  combustion  as  determined  with  the  bomb- 
calorimeter  may  be  used  without  appreciable  error  in  computing 
the  metabolism  of  energy  in  the  body.  The  only  substance  involved 
in  such  computations  for  which  the  correction  needs  to  be  made  is 
methane,  CH4,  the  heat  of  combustion  of  which  is  at  constant 
volume  13,246  cals.  per  gram  and  at  constant  pressure  13,344  cals. 

Units  of  Measurement. — The  unit  of  force  is  the  dyne,  which 
is  defined  as  the  amount  of  force  required  to  produce  in  a  mass  of 
one  gram,  in  one  second,  an  acceleration  of  one  centimeter  per 
second. 

When  a  .orce  acts  upon  a  mass,  the  amount  of  work  done  is 
measured  by  the  product  of  the  force  into  the  distance  (measured 
along  the  direction  of  the  force)  through  which  it  acts.  The  unit 
of  work  is  the  erg,  which  is  defined  as  the  work  done  by  a  force  of 
one  dyne  acting  through  one  centimeter. 

Energy  has  been  defined  as  the  power  of  doing  work,  and  is 
measured  by  the  amount  of  work  done,  that  is,  in  ergs.  Since, 
however,  the  erg  is  a  very  small  quantity,  it  is  often  more  con- 
venient in  practice  to  use  a  multiple  of  it.  For  this  purpose  the 
quantity  1010  erg=l  Kilojoule  (J)  is  a  convenient  unit.  Energy  is 
also  frequently  expressed  in  units  based  on  weight  instead  of  mass, 
the  most  common  being  the  gram-meter,  the  kilogram-meter,  and 
the  foot-pound.  The  gram-meter  is  the  work  done  against  gravity 
in  raising  a  weight  of  1  gram  through  1  meter.  Since,  however,  the 
force  of  gravity,  and  consequently  the  weight  of  a  given  mass,  varies 
at  different  points  on  the  earth's  surface,  it  is  necessary  to  state 
also  where  the  weight  is  taken.  At  the  level  of  the  sea,  in  temperate 
latitudes,  the  force  of  gravity  equals  980.5  dynes.  Under  these 
conditions,  then,  doing  1  gram-meter  of  work  would  be  equivalent 
to  exerting  a  force  of  980.5  dynes  through  100  cm.,  which  equals 
98,050  ergs.  The  kilogram-meter  (kgm.)  is  the  work  done  against 
gravity  in  raising  1  kilogram  through  1  meter,  and  is  accordingly 
1000  times  the  gram-meter  or  98,050,000  ergs.  The  foot-pound 
is  the  work  done  against  gravity  in  raising  1  pound  through  1  foot 
and  accordingly  equals  13,550,000  ergs. 

In  addition  to  mechanical  energy  the    animal  produces  heat. 


232  •       PRINCIPLES   OF  ANIMAL   NUTRITION. 

For  the  measurement  of  heat  various  units  are  in  use,  but  the  ones 
most  commonly  employed  in  physiology  are  the  small  and  the  large 
calorie.  The  small  calorie  (cal.)  is  defined  as  the  amount  of  heat 
required  to  raise  the  temperature  of  1  gram  of  water  through  1°  C. 
Since,  however,  the  specific  heat  of  water  varies  somewhat  with  the 
temperature,  it  is  necessary  to  specify  the  average  temperature 
of  the  water.  The  temperature  of  18°  C.  has  been  quite  commonly 
used  for  this  purpose,  the  resulting  unit  being  indicated  by  the 
abbreviation  cal18.  Atwater  &  Rosa,*  however,  in  their  work 
with  the  respiration-calorimeter,  have  employed  the  temperature 
of  20°  C,  designating  their  unit  by  cal20.  The  difference  between 
the  two  is  very  slight,  1  cal20  equaling  1.0002  cal18.  The  large 
calorie  (Cal.)  is  the  amount  of  heat  required  to  raise  the  tempera- 
ture of  one  kilogram  of  water  through  1°  C,  or  is  equal  to  1000  small 
calories.  The  temperature  at  which  the  large  calorie  is  measured 
may  be  indicated  as  in  case  of  the  small  calorie. 

The  calorie,  however,  while  commonly  used,  and  while  in  some 
respects  a  convenient  unit,  is  in  a  sense  not  a  rational  one.  Since 
heat  is  one  form  of  energy,  and  since,  in  accordance  with  the  law  of 
the  conservation  of  energy,  there  is  a  fixed  relation  between  it  and 
other  forms  of  energy,  a  rational  unit  would  be  one  bearing  a  simple 
numerical  relation  to  the  units  employed  to  measure  other  forms 
of  energy,  or  in  other  words,  the  erg  or  some  simple  multiple  of  it. 
As  already  noted,  the  Kilojoule  (J)  is  a  convenient  unit  for  this 
purpose.  It  has  two  advantages  over  the  Calorie :  first,  it  permits 
of  a  direct  comparison  of  heat  with  other  forms  of  energy  (expressed, 
of  course,  in  units  of  the  same  system) ;  and  second,  it  is  an  "  abso- 
lute" unit,  that  is,  it  is  based  on  the  fundamental  units  of  space, 
mass,  and  time,  and  has  a  perfectly  definite  magnitude,  while  the 
Calorie  has  not  unless  the  temperature  at  which  it  is  measured  is 
stated.  To  this  may  be  added  that  in  discussing  physiological 
relations  it  avoids  the  sometimes  confusing  implication  that  the 
quantities  of  energy  dealt  with  actually  exist  in  all  cases  as  heat. 

The  relation  between  the  Calorie  and  the  Kilojoule  is  as  follows: 

lCal18  =  4.183  J       =41,830,000,000  ergs; 
U       =  0 .  2391  Cal18  =  10,000,000,000  ergs. 

*  U.  S.  Dept.  Agr.,  office  of  Expt.  Stats.,  Bull.  63,  p.  55. 


FORCE  AND   ENERGY. 


233 


Since,  however,  most  of  the  results  of  investigations  upon  the 
physiological  relations  of  energy  are  expressed  in  calories  (often 
without  any  statement  of  temperature)  it  will  be  more  convenient 
in  the  following  pages  to  employ  this  unit  rather  than  the  more 
rational  Kilojoule. 

Finally,  since  measurements  of  mechanical  energy  (as  in  experi- 
ments with  working  animals)  have  been  commonly  made  in  weight 
units,  it  is  necessary  to  know  the  relation  of  these  to  the  calorie. 
These  relations  are  included  in  the  following  table,  the  force  of 
gravity  being  taken  as  980.5  dynes: 


EQUIVALENCE   OF   UNITS  OF   ENERGY. 


Ergs.* 

Kilojoules. 

Gram- 
meters. 

Kilogram- 
meters. 

1010 

980.5X102 
980.5X105 
135.5X105 
4.183X107 
4.183X1010 

101989 

iooo 

138.2 

426.6 

426600 

1  gram-meter  =  

1  kilogram-meter  =  .  .  .  . 
1  foot-pound  = 

980.5-r-lO8 
980.5-hIO5 
135. 5  -h105 

0.004183 

4.183 

0.001 

0.1382 
0  4266 

1  Cal.8 

426  6 

Foot- 
pounds. 

ca]I9. 

Cal,e. 

738.1 
0.007236 
7.236 

239.1 
0.002344 
2.344 
0.3239 

"iooo"" 

0  2391 

0  2344 -=-10* 

0  002344 

1  foot-pound  = 

0  000324 

1  caL  ■ 

3.087 
3087. 

0  001 

1  Cal,8 

*  From  Ostwald,  Grundriss  der  allgemeinen  Chemie. 


CHAPTER  VIII. 
METHODS   OF   INVESTIGATION. 

The  food  is  the  sole  known  source  of  energy  as  well  as  of  matter 
to  the  body  of  the  warm-blooded  animal,  and  the  total  income  of 
potential  energy,  according  to  the  principles  laid  down  in  the  pre- 
ceding chapter,  is  represented  by  the  heat  of  combustion  of  the 
food. 

A  portion  of  this  food,  as  we  have  seen  in  Part  I,  is  metabolized 
in  the  body,  while  part  of  it  escapes  complete  oxidation  and  is  re- 
jected as  undigested  matter  in  the  feces,  as  metabolic  products  in 
feces,  urine,  and  perspiration,  and  as  combustible  intestinal  gases. 
All  these  substances  still  contain  more  or  less  of  their  original  store 
of  potential  energy  and  collectively  constitute  one  main  division  of 
the  outgo  of  energy.  We  may  call  it,  for  brevity,  the  outgo  of 
potential  energy.  A  portion  of  the  food  may  also  be  applied  to  the 
production  and  storage  of  tissue  (protein  and  fat)  in  the  body,  and 
this,  from  our  present  point  of  view,  is  to  be  classed  with  the 
outgo  of  potential  energy. 

The  potential  energy  of  the  remaining  portion  of  the  food,  viz., 
that  which  is  completely  oxidized,  may  take  various  transitory 
forms  in  the  organism,  but  ultimately  it  leaves  it  in  one  of  two 
forms  of  kinetic  energy,  viz.,  as  mechanical  work  or  as  heat.  Here 
we  have  the  second  main  division  of  the  outgo  of  energy,  viz.,  the 
outgo  of  kinetic  energy.  These  relations  may  be  briefly  expressed 
in  tabular  form,  as  shown  at  the  head  of  the  opposite  page. 

As  in  the  corresponding  chapter  of  Part  I,  it  is  proposed  to  con- 
sider here  simply  the  general  principles  of  the  more  important 
methods  available  for  determining  the  income  and  outgo  of  poten- 
tial and  kinetic  energy,  without  entering  into  technical  details. 

234 


Potential  energy. 


METHODS  OF  INVESTIGATION.  235 

Income  : 

Food 
Outgo  : 

Feces 

Urine 

Perspiration 

Combustible  gases 

Storage  of  tissue 

Work  I  „. 

Heat  [Kinetic  energy. 

Determination  of  Potential  Energy. 

The  Energy  of  the  Food. — The  potential  energy  of  the  food  is 
conveniently  measured  by  converting  it  into  the  kinetic  form  of  heat; 
that  is,  by  determining  its  heat  of  combustion.  This  determina- 
tion is  effected  by  means  of  an  instrument  known  as  a  calorimeter, 
in  which  the  heat  produced  by  the  complete  combustion  of  a  known 
weight  of  the  substance  under  examination  is  absorbed  by  some 
calorimetric  substance  and  its  amount  measured  by  the  change  of 
temperature  or  of  physical  state  of  the  latter.  The  calorimetric 
substance  ordinarily  employed  is  water,  the  increase  in  tempera- 
ture of  a  known  weight  of  this  substance  giving  directly  the  amount 
of  heat  in  calories.  It  is,  of  course,  essential  either  that  all  the  heat 
produced  shall  be  transferred  to  the  calorimetric  substance  or  that 
it  shall  be  possible  to  correct  the  observed  results  for  any  heat  that 
may  escape  absorption. 

Another  essential  is  that  the  oxidation  shall  be  complete,  a 
condition  whose  fulfillment  it  is  by  no  means  easy  to  secure.  Two 
general  methods  have  been  employed  for  this  purpose.  The  first 
was  that  of  Thompson,*  as  used  by  Frankland  and  subsequently 
modified  by  Stohmann.f  in  which  the  oxidation  is  effected  by 
means  of  pure  potassium  chlorate,  corrections  being  made  for  the 
heat  evolved  in  the  decomposition  of  the  latter  substance.  The 
second  method,  which  has  almost  entirely  replaced  the  first,  con- 

*  Described  by  Frankland,  Proc.  Roy.  Inst,  of  Great  Biitain,  June  8, 
1866,  and  Phil.  Mag.  (4),  32,  182. 

t  Jour.  pr.  Chem.,  127,  115;  Landw.  Jahrb.,  13,  513. 


236  PRINCIPLES   OF  ANIMAL   NUTRITION. 

sists  in  burning  the  substance  without  any  admixture  in  highly 
compressed  oxygen  contained  in  a  lined  steel  bomb  as  first  devised 
by  Berthelot  *  and  subsequently  modified  by  Mahler,  Hempel,  and 
Atwater.  With  this  type  of  calorimeter  very  accurate  and  com- 
paratively rapid  work  may  be  done.f 

Frankland  was  the  first  to  undertake  determinations  of  the 
heats  of  combustion  of  foods  and  food  ingredients,  using  the  origi- 
nal form  of  the  Thompson  calorimeter.  Subsequent  investigators, 
of  whom  may  be  especially  mentioned  Stohmann,  v.  Reehenberg 
and  Danilewski,  Berthelot  and  his  associates,  Rubner,  and  Atwater, 
Gibson  &  Woods,  have  continued  these  investigations  with  im- 
proved apparatus  and  more  refined  methods,  J  and  we  now  possess 
a  considerable  mass  of  data  as  to  the  heats  of  combustion  of  the 
more  important  ingredients  of  animals  and  plants  and  of  the  prod- 
ucts of  metabolism.  Atwater  §  gives  the  following  summary  of 
the  results  on  record  up  to  July,  1894  (see  pp.  237-9). 

In  the  course  of  recent  investigations  into  the  energy  relations 
of  the  food  of  man  and  of  domestic  animals  a  considerable  amount 
of  data  has  also  been  secured  regarding  the  heats  of  combus- 
tion of  foods  and  feeding-stuffs.  A  summary  of  the  results  of 
such  determinations  on  276  samples  of  human  foods  of  various 
kinds  has  been  published  by  Atwater  &  Bryant.  ||  No  similar 
compilation  of  heats  of  combustion  of  feeding-stuffs  is  as  yet  avail- 
able. 

It  need  hardly  be  pointed  out  that,  taken  by  themselves, 
such  results  furnish  no  measure  of  the  relative  values  of  the 
various  feeding-stuffs.  Like  a  chemical  analysis,  they  supply 
but  a  single  factor,  albeit  an  important  one,  for  such   a    com- 

*  Ann.  de  Chim.  et  de  Phys.,  (5),  23,  160. 

t  For  the  technical  details  of  the  method  reference  may  be  had  to  tbe 
published  descriptions  of  the  apparatus  or  to  Wiley's  Principles  and  Prac- 
tice of  Agricultural  Chemical  Analysis,  Vol.  Ill,  p.  569. 

X  For  a  historical  sketch  of  the  development  of  calorimetry,  as  applied 
to  food  substances,  compare  Atwater,  "Chemistry  and  Economy  of  Food." 
U.  S.  Department  of  Agriculture,  Office  of  Experiment  Stations,  Bull.  21,  pp 
116-126. 

§  Ibid.,  pp.  127  and  128.  Compare  also  Rep.  Storrs  Expt.  Station,  1899, 
p.  73. 

II  Rep.  Conn.  Storrs  Expt.  Station,  1899,  p.  97. 


METHODS  OF  INVESTIGATION. 


'■37 


HEATS  OF   COMBUSTION  OF  ORGANIC  SUBSTANCES. 


Berthelot 
Method. 


Berthe- 
lot and 


Stoh- 
mann 
and 
Lang- 
bein. 


Thompson-Stohmann  Method. 


Stoh- 
mann 
and 
Asso- 
ciates. 


Dani- 

lewski. 


Rub- 
ner. 


Albuminoids,  etc. 

Gluten  

Elastin , 

Plant  fibrin 

Serum  albumin 

Syntonin . 

Hemoglobin 

Milk  casein 


5990.3 


Yolk  of  egg 

Legumin 

Vitellin   

Egg  albumin 

Muscle,  extractives  and  fat  re- 
moved   

Crystallized  albumin 

Muscle,  fat  removed 


Blood  fibrin   

Harnack's  albumen  . . 

Wool 

Congluten 

Fibrin  of  skin 

Peptone  

Fish  glue 

Chondrin 

Ossein 

Fibroin  

Chitin 

Tunicin 

Paraglobulin 

Amids,  etc. 

Urea 

Glycocoll 

Alanin 

Leucin 

Sarkosin 

Hippuric  acid 

Aspartic  acid 

Tyrosin 

Asparagin  

Kreatin  (cryst.) 

"        (water-free)  . 

Uric  acid 

Guanin 

Caffein 


5832.3 


5961 
5941 
5917 
5907 
5885 
5867 
5849 
8112.4  5840 
5793 
5745 
5735 


5910 
5626.4 


5780 . 6 
5687.4 


5728.4 


5529 . 1 


5564.2 


5720 
5672 
5662 
5640 
5637 
5553 
5510 
5479 
5355 
5298. 


5240 . 1 

5342.4 

5410.4 

5095.7 

4655 

4146.8 


2530.1 
3133.6 
4370.7 
6536.5 


5659.3 
2911.1 
5915.9 
3396.8 


5130 
5039. 
4979. 
4650, 


2754 


2541. 
3129 
4355. 
6525. 
4505. 
5668. 
2899 

3514 
3714. 

'4275. 
2749. 
3891. 
5231 . 


5717 


6141 
6231 


5785 
5573 


5579 


5598 
5324 


5511 
5362 


5950 


5778* 
5656* 


5637 


2465 
3053 


5642 
3428 


5709 


5069 
5493 
4909 


2537 


2523 


(3206) 


2621 


Calculated  ash-free. 


238 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


HEATS  OF   COMBUSTION   OF  ORGANIC 

SUBSTANCES   {Continued). 

Berthelot 
Method. 

Thompson-Stohmann  Method. 

Berthe- 
lot and 
Asso- 
ciates. 

Stoh- 
mann 
and 
Lang- 
bein. 

Stoh- 
mann 
and 
Asso- 
ciates. 

B. 

Dani- 
lewski. 

Rub- 

ner. 

Gibson. 

Fats. 
1    Animal: 

9476.9 
9485 . 7 
9493 . 6 

9380 
9357 
9406 
9410 
9330 
9345 
9324 
9398 
9192 

9686 

9423 

9515 

9427 

9530 

Fat  of  dog 

9215.8 

9185 

10001 

2.  Vegetable: 

9328 
9471 
9442 
9489 
9619 
(9130 
{9467 

3695 

3659 
3692 

3866 
3877 
3663 

9471 

Carbohydrates,  etc. 
1.  Pentoses: 

3714 
3739.9 

3722 

3746 
4340.9 
4379.3 
3909.2 

3714.5 
3721.5 
3742.6 
3755 

3955.2 

3951.5 

3736.8 

3949.3 

3721.8 

3947 

3550.3 

4020.8 
3400.2 
3913.7 

Xylose 

2.  Hexoses: 

3762 

3754 

4001 

3.  Heptoses: 

3732.8 
3961.7 

4.  Disaccharids : 

3921 

Milk       "                

3777.1 

3710 

5.  Trisaccharids: 

4020 

Melezit  ose 

METHODS   OF  INVESTIGATION. 


!39 


HEATS  OF  COMBUSTION  OF   ORGANIC  SUBSTANCES   (Coru 

inued). 

| 

Berthelot 
Method. 

Thompson-Stohmann  Method. 

Berthe- 
lot and 
Asso- 
ciates. 

Stoh- 
mann 
and 
Lang- 
bein. 

Stoh- 
mann 
and 
Asso- 
ciates. 

B. 

Dani- 
lewski. 

Rub- 
ner. 

Gibson. 

6.  Polysaccharids: 

4190.6 
4185.4 
4182.5 
4112.3 
4133.5 

4112.4 
3997.8 
3679.6 

9352.9 

4146 
4123 

4070 

4317 

3908 

9226 
9429 

1960 
3019 
1745 
2397 

Cellulose 

4200 
4228 
4180.4 
4187.1 

7068 

4164 

Dextran 

Inulin 

Alcohols. 
Ethyl  alcohol 

4001.2 
3676.8 

3490.4 

3959 

Inosite 

Acids. 
Acetic  

Oleic 

9494.9 

Malonic 

Succinic 

1998.2 
3006.2 

Citric 

2477.9 

parison.  Just  as  the  chemical  analysis  shows  the  total  amounts 
of  various  substances  or  classes  of  substances  present,  so  the  heat 
of  combustion  shows  the  total  amount  of  potential  energy  which 
has  been  stored  up  in  the  feeding-stuff.  In  both  cases  the  knowl- 
edge thus  acquired  must  be  combined  with  data,  secured  in  an 
entirely  different  way,  as  to  the  availability  of  these  ingredients  or 
this  energy  before  we  can  form  a  judgment  as  to  the  relative  values 
to  the  animal. 

Computation  of  Heats  of  Combustion. — The  heat  of  com- 
bustion of  a  mixture  of  various  organic  substances,  such  as  are 
contained  in  ordinary  foods  and  feeding-stuffs,  is  equal  to  the  sum  of 
the  heats  of  combustion  of  the  single  ingredients.  If  the  latter  are 
known  we  may  obtain  the  heat  of  combustion  of  the  material  in 
question  either  by  a  direct  calorimetric  determination  or  by  deter- 
mining chemically  the  proportions  of  the  several  ingredients  and 
multiplying  the  amount  of  each  into  its  known  heat  of  combustion. 


240  PRINCIPLES   OF  ANIMAL   NUTRITION. 

The  first  method,  when  available,  is  obviously  to  be  preferred, 
and  is  to  be  regarded  as  indispensable  in  all  exact  investigations 
into  the  energy  relations  of  the  food  of  man  or  of  animals.  With 
materials  whose  proximate  composition  is  fairly  well  known,  how- 
ever, the  agreement  between  the  computed  and  the  actual  heat  of 
combustion  is  very  close,  as  has  been  shown  by  Wiley  &  Bigelow  * 
and  Slosson  f  for  hulled  cereals  and  cereal  products.  Atwater  & 
Bryant,  in  their  publication  just  referred  to,  have  discussed  this 
question  very  fully  in  relation  to  human  foods  and  have  proposed 
a  series  of  factors  for  the  ingredients  of  the  various  classes  of  foods 
by  whose  use  they  obtain  a  most  satisfactory  agreement  with  the 
calorimetric  results. 

On  the  other  hand,  in  case  of  vegetable  products  containing 
much  woody  and  fibrous  material  the  actual  heat  of  combustion 
is  higher  than  that  computed  under  the  ordinary  interpretation  of 
the  results  of  chemical  analysis.  Thus  the  actual  heat  of  combus- 
tion of  unhulled  oats  was  found  by  Wiley  &  Bigelow  to  be  about 
4.5  per  cent,  higher  than  the  computed  value,  and  Merrill  \  has 
obtained  similar  results  for  wheat  middlings  and  bran  and  for  hay 
and  silage.  This  obviously  arises  from  the  presence  among  the 
ill-known  bodies  constituting  the  so-called  lignin  and  incrusting 
substances  of  compounds  having  higher  heats  of  combustion  than 
the  common  carbohydrates.  It  is  not  impossible  that  a  series  of 
factors  similar  to  those  of  Atwater  &  Bryant  might  be  worked  out 
for  different  classes  of  stock  foods,  so  that  their  heats  of  combustion 
might  be  computed  from  their  chemical  composition.  In  view, 
however,  of  the  comparative  ease  and  rapidity  with  which  direct 
calorimetric  results  can  be  accumulated  it  may  be  doubted  whether 
such  an  undertaking  would  repay  the  labor  involved. 

Methods  have  also  been  proposed  and  somewhat  extensively 
used  for  computing  the  heat  of  combustion  of  the  digested  portion 
of  the  food.  This  phase  of  the  subject,  however,  can  be  more 
profitably  considered  later. 

The  Energy  of  the  Excreta. — For  the  visible  excreta  (feces 
and  urine)  substantially  the  same  method  is  available  as  for  the  food, 

*  Jour.  Am.  Chem.  Soc,  20,  304. 
t  Wyoming  Expt.  Station,  Bull.  33. 
%  Maine  Expt.  Station,  Bull.  67,  p.  169. 


METHODS  OF  INVESTIGATION.  241 

viz.,  a  determination  of  the  heat  of  combustion.  An  element  of 
uncertainty,  however,  which  is  ordinarily  not  met  with  in  the  case 
of  the  food,  arises  from  the  ready  decomposability  of  the  excretory 
products,  which  is  liable  to  result  in  a  loss  of  energy  during  the 
drying  necessary  to  prepare  them  for  combustion.  The  urea  of 
the  urine,  in  particular,  is  very  readily  converted  into  the  volatile 
ammonium  carbonate.  Comparative  determinations  of  nitrogen 
in  the  fresh  and  in  the  dried  urine  will  show  the  amount  of  nitrogen 
lost  in  drying,  and  on  the  assumption  that  only  urea  is  decomposed 
the  loss  of  energy  can  be  readily  computed  from  the  known  heat  of 
combustion  of  that  substance.  Atwater  &  Benedict  *  have  found 
this  assumption  to  be  substantially  correct  for  human  urine,  and 
the  same  may  be  presumed  to  be  the  case  with  the  urine  of  carniv- 
ora.  It  has  usually  been  assumed  to  be  applicable  also  to  the 
more  complex  urine  of  herbivora,  although  without,  so  far  as  the 
writer  is  aware,  any  experimental  proof. 

A  greater  or  less  loss  of  nitrogen  has  also  been  observed  in  the 
drying  of  the  feces  of  domestic  animals,  particularly  of  horses  and 
sheep,  but  the  nature  of  the  material  decomposed  has  not  yet  been 
investigated,  and  the  same  is  true  of  the  possible  decomposition  of 
non-nitrogenous  materials  in  both  urine  and  feces.  Atwater  & 
Benedict  (loc.  cit.)  found  the  loss  of  nitrogen  from  human  feces  to 
be  insignificant. 

Computation  of  Energy. — The  computation  of  the  energy  of 
the  visible  excreta  is  much  less  satisfactory  than  in  the  case  of  the 
food  on  account  of  our  inferior  knowledge  of  the  proportions  and 
chemical  nature  of  their  ingredients. 

The  Urine. — Formerly  the  urine  was  assumed  to  be  substan- 
tially an  aqueous  solution  of  urea,  and  numerous  computations  of 
its  energy  content  were  made  on  this  basis,  particularly  in  connec- 
tion with  estimates  of  the  metabolizable  energy  of  the  proteids, 
while  the  same  method  has  been  applied  also  in  estimating  the 
metabolizable  energy  of  feeding-stuffs.  Rubner  f  was  the  first  to 
demonstrate  the  serious  nature  of  the  error  involved  in  this  assump- 
tion and  to  show  that  the  energy  of  the  urine  is  materially  greater 
than  the  amount  thus  computed.      In  the  urine  of  the  dog  he  found 

*  U.  S.  Dept.  Agr.,  Office  of  Experiment  Stations,  Bull.  69,  p.  22. 

t  Zeit.  f.  Biol.,  20,  265;  21,  250  and  337;  42,302.     Compare  Chapter  X. 


242 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


the  energy  content  to  be  from  6.7  to  8.5  Cals.  per  gram  of  nitrogen 
in  place  of  5.4  Cals.  as  computed  on  the  assumption  that  only  urea 
was  present,  while  for  human  urine  he  has  obtained  values  ranging 
from  6.42  Cals.  to  8.87  Cals.  per  gram  of  nitrogen,  and  Tangl  * 
has  reported  even  higher  figures. 

Kellner  f  has  shown  that  the  difference  is  still  greater  in  the 
urine  of  an  ox  receiving  only  coarse  fodder,  the  actual  energy  being 
about  six  times  that  computed  on  the  above  assumption  and  nearly 
175  per  cent,  of  that  computed  after  allowing  for  the  hippuric  acid 
present.  In  subsequent  investigations  %  he  finds  that  the  energy 
content  of  the  urine  of  cattle  is  much  more  nearly  proportional  to 
its  carbon  than  to  its  nitrogen,  being  approximately  10  Cals.  per 
gram  of  carbon. 

In  six  cases  reported  by  Atwater  k  Benedict  §  in  the  course  of 
their  investigations  with  the  respiration-calorimeter,  the  amount  of 
energy  found  in  human  urine  from  a  mixed  diet  as  compared  with 
that  computed  from  the  nitrogen  reckoned  as  urea  was: 


Total 

Nitrogen, 

Grms. 

Energy. 

Actual, 
Cals. 

Computed, 
Cals. 

Experiment  No. 

5 

72.43 
64.29 
70.60 
77.90 
71.72 
77.76 

511 
504 
569 
658 
597 
589 

392 

6  .  .                

348 

It                 a 

7                        

382 

it                 U 

8                      

421 

tt                 « 

9 

388 

U                 << 

10 

421 

Here,  too,  it  is  evident  that  a  computation  on  the  basis  of  the 
urea  yields  results  much  below  the  truth,  and  later  experiments  by 
the  same  authors  have  fully  confirmed  this  result. 

The  Feces. — Our  knowledge  of  the  proximate  principles  con- 
tained in  the  feces  is  so  small  that  no  satisfactory  computation  of 
their  energy  content  is  possible,  except  perhaps  in  the  case  of  car- 
nivora  on  a  purely  meat  diet,  where  the  total  amount  of  feces  is 


*  Arch.  f.  (Anat.  u.)  Physiol.,  1899,  p.  261. 

t  Landw.  Vers.  Stat.,  47,  275. 

%  Ibid.,  53,  437. 

§  U.  S.  Dept.  Agr.,  Office  of  Experiment  Stations,  Bull.  63. 


METHODS  OF  INVESTIGATION.  243 

small.  On  a  mixed  diet  containing  any  considerable  proportion  of 
vegetable  matter,  and  particularly  in  the  case  of  herbivorous  ani- 
mals consuming  large  amounts  of  coarse  fodders,  only  an  actual 
determination  of  the  heat  of  combustion  can  be  depended  upon. 
Since  the  feces  of  these  animals  contain  a  larger  proportion  of  the 
indigestible  lignin,  etc.,  than  does  their  food,  the  heat  of  combustion 
of  the  feces  is  correspondingly  higher,  but  its  actual  value  must 
obviously  depend  to  a  considerable  degree  on  the  character  of  the 
food. 

Combustible  Gases. — Since  it  is  impracticable  to  collect  sepa- 
rately the  combustible  intestinal  gases,  we  must  of  necessity  com- 
pute the  amount  of  potential  energy  carried  off  in  these  substances. 
This  computation  has  been  based  on  the  amount  of  carbon  con- 
tained in  these  gases,  determined  in  the  manner  indicated  on  p.  72, 
upon  the  assumption  that  only  methane  (CH4)  was  present.  It 
has  been  shown  that  this  gas  exists  in  considerable  amounts  in 
the  digestive  tract  of  herbivora,  and  it  is  probable  that  the  above 
assumption  is  substantially  accurate,  although  a  small  amount  of 
hydrogen  has  been  found  by  some  observers.  In  experiments  by 
Fries,*  at  the  Pennsylvania  Experiment  Station,  in  which  both 
the  carbon  and  hydrogen  of  the  combustible  gases  excreted  by  a 
steer  consuming  chiefly  timothy  hay  were  determined,  the  follow- 
ing ratios  of  hydrogen  to  carbon  were  obtained: 

Period  A. 
First  day,  1:2.900 

Second  day,  1:2.916 

Period  B. 
First  day,  1:2.978 

Second  day,  1:2.947 

Period  C. 
First  day,  1:2.899.' 

Second  day,  1:2.951 

Period  D. 
First  day,  1:3.051 

Second  day,  1:3.096 

Average,  1:2.967 

Computed  for  CH4,    1:2.976 
*  Proc.  Soc.  Prom.  Ag.  Sci.,  1902. 


244  PRINCIPLES   OF  ANIMAL   NUTRITION. 

These  results  tend  strongly  to  substantiate  the  belief  that  the 
combustible  gases  practically  consist  of  methane  only. 

Perspiration. — In  view  of  the  relatively  minute  amoimts  of 
organic  matter  contained  in  the  perspiration  it  has  generally  been 
regarded  as  a  negligible  quantity.  The  data  given  on  p.  48  for 
the  nitrogen  of  the  sensible  perspiration  would  afford  some  approxi- 
mate data  for  computing  the  amount  of  energy  contained  in  it. 

The  Energy  of  Tissue  Gained. — The  amount  of  potential  energy 
stored  up  in  a  gain  of  tissue,  or  the  amount  liberated  in  the  kinetic 
form  in  case  the  gain  is  negative,  cannot,  of  course,  be  made  the 
subject  of  a  direct  determination.  The  amounts  of  protein  and  of 
fat  gained  or  lost  can,  however,  be  determined  by  the  methods 
described  in  Chapter  III,  and  their  energy  content  computed  from 
average  figures.  The  errors  involved  are  those  incident  to  the 
method  of  computation  from  the  carbon  and  nitrogen  balance, 
which  have  already  been  considered  in  the  chapter  cited,  and  those 
arising  from  uncertainty  as  to  the  exact  energy  content  of  the 
material  gained  by  the  body. 

Protein. — Just  as  computations  of  the  gain  or  loss  of  protein 
by  the  body  have  been  based  upon  the  average  composition  of  the 
proteids,  so  computations  of  its  energy  content  have  been  based 
upon  the  average  heat  of  combustion  of  these  substances.  The 
compilation  by  Atwater  on  pp.  237-9  contains  the  available  data 
up  to  1894. 

For  approximate  computations  the  value  5.7  Cals.  per  gram  has 
been  commonly  used,  while  in  more  exact  computations  it  has 
been  assumed  that  the  gain  of  protein  by  the  animal  has  substan- 
tially the  heat  value  as  well  as  the  chemical  composition  of  fat-free 
muscular  tissue  (see  p.  63),  and  the  average  of  Stohmann's  two 
determinations,  viz.,  5.652  Cals.  per  gram,  has  been  employed. 
Kohler's  investigation*  of  the  composition  of  fat  and  ash-free 
muscular  tissue  (p.  64)  included  determinations  of  the  heats  of 
combustion  which  are  reproduced  on  the  opposite  page. 

Fat. — Rubner,  in  his  computations,  employs  the  round  number 
9.4  Cals.  per  gram  for  fat,  while  Kellner  uses  the  value  9.5  Cals. 
Benedict  &  Osterberg,f  whose  determinations  of  the  composition  of 

*  Zeit.  physiol.  Chem.,  81,  479. 
t  Amer.  Jour.  Physiol.,  4,  69. 


METHODS  OF  INVESTIGATION. 


245 


No.  of 
Samples. 

Heat  of  Combus- 
tion per  Gram, 
Cals. 

Cattle 

4 
2 
2 
3 
2 
2 

5 . 6776 

5 . 6387 

5.6758 

5.5990* 

Rabbit 

5.6166 

Hen 

5.6173 

human  fat  are  given  on  p.  61,  found  for  the  heat  of  combustion  of 
the  same  twelve  samples  values  ranging  from  9.474  Cals.  to  9.561 
Cals.  per  gram,  the  average  being  9.523  Cals.  Other  results  are 
noted  in  the  table  on  pp.  237-9. 


Determination  of  Kinetic  Energy. 

Mechanical  Work. — The  energy  of  the  mechanical  work  done 
by  the  animal  upon  its  surroundings  is  derived,  as  was  seen  in  Part 
I,  immediately  from  body  materials  and  mediately  from  the  food, 
and  is  one  of  the  two  forms  in  which  kinetic  energy  leaves  the  body. 

The  energy  of  the  mechanical  work  done  by  the  animal  may  be 
measured  in  various  ways,  the  consideration  of  which  belongs 
to  the  domain  of  mechanics  and  lies  outside  the  scope  of  the 
present  work.      In  general  two  classes  of  appliances  have  been  used : 

First,  dynamometers  proper,  in  which  the  work  is  expended  in 
overcoming  a  known  resistance,  produced  either  by  friction  or  by 
a  magnetic  field,  the  work  done  being  measured  by  the  tension  of  a 
spring  or  by  the  amount  of  electric  energy  produced. 

Second,  the  tread  power,  in  which  the  work,  aside  from  that  of 
locomotion,  consists  in  lifting  the  body  vertically  and  is  propor- 
tional to  the  product  of  the  mass  of  the  body  into  the  distance 
through  which  it  is  raised. 

Heat. — The  second  form  in  which  kinetic  energy  leaves  the 
body  is  heat.  In  an  animal  doing  no  work  all  the  energy  arising 
from  the  metabolism  in  the  body  ultimately  takes  this  form,  and 
even  when  mechanical  work  is  done  the  larger  share  of  the  outgo 
of  kinetic  energy  consists  of  heat.  Part  of  this  heat  is  imparted  to 
the  surroundings  of  the  animal  by  conduction  and  radiation  and  a 

*  Contained  an  average  of  3 .  65  per  cent,  glycogen. 


246  PRINCIPLES   OF  ANIMAL   NUTRITION. 

part  is  expended  in  the  evaporation  of  water  from  skin  and  lungs 
and  takes  the  form  of  the  latent  heat  of  water  vapor. 

Animal  Calorimeters. — The  direct  determination  of  the  heat 
produced  by  an  animal,  especially  a  large  animal,  is  not  an  easy 
task.  It  requires  in  the  first  place  a  calorimeter  large  enough  to 
contain  the  animal  and  in  the  second  place,  for  experiments  of  any 
length,  the  maintenance  of  a  sufficient  ventilating  current  of  air 
under  such  conditions  as  shall  not  affect  the  accuracy  of  the  calori- 
metric  determination,  while  the  latent  heat  of  the  water  vapor 
carried  out  in  the  air-current  must  also  be  taken  account  of.  In 
other  words,  such  an  apparatus  must  be  at  once  a  respiration  appa- 
ratus and  a  calorimeter,  and  hence  the  name  respiration-calorimeter 
has  come  to  be  applied  to  it.  Various  forms  of  animal  calorimeters 
have  been  devised,  some  of  which  may  be  briefly  mentioned. 

Lavoisier  &  Laplace,*  in  their  investigations  upon  the  origin  of 
animal  heat,  employed  an  ice-calorimeter,  in  which  the  heat  is 
measured  by  the  amount  of  ice  melted.  Crawford  t  investigated 
the  same  subject  using  a  water-calorimeter,  as  did,  later,  Dulong  \ 
and  Despretz,§  while  more  recently  Wood,||  and  still  later 
Reichert,^[  have  also  employed  the  water-calorimeter. 

The  ice-calorimeter,  however,  necessarily  subjects  the  animal 
to  an  abnormally  low  temperature,  while  with  the  water-calorim- 
eter it  has  been  found  very  difficult  to  secure  a  uniform  heating  of 
the  different  strata  of  water.  These  facts  led  to  the  employment 
of  air  as  the  calorimetric  substance,  the  heat  being  measured  either 
by  the  increase  in  the  volume  of  a  confined  body  of  air  at  a  constant 
pressure  or  the  increase  in  the  pressure  at  constant  volume,  and 
until  quite  recently  the  most  exact  methods  have  been  based  on 
this  principle. 

Scharling,**  Vogel,tt  and  Him, %\  between  1849  and  1864,  used 

*  Hist.  Acad.  Roy.  d.  Sc,  1780,  355. 

t  Experiments  and  Observations  on  Animal  Heat.     London,  1788. 

%  Ann.  de  Chim.  et  de  Phys.  (3),  1,  440. 

§  Ibid.,  (2),  26,  337. 

||  Smithsonian  Contributions,  1880. 

f  Univ.  Med.  Mag.,  Phila.,  2,  173. 

**Jour.  pr.  Chem.,  48,  435. 

ft  Arch.  d.  Ver.  f.  Wiss.  Heilk.,  1864,  p.  442. 

XX  Recherches  sur  Equivalent  mdchanique  de  la  chaleur.     Paris,  1858. 


METHODS  OF  INVESTIGATION.  247 

crude  forms  of  the  air-calorimeter.  In  1885  Richet  *  described  an 
air-calorimeter  for  small  animals,  the  heat  being  measured  by  the 
increase  in  the  volume  of  a  confined  portion  of  air  at  constant  press- 
ure. His  experiments  were  of  short  duration  (1  to  1^  hours)  and 
no  specific  statement  is  made  regarding  ventilation  and  no  mention 
of  any  determinations  of  the  latent  heat  of  the  water  vapor.  - 

In  1886  d'Arsonval  t  described  a  differential  air-calorimeter, 
and  in  1890  J  two  other  forms  of  animal  calorimeter,  the  first  being 
a  water-calorimeter  of  constant  temperature  with  automatic  regu- 
lation of  the  flow  of  water,  for  which  a  high  degree  of  accuracy  is 
claimed,  and  the  second  an  air-calorimeter,  but  he  reports  no  ex- 
periments with  either  form.  In  the  same  year  Laulanie  §  (see  p.  70) 
described  briefly  a  Regnault  respiration  apparatus  which  was  also 
used  as  a  calorimeter,  and  has  subsequently  reported  some  results 
obtained  by  its  use. 

One  of  the  best  known  forms  of  animal  calorimeter  is  that  of 
Rubner.||  This  is  essentially  a  Pettenkofer  respiration  apparatus, 
the  walls  of  the  chamber  being  double  and  the  whole  surrounded 
by  an  air  space  which  in  its  turn  is  surrounded  by  a  jacket  con- 
taining water  kept  at  a  constant  temperature.  The  amount  of 
heat  given  off  to  the  calorimeter  is  measured  by  the  expansion 
under  constant  pressure  of  the  confined  volume  of  air  between  the 
two  walls  of  the  respiration  chamber,  while  from  comparative  de- 
terminations of  moisture  in  the  ingoing  and  outcoming  air  the  heat 
removed  in  the  latent  form  is  computed. 

Rosenthal  *[[  has  constructed  a  somewhat  similar  instrument  in 
which  the  respiratory  portion  is  a  Regnault  apparatus,  while  the 
heat  is  measured  by  the  increase  in  pressure  of  the  air  at  constant 
volume,  instead  of  by  the  increase  in  its  volume  as  in  Rubner's 
apparatus.  Both  instruments  are  therefore  air-calorimeters,  and  the 
numerical  values  of  their  readings  must  be  determined  experimen- 
tally for  each  instrument.  These  two  forms  of  apparatus  are  of  a  size 
sufficient  for  experiments  with  small  animals  (rabbits  or  small  dogs). 

*  Archives  de  Physiol ,  1885,  II,  237. 

t  Jour,  de  l'Anat.  et  Physiol.,  1886. 

%  Archives  de  Physiol.,  1890,  pp.  610  and  781. 

§Ibid.,  p.  571. 

||  Calormetrische  Methodik,  Marburg,  1891 ;  Zeit.  f.  Biol.,  30,  91. 

\  Arch.  f.  (Anat.  u.)  Physiol.,  1894,  p.  223. 


248  PRINCIPLES   OF  ANIMAL   NUTRITION. 

In  1894  Haldane,  White  &  Washbourne  *  described  a  form  of  air- 
calorimeter  in  which  the  expansion  caused  by  the  heat  produced  by 
the  animal  in  one  chamber  is  balanced  by  that  produced  by  a  flame 
of  hydrogen  burning  in  a  second  similar  chamber.  The  calorimeter 
is  essentially  one  of  constant  volume,  but  the  heat  is  computed 
from  the  amount  of  hydrogen  burned. 

Laulanie  f  in  1895  described  a  Pettenkofer  apparatus  with  small 
ventilation  (see  p.  71)  which  served  also  as  an  air-calorimeter,  and 
still  later  J  has  described  a  differential  water-calorimeter.  Kauf- 
mann,§  as  mentioned  on  p.  69,  has  determined  the  respiratory 
exchange  of  animals  during  short  periods  in  a  confined  volume  of 
air.  The  apparatus  consisted  simply  of  a  zinc  receptacle  which 
served  also  as  a  radiation  calorimeter.  The  internal  temperature 
and  that  of  the  surrounding  air  were  measured  by  recording  ther- 
mometers and  the  loss  of  heat  calculated  according  to  Newton's 
law.  The  atmosphere  in  the  apparatus  was  saturated  with  water- 
vapor  at  the  start,  so  that  the  moisture  excreted  by  the  animal  was 
condensed  and  no  correction  for  the  heat  of  vaporization  was  neces- 
sary. 

By  far  the  most  important  form  of  respiration-calorimeter  yet 
devised,  however,  not  only  as  regards  accuracy  but  particularly 
in  view  of  the  range  of  work  of  which  it  is  capable,  is  that  of  Atwater 
&  Rosa,|  the  respiratory  part  of  which  has  already  been  mentioned 
(pp.  72  and  79).  In  this  apparatus  water  is  used  as  the  calori- 
metric  substance,  but  in  the  form  of  a  constant  current  instead  of  a 
large  stationary  mass.  As  described  by  the  authors  the  appara- 
tus consists  of  a  Pettenkofer  respiration  apparatus  provided  with 
special  devices  for  the  accurate  measurement,  sampling,  and  analy- 
sis of  the  air-current.  A  current  of  cold  water  is  led  through  copper- 
absorbing  pipes  near  the  top  of  the  respiration  chamber  and  takes 
up  the  heat  given  off  by  the  subject.  The  volume  of  the  water  used 
being  measured,  and  its  temperature  when  entering  and  leaving 
being  taken  at  frequent  intervals,  the  amoimt  of  heat  brought  out 

*  Jour.  Physiol.,  16,  123. 

t  Archives  de  Physiol.,  1895,  p.  619. 

t  Ibid.,  1898,  pp.  538  and  613. 

§  Ibid.,  1896,  p.  329. 

||  U.  S.  Dept,  Agr.,  office  of  Experiment  Stations,  Bulletins  63  and  69. 


METHODS  OF  INVESTIGATION.  249 

in  the  water-current  is  readily  calculated.  To  this  is  added  the 
latent  heat  of  the  water-vapor  brought  out  in  the  ventilating  air- 
current.  By  means  of  ingenious  electrical  devices,  a  description 
of  which  would  occupy  too  much  space  here,  the  temperature  of  the 
interior  of  the  apparatus  is  kept  constant,  and  any  loss  of  heat  by 
radiation  through  the  walls  or  in  the  air-current  is  prevented.  In 
test  experiments  the  apparatus  has  given  extremely  accurate  re- 
sults. 

An  especial  advantage  of  this  apparatus  is  that  it  is  practicable 
to  make  it  of  large  size,  and  also  to  continue  the  experiments  for  an 
indefinite  length  of  time.  The  original  apparatus  was  of  a  size 
sufficient  for  experiments  on  man,  while  all  previous  forms  were 
restricted  to  experiments  on  small  animals.  Recently  a  modified 
Atwater-Rosa  apparatus  has  been  completed  under  the  writer's 
direction  at  the  Pennsylvania  Experiment  Station,  with  the  co- 
operation of  the  Bureau  of  Animal  Industry  of  the  United  States 
Department  of  Agriculture,  of  a  size  sufficient  for  investigations 
with  cattle,  and  still  larger  ones  are  in  process  of  construction. 

Computation  of  Heat  Production. — The  respiration-calorim- 
eter, in  its  more  perfected  forms,  is  a  complicated  and  costly  appara- 
tus both  in  construction  and  use,  and,  moreover,  is  a  rather  recent 
development.  It  was  natural,  therefore,  that  attempts  should  be 
made  to  determine  the  heat  production  indirectly  by  computations 
based  on  the  kind  and  amount  of  matter  oxidized  in  the  body. 

We  may  conveniently  distinguish  three  distinct  although  closely 
related  methods  of  attacking  the  problem,  all  of  which  assume  as  a 
fundamental  postulate  that  the  oxidation  of  a  given  substance  in 
the  body  liberates  the  same  amount  of  energy  as  does  its  oxidation 
outside  the  organism.  In  the  next  chapter  we  shall  examine  into 
the  correctness  of  this  postulate;  for  the  present  we  are  con- 
cerned simply  with  the  methods  of  computation  based  on  it. 

Computation  from  Gaseous  Exchange. — From  a  knowledge  of  the 
ultimate  composition  and  heat  of  combustion  of  a  substance  it  is 
easy  to  compute  the  amount  of  heat  which  will  be  produced  by  the 
oxidation  of  an  amount  of  it  sufficient  to  yield  a  unit  of  carbon 
dioxide  or  to  consume  a  unit  of  oxygen.  Conversely,  then,  we  can 
compute  from  the  carbon  dioxide  evolved  or  the  oxygen  consumed 
in  a  given  time  the  corresponding  amount  of  energy  liberated. 


25° 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


Such  computations  have  been  made  by  different  authors  for  the 
three  principal  classes  of  nutrients,  viz.,  the  proteids,  carbohydrates, 
and  fats,  the  results  of  a  few  of  which  are  as  follows: 


Magnus- 
Levy.* 

Zuntz.t 

Kaufmann.t       Laulanie\§ 

Per 

Liter 
COo 
Cals. 

Per 
Liter 

o., 

Cals. 

Per 

Liter 
CO, 
Cals. 

Per 

Liter 

O, 

Cals. 

Per 
Liter 
COa 
Cals. 

Per       Per 
Liter     Liter 

o,      co2 

Cals.  |   Cals. 

Per 

Liter 

0, 

Cafe. 

5.464 
6.586 
4.915 
4.976 
5.090 

4.289 
4.676 
4.915 
4.976 
5.090 

5.644 
6.628 

4.476  5.569 
4.686  6  648 

4.647  

4.650  6.571 
5.056 

4.95 

4.6 

Fat 

4.6 

5.056 

Starch  

5.047 

5.047 

"  Carbohydrates" 

4.95 

1 

Kaufmann  also  computes  from  his  theoretical  equations  already 
given  in  Part  I  (pp.  38  and  51)  the  evolution  of  heat  per  liter  of 
oxygen  in  the  various  processes  of  partial  oxidation  which  he  be- 
lieves to  take  place  in  the  body,  with  the  following  results : 

Albumen  to  fat  and  urea 4 .  646  Cals. 

"         "  dextrose  and  urea 4.460    " 

Fat  (stearin)  to  dextrose 4.067    " 

Disregarding  the  minor  differences  in  the  figures  of  different 
authorities,  it  is  evident  that  the  amount  of  heat  produced  bears  a 
much  more  constant  relation  to  the  oxygen  consumed  than  to  the 
carbon  dioxide  produced.  For  the  fats  and  proteids,  especially,  the 
difference  is  comparatively  small.     In  the  case  of  an  animal  metab- 

*  Arch.  ges.  Physiol.,  55,  9. 

t  Ibid.,  68,  191. 

X  Archives  de  Physiol.,  1896,  pp.  329,  342,  757. 

§  Ibid.,  1898,  p.  748. 

||  As  pointed  out  on  pp.  74-75  the  determination  of  the  respiratory  exchange 
corresponding  to  a  unit  of  proteids  is  not  a  simple  matter.  In  the  table 
Kaufmann's  and  Laulanie's  figures  are  based  upon  the  theoretical  equation 
(p.  75)  for  the  conversion  of  albumin  into  carbon  dioxide,  water,  and  urea, 
while  those  of  Magnus-Levy  and  Zuntz  are  derived  largely  from  determina- 
tions and  estimates  by  Rubner  (Zeit.  f.  Biol.,  21,  363)  and  others  of  the 
proximate  composition  of  the  urine  of  meat-fed  animals.  As  will  appear 
later,  these  figures  are  not  applicable  to  the  urine  of  herbivora, 


METHODS  OF  INVESTIGATION. 


:5i 


olizing  substantially  proteids  and  fat,  then,  such  as  a  fasting  animal 
or  one  consuming  only  those  two  nutrients,  a  determination  by  any 
of  the  methods  indicated  in  Chapter  III  of  the  amount  of  oxygen 
consumed  will  afford  the  basis  for  at  least  an  approximately  correct 
computation  of  the  energy  liberated  during  the  same  time,  par- 
ticularly when,  as  is  often  the  case,  the  proteid  metabolism  consti- 
tutes but  a  small  proportion  of  the  total  metabolism.  For  the 
carbohydrates  the  figures  are  somewhat  higher,  and  where  these 
bodies  constitute  a  considerable  portion  of  the  food  the  error  will 
be  more  serious,  but  even  then  the  results  will  be  of  value  and 
especially  will  afford  relatively  correct  figures  for  the  heat  produc- 
tion on  the  same  diet  at  different  times. 

The  computation  from  the  gaseous  exchange  of  the  amount  of 
energy  liberated  assumes  a  more  exact  form  in  case  it  is  desired 
to  determine  the  increment  arising  from  some  change  in  the 
conditions  of  the  experiment,  notably  from  an  increase  in  the 
muscular  work  done.  In  the  latter  case,  as  we  have  seen  (Chap- 
ter VI),  the  increased  metabolism  is  largely  or  wholly  that  of  non- 
nitrogenous  matter.  Such  being  the  case,  we  can  compute  in  the 
manner  indicated  on  p.  76  from  the  increments  of  carbon  dioxide 
and  oxygen  caused  by  the  work  the  proportion  of  each  gas  corre- 
sponding respectively  to  the  oxidation  of  fat  and  of  carbohydrates, 
and  from  this  it  is  easy  to  compute  the  corresponding  amounts  of 
energy.  Thus,  to  take  the  example  from  Zuntz's  investigations 
there  given,  the  increments  of  oxygen  and  of  carbon  dioxide  pro- 
duced by  the  performance  of  1  kgm.  of  work  in  the  case  of  a  dog 
were  computed  to  be  divided  as  follows: 


Oxygen 

Consumed, 
c.c. 

Carbon  Dioxide 

Produced, 

c.c. 

By  fat 

0.6939 
0.9765 

0.4905 

0.9765 

Total 

1.6704 

1.4670 

From  this,  using  Zuntz's  factors  and  assuming  that  there  was  no 
change  in  the  proteid  metabolism,  the  total  excess  of  energy  liber- 
ated in  the  body  during  work  over  that  metabolized  during  rest  is 
computed  as  follows: 


252  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Energy  from  fat 4.686  cals.  X  0.6939  =  3.252  cals. 

"       carbohydrates  ...  5 .  047  cals.  X  0 .  9765  =  4 .  927     < 


Total 8.179      " 

It  is  obvious  that  this  method  of  computation  affords  the  means 
of  comparing  the  total  energy  metabolized  during  the  performance 
of  a  measured  amount  of  work  with  the  quantity  recovered  in  the 
work  itself.  It  has  been  extensively  used  for  this  purpose  by  Zuntz 
and  his  associates,  especially  in  his  investigations  in  conjunction 
with  Lehmann  and  Hagemann*  upon  work  production  in  the  horse, 
which  will  be  considered  in  a  subsequent  chapter.  The  same 
authors  f  show  that  the  error  introduced  by  the  assumption  of 
unchanged  proteid  metabolism  is  too  small  to  be  of  any  significance. 
Computation  from  Total  Excreta. — The  method  just  described 
naturally  leads  up  to  a  computation  based  on  the  gaseous  exchange 
combined  with  a  determination  of  the  urinary  products,  particu- 
larly nitrogen.  The  latter  shows  the  total  amount  of  proteids 
metabolized.  If  we  also  know,  or  can  compute  with  sufficient 
accuracy,  the  carbon,  hydrogen,  and  oxygen  of  the  urinary  solids 
we  have  the  data  from  which  to  compute  the  portion  of  the  respira- 
tory exchange  due  to  the  protein  (see  p.  75)  and  the  corresponding 
amount  of  energy  liberated.  The  residues  of  carbon  dioxide  and 
oxygen  can  then  be  distributed  between  the  fats-  and  carbohy- 
drates in  the  manner  already  described.  This  method  has  been 
extensively  employed  by  Kaufmann.J  As  already  stated,  he  com- 
putes the  gaseous  exchange  of  the  proteids  on  the  assumption  of  an 
oxidation  to  carbon  dioxide,  water,  and  urea  only,  an  assumption 
which,  as  we  have  seen,  is  in  some  cases  considerably  wide  of  the 
truth. 

It  is,  of  course,  essential  that  experiments  by  this  method  shall 
cover  a  sufficient  length  of  time  to  ensure  that  the  nitrogen  excretion 
corresponds  with  the  actual  proteid  metabolism.  It  is  therefore 
inapplicable  to  periods  of  from  a  few  minutes  to  an  hour  or  so,  such 
as  have  been  generally  employed  in  experiments  based  on  the  gas- 
eous exchange  only.     KaufmamYs  experiments  extended  over  five 

*  Landw.  Jahrb.,  18.  1;   23,  125;  27,  Supp.  Ill 

f  Ibid.,  27,  Supp.  III.,  p.  251. 

t  Archives  de  Physiol.,  1896,  pp.  329,  342,  757. 


METHODS  OF  INVESTIGATION.  253 

hours,  but  it  is  open  to  serious  question  whether  such  a  period  is 
sufficiently  long. 

Rubner  *  has  made  extensive  use  of  a  method  substantially  the 
same  as  that  just  outlined,  but  differing  in  details.  The  computa- 
tion is  based  upon  the  total  nitrogen  and  carbon  (determined  or 
estimated)  of  urine,  feces,  and  respiration  for  twenty-four  (or 
twenty-two)  hours,  the  feces  being  regarded  as  substantially  a 
metabolic  product.  The  oxygen  consumption  is  not  determined. 
From  the  results  for  nitrogen  and  carbon  the  proteid  and  fat  meta- 
bolism is  computed  in  the  manner  explained  in  Chapter  III  (p.  78). 
For  each  gram  of  carbon  in  the  fat  metabolized  Rubner  reckons 
12.31  Cals.  of  energy,  equivalent  to  9.4  Cals.  per  gram  of  fat,  while 
for  each  gram  of  excretory  nitrogen  (urine  and  feces)  he  uses  an 
energy  value  based  on  previous  experiments  f  in  which  the  actual 
heats  of  combustion  of  proteids  and  the  products  of  their  meta- 
bolism were  determined.  These  results  will  be  considered  in  another 
connection  (Chapter  X).  The  resulting  values  for  the  evolution 
of  energy  corresponding  to  each  gram  of  excretory  nitrogen  are : 

Fasting  (mammals) 24 .  94  Cals. 

(birds) 24.35     " 

Leanmeatfed 25.98     ", 

Extracted  lean  meat  fed 26.66     " 

These  factors  were  obtained  in  experiments  on  dogs  and  in 
strictness  apply  only  to  carnivorous  animals.  By  their  use,  espe- 
cially if  average  figures  are  assumed  for  some  of  the  minor  quanti- 
ties, such  as  the  carbon  of  the  feces  and  urine,  the  determination 
of  the  heat  production  of  a  quiescent  animal  in  this  indirect  way 
becomes  a  relatively  simple  matter,  while  comparisons  with  direct 
calorimetric  results  have  shown  it  to  be  quite  accurate.  As  was 
pointed  out  on  p.  78,  however,  when  carbohydrates  enter  largely 
into  the  diet  the  results  are  ambiguous,  and  this  fact  as  well  as 
the  marked  differences  in  the  character  of  the  excreta  forbid  its 
application  to  herbivorous  animals. 

Cleavages.  Hydrations,  etc. — Both  the  above  methods  of  comput- 
ing the  heat  production  of  an  animal  assume  that  the  gaseous  ex- 

*  Zeit.  f.  Biol.,  19,  313;   22,  40;   30,  73. 
t  Ibid.,  21,  250  and  337. 


254  PRINCIPLES   OF  ANIMAL   NUTRITION. 

change  is  brought  about  by  what  is,  in  effect,  a  process  of  oxidation 
simply.  That  many  other  chemical  processes  take  place  in  the 
body  is,  however,  well  known,  and  Berthelot  *  in  particular  lays 
special  stress  upon  the  possibility  of  numerous  cleavages,  syntheses, 
hydrations,  and  dehydrations  in  which  the  respiratory  quotient 
may  vary  between  wide  limits  and  in  which  the  heat  production  is 
not  necessarily  proportional  to  either  the  oxygen  consumed  or  the 
carbon  dioxide  generated.  An  example  of  such  a  process  is  the 
formation  of  fat  from  carbohydrates,  which,  as  we  have  seen,  may 
be  regarded  in  the  light  of  an  intra-molecular  combustion  in  which 
no  oxygen  from  outside  is  consumed,  but  in  which  there  is  an  evolu- 
tion of  heat.  As  an  illustration  of  the  opposite  possibility — an 
evolution  of  heat  without  production  of  carbon  dioxide — Berthelot 
instances  f  the  oxidation  of  a  molecule  of  ethyl  alcohol  by  suc- 
cessive atoms  of  oxygen  to  ethyl  aldehyde,  acetic  acid,  glycollic 
acid,  oxyglycollic  acid,  oxalic  acid,  and  finally  carbon  dioxide  and 
water.  Only  in  the  last  of  these  stages  is  there  an  evolution  of 
carbon  dioxide,  yet  in  each  stage  there  is  an  evolution  of  heat  vary- 
ing from  39.9  Cals.  to  73.3  Cals.  per  atom  of  oxygen. 

But  while  the  possibility  and  even  probability  of  similar  reac- 
tions in  the  body  of  the  animal  cannot  be  denied,  it  certainly 
seems  very  questionable,  in  the  light  of  the  results  to  be  considered 
in  the  next  chapter,  whether  they  have  any  material  bearing  upon 
the  determination  of  the  general  balance  of  energy.  We  know  at 
least  approximately  the  final  products  of  metabolism,  and  accord- 
ing to  the  law  of  initial  and  final  states  (p.  228)  the  intermediate 
reactions  can  only  affect  the  total  amount  of  energy  liberated  in 
case  some  of  the  intermediate  products  are  retained  in  the  organism. 
The  only  material  which  we  know  to  be  stored  up  in  any  consider- 
able quantity  in  the  normal  body,  however,  is  fat,  and  the  amount 
of  this  we  can  at  least  approximately  determine.  It  is  of  course 
possible  that  in  an  experiment  covering  a  few  minutes  only,  these 
intermediate  reactions  may  seriously  affect  the  result,  but  in  an 
experiment  covering  several  hours  or  a  whole  day  we  can  hardly 
conceive  such  to  be  the  case.  Indeed  we  may  probably  go  still 
further.  It  seems  to  be  a  general  physiological  law  that  the  func- 
tions of  the  organism  are  adjusted  to  a  certain  average  composition 
*  Chaleur  Animale,  Part  I.  t  Loc.  cit.,  p.  44. 


METHODS  OF  INVESTIGATION.  255 

of  its  tissues  and  fluids,  and  that  even  a  comparatively  small  varia- 
tion in  the  latter  calls  into  action  compensatory  processes.  A 
striking  illustration  of  this  is  seen  in  the  promptness  with  which  the 
respiratory  and  vascular  mechanism  reacts  to  the  changes  produced 
in  the  blood  by  muscular  activity  (compare  Chapter  VI).  It  seems 
improbable,  therefore,  that  any  sufficient  accumulation  of  the  in- 
termediate products  of  metabolism  can  take  place  to  seriously  in- 
fluence the  results  of  any  but  very  short  experiments.  That  the 
methods  employed  involve  other  sources  of  error  has  already  ap- 
peared, but  with  due  allowance  for  these  it  would  appear  that  the 
results  are  worthy  of  a  large  degree  of  confidence. 

Computation  from  Carbon  and  Nitrogen  Balance. — The  method 
of  computing  the  heat  production  from  the  total  excreta,  as  em- 
ployed by  Rubner  and  others  for  carnivorous  animals,  we  have  seen 
to  be  inapplicable  to  herbivora.  It,  however,  shades  naturally  into 
a  third  method,  of  general  applicability,  which  consists  in  combining 
with  a  determination  of  the  carbon  and  nitrogen  balance  by  means 
of  the  respiration  apparatus  direct  determinations  of  the  potential 
energy  of  the  food  and  of  the  visible  excreta  by  the  methods  already 
indicated.  Kellner  has  made  extensive  use  of  this  method,  and  the 
following  example,  taken  from  his  earliest  investigations,*  will 
serve  to  show  clearly  the  nature  of  the  method.  The  ox  experi- 
mented upon  was  fed  daily  8.5  kgs  of  meadow  hay.  Respiration 
experiments  showed  that  on  this  ration  there  was  a  daily  gain  by 
the  animal  of  6.2  grams  of  nitrogen  and  127.2  grams  of  carbon, 
equivalent  to  37.2  grams  of  protein  and  1408  grams  of  fat,  the 
potential  energy  of  which  can  be  computed  from  the  data  on  p.  244. 

From  determinations  of  the  heats  of  combustion  of  food,  feces, 
and  urine,  assuming  the  combustible  gases  excreted  to  consist  only 
of  methane,  the  balance  of  energy  is  computed  as  in  the  table  on 
p.  256.f 

Having  included  under  the  head  of  outgo  all  the  known  forms 
in  which  potential  energy  as  such  may  be  disposed  of,  the  balance 
of  14,819.5  Cals.  is  regarded  as  having  been  liberated  as  kinetic 
energy,  and,  since  no  external  work  was  performed,  to  have  taken 
finally  the  form  of  heat.     Short  of  an  actual  calorimetric  experi- 

*  Landw.  Vers.  Stat.,  47,  275. 

t  The  figures  are  the  corrected  ones  given  in  Landw.  Vers.  Stat.,  53,  9. 


*5<5 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Income, 

Cab. 

Outgo, 
Cals. 

32,177.3 

Feces 

11,750.3 
1,945  0 

2,113.7 
211  2 

Fat             "      

1,337.6 
14,819.5 

32,177.3 

32,177.3 

merit,  this  is  the  most  accurate  method  available  for  determining 
the  heat  production  of  an  animal  during  a  considerable  period  of 
time.     To  short  periods  it  is  inapplicable  for  obvious  reasons. 

Heat  Production  and  Heat  Emission. — In  conclusion,  it  is 
important  to  remember  that  what  is  determined  more  or  less  accu- 
rately by  all  these  indirect  methods  is  the  amount  of  energy  which 
takes  the  kinetic  form,  and  in  the  absence  of  mechanical  work 
finally  appears  as  heat.  In  other  words,  what  is  determined  is  the 
heat  'production  by  the  animal.  On  the  other  hand,  the  results  ob- 
tained with  an  animal  calorimeter  show  the  amount  of  heat  given 
off  by  the  animal  during  the  experiment,  that  is,  the  heat  emission. 
But  these  two,  heat  production  and  heat  emission,  are  by  no  means 
necessarily  equal.  On  the  one  hand,  heat  produced  may  be  tem- 
porarily stored  in  the  body,  or,  on  the  other  hand,  heat  retained  in 
the  body  from  a  previous  period  may  be  given  off  along  with  that 
actually  produced  during  the  experiment. 

This  is  sufficiently  obvious  in  case  of  changes  in  the  body  tem- 
perature, but  even  when  the  latter  remains  constant  the  possibility 
of  a  temporary  storage  of  the  materials  of  the  food,  and  especially 
of  water,  in  the  body,  must  be  considered.  If,  for  example,  the  con- 
sumption of  water  in  an  experiment  exceeds  the  total  amount  given 
off  in  the  visible  and  gaseous  excreta,  the  quantity  of  heat  required 
to  warm  the  excess  of  water  to  the  temperature  of  the  body  remains 
in  the  animal  as  sensible  heat.  The  heat  is  produced  but  not 
emitted.  If,  on  the  other  hand,  the  excretion  of  water  exceeds  the 
consumption,  sensible  heat  is  removed  from  the  body  in  this  excess 
and  the  emission  of  heat  exceeds  the  production  by  a  corresponding 
amount.  What  is  true  of  water  is  of  course  true  also,  ceteris  paribus, 
of  the  total  income  and  outgo  of  matter,  although  the  water,  on 


METHODS  OF  INVESTIGATION.  257 

account  of  its  large  amount  and  high  specific  heat,  constitutes  the 
most  important  factor.  The  skillful  investigator  will,  of  course, 
seek  to  plan  his  experiments  so  as  to  avoid  these  fluctuations  so  far 
as  possible,  but  they  can  rarely  be  completely  eliminated  and 
therefore  we  cannot  expect  that  the  emission  of  heat  will  correspond 
exactly  to  the  production. 


CHAPTER  IX. 
THE  CONSERVATION  OF  ENERGY  IN  THE  ANIMAL  BODY. 

Throughout  the  preceding  chapter,  particularly  in  considering 
the  indirect  methods  of  animal  calorimetry,  it  has  been  assumed 
that  the  law  of  the  conservation  of  energy  applies  to  the  animal 
body.  This  is  the  fundamental  postulate  upon  which  all  study  of 
nutrition  from  the  standpoint  of  energy  is  based,  and  it  is  of  prime 
importance,  therefore,  to  examine  into  the  experimental  evidence 
upon  which  it  is  based. 

The  processes  of  metabolism  are  essentially  chemical  processes, 
and,  like  other  chemical  reactions,  are  accompanied  by  thermal 
changes,  resulting  as  a  whole  in  a  liberation  of  kinetic  energy. 
From  this  point  of  view,  then,  the  subject  may  be  regarded  as  a 
branch  of  thermo-chemistry. 

The  applicability  of  the  law  of  the  conservation  of  energy,  and 
in  particular  of  the  law  of  initial  and  final  states,  to  the  most  diverse 
chemical  reactions  has  been  amply  demonstrated  by  the  investiga- 
tions of  Hess,  Berthelot,  Thomsen,  and  others.  It  might  seem,  then, 
in  view  of  the  chemical  nature  of  metabolism,  that  we  were  justified 
in  assuming  the  same  law  to  apply  also  to  the  reactions  taking  place 
in  the  body,  especially  since  investigations  in  other  fields  of  science 
have  led  us  to  regard  it  as  one  of  the  fundamental  laws  of  the  uni- 
verse. On  the  other  hand,  however,  the  reactions  occurring  in  the 
body  are  vast  in  number,  are  of  the  most  varied  character — oxida- 
tions, reductions,  syntheses,  cleavages,  hydrations,  etc. — are  infi- 
nitely more  complex  than  those  which  the  chemist  can  produce  in  his 
laboratory,  and  finally,  our  knowledge  of  them  is  as  yet  but  very 
partial  and  fragmentary.  Moreover,  the  matter  composing  the 
body  is  living  matter,  and  whatever  view  we  may  take  as  to  the 
nature  of  life  the  properties  of  living  matter  differ  from  those  of 

258 


THE  CONSERVATION  OF  ENERGY  IN   THE  ANIMAL   BODY.    259 

dead  matter,  and  we  have  no  scientific  right  to  assume  in  advance 
of  the  evidence  that  no  special  forces  are  operative  in  the  former. 
In  brief,  whatever  may  be  the  probabilities  in  the  case  the  applica- 
bility of  the  law  to  living  beings  as  logically  requires  experimental 
demonstration  as  did  its  applicability  in  physics  or  chemistry,  and 
no  little  labor  has  been  within  the  past  few  years  devoted  to  this 
problem. 

Nature  of  Evidence. — Before  proceeding  to  a  consideration 
of  the  experiments  bearing  upon  this  question  it  will  be  well  to 
make  clear  the  nature  of  the  evidence  required. 

If  the  law  of  the  conservation  of  energy  applies  to  the  animal, 
the  following  are  necessary  consequences  of  it : 

1.  In  an  animal  doing  no  work  on  its  surroundings  and  neither 
gaining  nor  losing  body  substance,  the  potential  energy  (heat  of 
combustion)  of  the  food  will  be  equal  to  the  potential  energy  of  the 
excreta  plus  the  kinetic  energy  given  off  in  the  form  of  heat  plus 
the  energy  expended  in  producing  physical  and  chemical  changes  in 
the  body.* 

2.  In  an  animal  doing  work  on  its  surroundings,  but  neither 
gaining  nor  losing  body  substance,  the  potential  energy  of  the  food 
will  be  equal  to  the  potential  energy  of  the  excreta  plus  the  energy 
of  the  heat  given  off  plus  the  energy  of  the  work  done  plus  the 
energy  expended  in  producing  physical  and  chemical  changes  in 
the  body. 

3.  In  an  animal  doing  no  work  on  its  surroundings,  but  gaining 
or  losing  body  substance,  the  potential  energy  of  the  food  will  equal 
the  potential  energy  of  the  excreta  plus  the  energy  of  the  heat  given 
off  plus  the  potential  energy  of  the  gain  by  the  body  (a  loss  by 
the  body  being  regarded  as  a  negative  gain)  plus  the  energy  ex- 
pended in  producing  physical  and  chemical  changes  in  the  body. 

4.  In  an  animal  doing  work  on  its  surroundings  and  gaining  or 
losing  body  substance  the  potential  energy  of  the  food  will  equal 
the  potential  energy  of  the  excreta  plus  the  energy  of  the  heat  given 
off  plus  the  energy  of  the  work  done  plus  the  potential  energy  of  the 
gain  by  the  body  (a  loss  by  the  body  being  regarded  as  a  negative 

*  Such  as  changes  of  temperature  or  aggregation,  cleavages,  syntheses, 
etc.  In  case  these  resulted  in  an  evolution  of  energy,  this  term  of  the  equa- 
tion would,  of  course,  have  a  negative  sign. 


260  PRINCIPLES   OF  ANIMAL   NUTRITION. 

gain)  plus  the  energy  expended  in  producing  chemical  and  physical 
changes  in  the  body. 

In  actual  experimentation  it  is  practically  impossible  to  so 
adjust  the  food  that  there  shall  be  absolutely  no  gain  or  loss  of  body 
substance,  although  its  amount  can  be  made  relatively  small. 
Experiments  on  this  subject,  then,  necessarily  fall  under  Cases  3 
or  4,  and  as  a  matter  of  fact,  in  all  the  experiments  hitherto 
made,  the  subject  has  either  done  no  mechanical  work  or  this 
work  has  been  converted  into  heat  inside  the  calorimeter  and 
measured  along  with  that  directly  given  off  by  the  body,  so  that 
all  these  experiments  fall  under  Class  3. 

The  quantities  to  be  determined,  then,  are 

1.  Potential  energy  of  food. 

2.  Potential  energy  of  excreta  (feces,  urine,  hydrocarbons,  etc.). 

3.  The  heat  produced  (including  that  into  which  any  mechani- 
cal work  is  converted). 

4.  The  potential  energy  of  the  gain  or  loss  of  body  substance. 

5.  The  energy  expended  (or  evolved)  in  producing  changes  in 
the  body. 

If  we  can  determine  accurately  these  five  factors,  and  having 
done  so  find  the  equality  stated  under  3  to  exist  in  a  large  number 
of  cases,  we  shall  be  justified  in  the  conclusion  that  the  law  of  the 
conservation  of  energy  applies  to  the  animal  organism. 

The  methods  by  which  the  first  four  of  the  above  factors  may  be 
determined  formed  the  subject  of  the  preceding  chapter.  As  re- 
gards the  fifth,  it  has  commonly  been  assumed  that  in  an  experi- 
ment begun  and  ended  at  the  same  hour  of  the  day  and  under  com- 
parable conditions,  which  has  been  preceded  by  a  considerable 
period  of  uniform  feeding  and  other  conditions,  and  in  which  the 
subject  was  in  apparent  good  health,  the  initial  and  final  states  of 
the  body  are  substantially  the  same.  While  it  seems  highly  prob- 
able that  this  is  true,  an  actual  demonstration  of  its  truth  is  not  an 
easy  matter.  With  respect  to  the  body  temperature  in  particular 
it  is  worthy  of  note  that  even  a  slight  variation  would  materially 
affect  the  results.  Thus  in  a  1000-pound  ox,  assuming  an  average 
specific  heat  of  0.8,  a  variation  of  one  fifth  of  a  degree  Celsius  would 
correspond  to  160  Cals.  The  rectal  temperature  affords  the  best 
available  means  of  control  on  this  point,  and  a  very  ingenious 


THE   CONSERVATION  OF  ENERGY  IN   THE  ANIMAL   BODY.    26 1 

method  for  its  determination  at  frequent  intervals  has  been  de- 
scribed by  Benedict  &  Snell.*  While  it  is  true  that  the  rectal 
temperature  is  not  necessarily  the  average  of  that  of  the  whole  body, 
we  may  probably  assume  with  safety  that  the  variations  of  the  two 
will  substantially  correspond  and  therefore  that  the  error  introduced 
by  the  use  of  the  former  will  be  insignificant. 

The  question  of  possible  chemical  and  physical  changes  in  the 
make-up  of  the  tissues  has  already  been  considered  in  the  preceding 
chapter,  where  it  was  pointed  out  that  their  effect  is  in  all  proba- 
bility negligible  in  experiments  of  any  considerable  duration. 

Early  Experiments.! — From  a  slightly  different  point  of  view 
the  question  under  consideration  may  be  stated  as  that  of  the  source 
of  animal  heat.  Is  the  energy  given  off  by  the  animal  in  this  form 
(in  the  absence  of  external  muscular  work)  equivalent  to  the  heat 
produced  by  the  oxidation  of  the  same  materials  outside  the  body  ? 
In  this  form  the  question  could  scarcely  fail  to  attract  attention  as 
soon  as  man  began  to  observe  and  reflect  upon  the  phenomena  of 
nature. 

The  ancients  regarded  the  "animal  heat"  or  "vital  heat"  as 
"  innate  "  and  having  its  source  in  the  heart.  In  more  recent  times 
it  was  attributed  in  a  vague  way  to  chemical  action,  and  later  was 
also  explained  as  resulting  from  mechanical  action  and  in  particular 
from  the  pulsation  of  the  blood  in  the  blood-vessels.  Our  real 
knowledge  of  the  subject,  however,  dates  from  the  discovery  of 
oxygen  and  from  those  researches  by  Lavoisier  and  others  which 
established  the  true  nature  of  combustion  and  laid  the  foundations 
of  modern  chemistry. 

Black  I  discovered  that  carbon  dioxide  was  produced  in  animals 
by  a  process  of  combustion,  and  Lavoisier. §  along  with  his  more 
purely  chemical  researches,  studied  the  question  of  animal  heat  and 
advanced  the  hypothesis  that  respiration  consists  essentially  of  a 
slow  oxidation  of  the  carbon  and  hydrogen  of  the  food  by  the  oxygen 
of  the  air,  and  that  this  slow  combustion  is  the  source  of  the  animal 
heat. 

*  Arch,  ges  Physiol.,  90,  33. 

t  This  paragraph  follows  substantially  the  historical  introduction  to 
Rubner's  paper,  "  Die  Quelle  der  thierischen  Warme."  cited  below. 

X  Lectures  on  Chemistry,  edited  by  Robison,  Edinburgh,  1803 

§  Hist.  Acad.  Roy.  d.  Sci..  Paris,  1780,  355. 


262  PRINCIPLES   OF  ANIMAL   NUTRITION. 

The  first  part  of  this  hypothesis  was  readily  susceptible  of  verifi- 
cation by  a  quantitative  determination  of  the  oxygen  taken  up  and 
the  carbon  dioxide  given  off,  but  the  second  portion  was  too  bold  to 
secure  general  acceptance.  Lavoisier,  therefore,  with  the  aid  of 
Laplace,  subsequently  attempted  to  secure  experimental  evidence 
as  to  its  truth.  To  this  end  they  determined  the  amount  of  heat 
given  off  by  a  guinea  pig  in  an  ice-calorimeter,  while  in  a  second 
experiment  the  animal  was  placed  under  a  bell-jar  and  the  produc- 
tion of  carbon  dioxide  determined.  Having  previously  determined 
by  means  of  the  ice-calorimeter  the  heat  of  combustion  of  carbon, 
the  results  of  these  two  trials  gave  them  data  for  comparing  this 
amount  with  that  produced  by  the  animal.  The  computed  amount 
of  heat  was  25.41  Cals.;  that  produced  by  the  animal  31.82  Cals. 

Several  sources  of  error  were  inherent  in  the  experimental 
methods  adopted,  of  some  of  which  Lavoisier  was  aware,  which 
tended  to  make  the  computed  amount  of  heat  too  small.  Taking 
these  into  consideration,  Lavoisier  considered  that  the  experiment 
substantially  confirmed  his  hypothesis. 

At  about  the  same  time  Crawford  *  was  investigating  the  same 
subject,  and  while  his  methods  were  rather  primitive  and  his  results 
less  accurate  than  those  of  Lavoisier  and  Laplace,  his  general  con- 
clusions were  the  same.  Of  later  experiments  may  be  mentioned 
especially  those  of  Despretz  f  and  of  Dulong4  Both  investigators 
employed  very  similar  apparatus,  viz.,  a  water-calorimeter  through 
which  a  current  of  air  was  passed,  the  respiratory  products  and  the 
heat  being  determined  in  the  same  experiment.  The  proportion  of 
the  oxygen  consumed  which  united  with  hydrogen  was  also  deter- 
mined. Both  investigators  found  more  heat  than  they  could  ac- 
count for  by  the  oxidation  of  tissue  and  concluded  that  chemical 
action  is  the  chief  but  not  the  only  source  of  animal  heat.§ 

.With  the  advance  of  physiological  knowledge  and  the  recogni- 
tion of  the  multiplicity  and  complexity  of  the  processes  taking  place 
in  the  body,  the  combustion  theory  of  the  origin  of  animal  heat 
lost  rather  than  gained  ground.     A  few  clear-sighted  physiologists 

*  Experiments  and  Observations  on  Animal  Heat,  1788. 

t  Ann.  de  Chim.  et  de  Phys.  (2),  26,  337. 

%  Ibid.  (3),  1,  440. 

§  Compare  Liebig's  discussion  of  their  experiments,  Thierehemie,  p.  28- 


THE   CONSERVATION  OF  ENERGY  IN    THE  ANIMAL   BODY.    263 

still  adhered  to  the  unity  and  simplicity  of  the  combustion  theory, 
but  in  general  various  subsidiary  hypotheses  were  brought  in  to 
account  for  the  observed  surplus,  such  as  the  motion  of  the  blood, 
friction,  imbibition,  etc. 

Rubner's  Experiments. — The  demonstration  of  the  law  of  the 
correlation  and  conservation  of  energy  in  the  inorganic  world  sup- 
plied the  clue  to  a  rational  explanation  of  the  energy  manifestations 
in  the  living  organism,  while  the  subsequent  developments  of  thermo- 
chemistry served  also  to  demonstrate  a  material  source  of  error  in 
the  older  experiments  on  animals.  In  those  experiments  the  com- 
puted heat  production  was  based  upon  the  amounts  of  carbon  and 
hydrogen  oxidized  and  the  heats  of  combustion  of  those  elements, 
the  nitrogenous  compounds  not  being  considered.  The  body,  how- 
ever, does  not  oxidize  free  carbon  and  hydrogen,  but  various  organic 
compounds,  while  among  its  excreta  are  likewise  incompletely 
oxidized  bodies.  The  computed  heat  production,  therefore,  in  the 
early  experiments  could  not  fail  to  be  seriously  erroneous.  From 
the  new  point  of  view,  therefore,  there  appeared  no  reason  to  seri- 
ously doubt  that  the  animal  heat  has  its  sole  source  in  the  metab- 
olism of  food  and  tissue,  or,  in  other  words,  that  the  law  of  the  con- 
servation of  energy  applies  to  the  animal  body.  The  first  to  under- 
take an  experimental  demonstration  of  this  fact  by  modern  methods 
was  Rubner.* 

His  object  being  primarily  to  investigate  the  source  of  animal 
heat,  his  experimental  method  could  be  somewhat  abbreviated  from 
the  general  method  outlined  on  p.  260.  No  external  mechanical 
work  having  been  done  by  the  animals,  we  have  Case  3  of  the 
four  possible  ones  there  mentioned.     If  we  let 

F  =  potential  energy  of  food, 
E=         "  "       "  excreta, 

G=         "  "       "  gain  by  body, 

H  =  hea,t  produced, 

then,  assuming  the  initial  and  final  states  of  the  body  to  be  the  same, 
we  have 

F  =  E  +  G+H, 
*  Zeit.  f.  Biol.,  30,  73. 


264 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


which  may  also  be  given  the  form 

H  =  (F-G)-E. 

Rubner  determined  summarily  the  value  of  the  quantity  F—G 
in  the  second  member  of  the  last  equation  by  the  method  described 
in  Chapter  VIII,  p.  253,  while  the  actual  heat  production  was  deter- 
mined by  means  of  his  respiration-calorimeter. 

The  quantities  actually  determined  in  these  experiments  were 
the  weight  and  nitrogen  content  of  feces  and  urine,  the  carbon 
dioxide  of  respiration,  and  the  heat  produced.  The  carbon  of  feces 
and  urine  was  estimated  from  their  nitrogen  and  the  absence  of 
combustible  gases  in  the  respiratory  products  was  assumed.  From 
the  total  excretion  of  nitrogen  and  carbon  the  amounts  of  protein 
and  fat  metabolized  are  computed,  it  being  assumed  that  all  the 
excretory  carbon  is  derived  from  these  two  substances.  The  corre- 
sponding amount  of  potential  energy,  equivalent  to  the  expression 
F  —  G  in  the  equation  above,  can  readily  be  computed  from  the  heats 
of  combustion  of  fat  and  protein.  From  this  the  potential  energy 
of  the  excreta  must  be  subtracted,  and  this  Rubner  virtually  com- 
putes from  their  total  nitrogen  on  the  basis  of  results  obtained  in 
previous  experiments  with  similar  food. 

A  comparison  of  the  heat  production  as  thus  computed  with  that 
actually  measured  by  means  of  the  calorimeter  gave  the  following 
results : 


Length 
of  Experi- 
ment. 
Days. 

Total  Heat. 

Computed. 
Cals. 

Measured , 
Cals. 

Difference. 

Fasting j 

Fat     

5 
2 
5 

8 

12 

6 

7 

1296.3 
1091.2 
1510.1 
2492.4 
3985.4 
2249.8 
4780.8 

1305.1 
1056.6 
1495.3 
2488.0 
3958.4 
2276.9 
4769.3 

+  0.69 
-3.15 
-0.97 

-0.17 

Meat | 

—0.68 
+  1.20 
-0.24 

Total 

45 

17406.0 

17349.7 

-0.32 

While  some  of  the  individual  experiments  show  not  inconsider- 
able discrepancies,  the  averages  of  computed  and  measured  heat 


THE  CONSERVATION  OF  ENERGY  IN   THE  ANIMAL  BODY.   265 

agree  very  closely  and,  granting  the  entire  validity  of  the  numerous 
assumptions  involved  in  this  method,  would  seem  to  approach  a 
demonstration  of  the  applicability  of  the  law  of  the  conservation  of 
energy  to  the  metabolism  of  the  animal.  Aside  from  errors  in  the 
estimation  of  the  carbon  of  the  excreta  from  their  nitrogen,  which 
are  probably  small,  the  chief  elements  of  uncertainty  are  the 
assumptions  as  to  the  nature  of  the  material  metabolized  in  the 
body  and  as  to  the  heat  of  combustion  of  the  excreta.  As  regards 
the  former  point,  Rubner  himself  points  out  (loc.  cit.,  pp.  118—121^ 
that  a  portion  of  the  carbon  of  the  respiration  may  be  derived  from 
glycogen,  and  even  bases  upon  the  calorimetric  results  in  one  case 
a  computation  of  the  extent  to  which  this  may  have  occurred.  The 
latter,  however,  is  obviously  begging  the  question,  and  in  his  main 
computations  Rubner  assumes  that  only  protein  and  fat  were  meta- 
bolized. 

Laulanie's  Experiments. — By  means  of  his  differential  water- 
calorimeter,  Laulanie  *  has  determined  the  respiratory  exchange 
and  the  heat  production  of  animals,  both  fasting  and  fed.  The 
nitrogen  excretion  does  not  appear  to  have  been  determined. 
From  the  respiratory  exchange  the  heat  production  is  computed, 
using  the  data  given  on  p.  250,  and  compared  with  that  obtained 
calorimetrically.  In  the  fasting  experiments  an  evolution  of  4.6 
Cals.  of  heat  is  computed  per  liter  of  oxygen  consumed.  In  the 
experiments  in  which  food  was  given  the  author  computes  from  the 
respiratory  quotient  the  distribution  of  the  oxygen  between  fat  and 
carbohydrates,  neglecting  the  protein  because  it  yields  the  same 
amount  of  heat  per  unit  of  oxygen  as  does  fat,  and  thence  calculates 
the  heat  production.  Preliminary  tests  of  the  calorimeter,  by 
allowing  water  to  cool  in  it,  gave  respectively  101.3  per  cent.,  100.9 
per  cent.,  and  99.7  per  cent,  of  the  theoretical  results.  The  experi- 
ments show  a  close  agreement  between  the  observed  and  computed 
amounts  of  heat,  as  appears  from  the  table  at  the  top  of  page  266. 

Atwater  &  Benedict's  Investigations.  —  By  far  the  most 
extensive  and  complete  data  regarding  the  conservation  of  energy 
in  the  animal  body  are  those  afforded  by  the  investigations  of 
Atwater  &  Benedict  f  upon  man.     The  experiments  were  made 

*  Archives  de  Physiol ,  1898,  p.  748. 

t  U.  S.  Dept.  Agr,  Office  of  Experiment  Stations,  Bull.  109;  Memoirs 
Nat  Acad.  Sci.,  8,  235. 


266 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Food. 

Legth 
of 

Experi- 
ment, 

Hours. 

Oxygen 
Con- 
sumed, 
Liters. 

Resp. 
Quo- 
tient. 

Heat  Production. 

Subject. 

Ob- 

serv'd, 
Cals. 

Com- 
puted 
Cals. 

Comp. 
+  Obs. 

% 

Two  guinea-pigs. . . 

Rabbit 

Duck 

Dog(2expts.) 

Average    of    all 

Third  day  of  fasting 

Second  day  of  fasting. . . 

Fasting  for  2  days 

"         "   2  and  4  days. 

8 
6* 

4 

m 

8.112 
7.650 
8.800 
30.205 

82  812 
51.683 
31.787 
46.445 
21.603 
180.398 

0.791 
0.752 
0.750 
0.758 

0.766 
0.816 
0.917 

0.893 
0.885 
0.973 

37.106 
35.254 
40.375 
141.366 

385.403 
239.431 
155.787 
227.086 
105.911 
882.580 

37.315 
35.190 
40.480 
138.943 

380.935 
237.741 
154.230 
224.050 
104.040 
887.197 

100.6 
99.8 

100.3 
98.2 

98.8 

300  grms.  of  meat 

(    Mixed  diet  rich  in    J 
f      carbohydrates      "j 

20 

"■36" 

8 
66 

99.2 

Guinea-pigs 

Rabbits 

99.0 
98  6 
98.2 

100.5 

with  the  aid  of  the  respiration-calorimeter  of  Atwater  &  Rosa  (p. 
248),  and  in  addition  to  the  great  pains  bestowed  to  obtain  accurate 
results  are  especially  distinguished  by  the  fact  that  all  the  quantities 
involved  were,  so  far  as  possible,  subjected  to  direct  measurement, 
estimates  being  avoided  with  the  necessary  exception  of  the  poten- 
tial energy  of  the  gain  or  loss  by  the  body.  The  sublingual  or 
axillary  temperature  of  the  subject  was  also  measured  in  every  case. 
The  following  results  of  one  of  the  earliest  experiments  (No.  5)  may 
serve  to  illustrate  the  general  features  of  them  all : 


Income, 
Cals. 

Outgo, 
Cals. 

2655 
2655 

"       "  feces  

143 

"        "  urine      

128 

Loss  by  body : 

-24 

Fat                     

-73 

2379 

Balance 

102 
2655 

Aside  from  the  loss  of  97  Cals.  by  the  body  as  computed  from  the 
carbon  and  nitrogen  balance,  all  the  quantities  in  the  above  state- 
ment represent  actual  determinations  of  energy  and  the  account 
balances  within  102  Cals.,  which  is  3.8  per  cent,  of  the  total  energy 
of  the  food  or  4.1  per  cent,  of  the  computed  heat  production.  To 
put  the  matter  in  a  slightly  different  way,  the  heat  production  as 
computed  by  Kellner's  method  (p.  255)  from  the  carbon  and  nitrogen 
balance  and  the  energy  of  food  and  excreta  exceeds  by  102  Cals. 


THE   CONSERVATION  OF  ENERGY  IN   THE  ANIMAL   BODY.   267 


the  heat  production  actually  measured  by  the  calorimeter.  This 
experiment  was  one  of  the  two  showing  the  greatest  percentage 
difference  between  the  computed  and  the  observed  heat  production. 
In  the  following  statement  are  tabulated  the  results  of  all  the  ex- 
periments thus  far  reported,  arranged  without  regard  to  the  subject 
of  the  experiment  or  the  nature  of  the  diet,  but  divided  into  two 
groups  according  as  active  muscular  work  was  or  was  not  performed. 


Gain 

by 

Body, 

Cals. 


Heat 
Production. 


Com-        Ob- 
puted,     served, 
Cals.        Cals. 


Cal- 
ories. 


Per 

Cent. 


Work 
Done, 

Cals. 


Rest  Experiments  : 

No.    5 

"      7 


Totals 


Work  Experiments  : 

No.    6 

"    11 

"    12 

"    29 

"    30 

"    31 

"    32 

"    33 

"    34 


Totals 

Totals,  rest  and  work. 


-  97 
-204 

+  266 
+  150 
+  159 
+  186 
+  158 
+  70 
+ 
+  138 
+  166 
+  330 
+  211 
-266 
+  597 
+  75 
+  571 
+  396 
+  213 
+  137 
+  182 


2481 
2434 
2361 
2277 
2268 
2112 
2131 
2357 
2336 
2289 
2367 
2220 
2339 
2304 
2180 
2216 
2238 
2242 
2043 
2125 
2067 


2379 
2394 
2287 
2309 
2283 
2151 
2193 
2362 
2332 
2276 
2488 
2279 
2303 
2279 
2258 
2176 
2272 
2244 
2085 
2123 
2079 


-102 

-  40 

-  74 
+  32 
+  15 
+  39 
+  62 
+  5 

-  4 

-  13 
+  121 
+  59 


-4.1 
-1.6 
-3.2 
+  1.4 
+  0.7 
+  1.8 
+  2.9 
+  0.2 
-0.2 
-0.6 
+  5.1 
+  2.7 
-1.5 
-1.1 
+  3.6 
-1.8 
+  1.5 
+  0.1 
+  2.0 
-0.1 
+  0.6 


+  3525 


■415 
-391 
-308 
-255 
-234 
-164 
-347 
-451 
-388 


47387 


3829 
3901 
3922 
3515 
3479 
3439 
3573 
3669 
3629 


-2953 

+  572 


32956 
80343 


47552 


3726 
3932 
3927 
3589 
3470 
3420 


165 


-103 
+  31 
+  5 

+  74 


3565 
3632 
3487 


32748 
80300 


19 


37 
142 


-208 
-  43 


0.35 


-2.7 
+  0.8 
+  0.1 
+  2.1 
-0.3 
-0.6 
-0.2 
-1.0 
-1.2 


250 

186 
200 
255 
249 
249 
196 
197 
250 


-0.63  2032 
-0.05 


268  PRINCIPLES   OF  ANIMAL   NUTRITION. 

In  the  former  case  the  observed  heat  production  includes  the  heat 
into  which  the  work  was  converted. 

The  total  of  all  the  experiments  shows  an  almost  absolute  agree- 
ment between  the  computed  and  the  observed  results.  To  a  trifling 
extent,  however,  this  arises  from  a  compensation  between  the  rest 
and  work  experiments,  the  computed  heat  tending  to  be  slightly 
too  small  in  the  former  and  slightly  too  great  in  the  latter,  but  the 
agreement  in  each  series  is  so  close  as  to  amount  to  a  demonstra- 
tion of  the  applicability  of  the  law  of  the  conservation  of  energy  to 
the  metabolism  of  the  animal  organism. 


CHAPTER  X. 

THE    FOOD    AS   A   SOURCE   OF    ENERGY— METABOLIZABLE 
ENERGY. 

With  the  establishment  of  the  law  of  the  conservation  of 
energy  in  its  application  to  the  animal  body,  and  with  the 
development  of  the  methods  of  calorimetric  research  briefly  out- 
lined in  Chapter  VIII,  it  has  become  possible  to  study  success- 
fully the  problems  of  animal  nutrition  from  a  new  standpoint,  re- 
garding the  food  as  primarily  a  source  of  energy  to  the  body  and 
tracing,  to  some  extent  at  least,  the  transformations  which  that 
energy  undergoes  in  the  organism  and  particularly  the  extent  to 
which  the  latter  utilizes  it  for  various  purposes. 

Some  data  regarding  the  total  energy  of  foods  and  their  constitu- 
ents have  already  been  given  in  Chapter  VIII.  It  was  there  pointed 
out,  however,  that  the  total  energy,  taken  by  itself,  does  not  fur- 
nish a  measure  of  the  nutritive  value  of  a  substance.  It  is  now 
necessary  to  enter  upon  the  question  of  the  availability  of  this 
energy  to  the  organism. 

Total  and  Metabolizable  Energy. — The  heat  of  combustion 
of  the  food  represents  to  us  its  total  store  of  potential  energy.  By 
no  means  all  of  this  potential  energy,  however,  is  accessible  to  the 
organism.  A  part  of  what  the  animal  eats  is  not  food  at  all  in  a 
physiological  sense,  but  is  simply  waste  matter  which  passes  through 
the  digestive  tract  unacted  upon.  Furthermore,  that  part  of  it 
which  is  digested  and  resorbed  is  not  completely  oxidized  in  the 
body,  but  gives  rise  to  the  formation  of  excretory  products  which  are 
still  capable  of  liberating  energy  by  oxidation.  We  have,  there- 
fore, at  the  outset,  to  distinguish  between  the  total,  or  gross, 
energy  of  the  food  eaten,  represented  by  its  heat  of  combustion, 
and  the  portion  of  that  energy  which  can  be  liberated  and  utilized  in 

269 


2  7©  PRINCIPLES   OF  ANIMAL   NUTRITION. 

the  organism.  It  is  only  this  latter  portion,  of  course,  of  which  the 
body  can  avail  itself,  and  the  term  available  energy  has,  therefore, 
very  naturally  been  proposed  for  it. 

As  will  appear  later,  however,  the  terms  available  and  availa- 
bility may  also  be  employed,  and  have  actually  been  used,  in  a  more 
restricted  sense  to  designate  that  part  of  the  energy  of  the  food 
which  can  be  applied  directly  by  the  organism  to  purposes  other 
than  simple  heat  production.  In  order  to  avoid  the  confusion  of 
terms  thus  arising  it  has  been  proposed  to  modify  the  term  available 
by  the  words  gross  and  net.  The  gross  available  energy,  according 
to  this  terminology,  signifies  all  of  the  total  energy  of  the  food 
which  can  be  utilized  by  the  body  for  any  purpose  whatever; 
that  is,  it  is  available  energy  in  the  first  of  the  two  senses  defined 
above.  Similarly,  the  net  available  energy  signifies  the  available 
energy  in  the  second  sense,  or  energy  available  for  other  purposes 
than  simple  heat  production.  The  term  "  fuel  value  "  has  also  been 
employed  by  some  writers,  notably  by  Atwater,  to  designate  the 
gross  available  energy. 

It  appears  to  the  writer  desirable,  however,  to  avoid  the  double 
use  of  the  word  available,  even  with  the  somewhat  awkward  modi- 
fying terms  proposed.  Strictly  speaking,  what  is  meant  by  gross 
available  energy  in  the  above  sense  is  that  portion  of  the  potential 
energy  of  the  food  which  the  digestive  and  metabolic  processes  of 
the  organism  can  convert  into  the  kinetic  form,  and  its  measure, 
according  to  the  principles  enunciated  in  Chapter  VII,  is  the  differ- 
ence between  the  potential  energy  of  the  food  and  the  potential 
energy  of  the  various  forms  of  unoxidized  matter  rejected  by  the 
organism.  In  other  words,  it  is  that  fraction  of  the  energy  of  the 
food  which  can  enter  into  the  metabolism  of  energy  in  the  body. 
The  writer,  therefore,  tentatively  proposes  for  it  the  term  metabo- 
lizable  energy,  as  expressing  the  facts  without  any  implication  as  to 
the  uses  made  by  the  body  of  the  energy  thus  metabolized. 

Metabolizable  energy,  then,  may  be  briefly  defined  as  potential 
energy  of  food  minus  potential  energy  of  excreta,  including  under 
excreta,  of  course,  all  the  wastes  of  the  body,  visible  and  invisible. 
The  method  is  analogous  to  that  of  the  determination  of  digestibility. 
In  both  cases  it  is  a  calculation  by  difference,  and  the  result  shows 
simply  the  maximum  amount  of  matter  or  of  energy  put  at  the  dis- 


THE  FOOD  AS  A  SOURCE  OF  ENERGY.  271 

of  the  organism  without  affording  any  clue  to  the  use  made 
of  it  by  the  latter,  that  is,  to  its  availability  in  the  more  restricted 
sense. 

In  actual  investigation,  of  course,  the  metabolizable  energy  of 
the  food  is  most  accurately  found  by  means  of  direct  determinations 
of  the  heats  of  combustion  of  the  food  and  the  waste  products. 
Except  in  the  case  of  the  intestinal  gases  no  serious  difficulties 
stand  in  the  way  of  these  determinations,  and  with  the  present  im- 
proved and  simplified  methods  of  calorimetry  it  may  fairly  be 
expected  that,  in  exact  experiments,  at  least  the  energy  of  the  food, 
feces,  and  urine  will  be  directly  determined,  while  it  is  not  impossi- 
ble that  more  extended  investigations  than  are  now  available  may 
enable  us  to  make,  for  different  classes  of  materials,  a  fairly  accurate 
estimate  of  the  intestinal  gases.  As  results  accumulate  from  such 
investigations  we  shall  gradually  acquire  a  fund  of  information 
regarding  the  amount  of  metabolizable  energy  contained  in  foods 
and  feeding-stuffs  which  it  is  perhaps  not  chimerical  to  suppose  may 
one  day  largely  take  the  place  of  our  present  tables  of  composition 
and  digestibility. 

Up  to  the  present  time,  however,  but  a  comparatively  small 
number  of  experiments  upon  domestic  animals  are  on  record  in  which 
the  metabolizable  energy  of  the  food  has  been  actually  determined. 
In  a  somewhat  larger  number  of  cases  the  loss  of  energy  in  feces 
and  urine  has  been  determined,  and  in  others  that  in  the  feces  only. 
As  regards  human  food  the  data  are  somewhat  more  abundant, 
but  nevertheless  by  far  the  greater  part  of  our  scientific  knowledge 
of  foods  and  feeding- stuffs  is  expressed  in  terms  of  (conventional) 
chemical  composition  and  apparent  digestibility.  If,  therefore,  Ave 
would  not  forego  the  advantages  which  may  be  anticipated  from  a 
study,  from  the  new  point  of  view,  of  the  accumulated  results  of 
the  last  half -century  of  experimental  work  in  this  domain,  it  is  im- 
portant that  we  be  able  to  estimate  as  accurately  as  may  be  the 
metabolizable  energy  of  the  food  from  its  known  or  estimated  com- 
position and  digestibility.  Not  a  little  labor  has  been  expended 
upon  both  aspects  of  the  subject,  particularly  by  Rubner  in  relation 
to  the  carnivora  and  man,  by  Atwater  and  his  associates  with  rela- 
tion to  human  nutrition,  by  Kellncr  as  regards  ruminants,  and  by 
Zuntz  and  his  associates  in  the  case  of  the  horse. 


272  PRINCIPLES  OF  ANIMAL   NUTRITION. 

§  i.  Experiments  on  Carnivora. 

The  comparative  simplicity  and  completeness  of  the  digestive 
processes  of  carnivora,  together  with  the  great  variations  which  can 
be  made  in  their  diet,  have  made  them  favorite  subjects  for  physio- 
logical experiments.  It  is  possible  to  feed  a  dog  or  cat  on  what  are 
close  approximations  to  simple  nutrients  for  a  sufficient  length  of 
time  to  permit  an  accurate  determination  of  the  waste  products, 
while  with  herbivora  this  is  impracticable  for  obvious  reasons. 

While  earlier  experimenters,  among  whom  may  be  mentioned 
Frankland,*  Traube.f  and  Zuntz,|  have  concerned  themselves  with 
the  question  of  the  energy  values  of  foods  and  nutrients,  it  is  to  the 
fundamental  researches  of  Rubner  that  we  owe  not  merely  more 
accurate  determinations  of  metabolizable  energy,  but  in  particular 
a  clearer  conception  of  its  actual  significance  in  nutrition.  Rubner's 
experiments  §  were  made  chiefly  with  dogs  and  were  directed 
toward  the  determination  of  what  he  designates  as  the  physiological 
heat  value  of  the  more  important  proteid  foods,  corresponding 
substantially  to  what  is  here  called  the  metabolizable  energy. 

Proteids. — As  regards  the  non-nitrogenous  ingredients  of  the 
food,  Rubner  assumes  that,  so  far  as  they  are  digested,  their  metab- 
olizable energy  is  the  same  as  their  gross  energy,  or,  in  other  words, 
that  there  are  no  waste  products.  For  example,  if  a  dog  is  given  a 
certain  amount  of  starch  and  none  appears  in  the  feces  it  is  assumed 
that  the  starch  has  simply  undergone  hydration  and  solution  in  the 
digestive  tract  without  material  loss  of  energy  and  that  conse- 
quently the  full  amount  of  energy  contained  in  the  starch  is  avail- 
able in  the  resorbed  sugar  for  the  metabolism  of  the  body.  In 
herbivora  we  know  that  there  is  a  considerable  production  of  gas- 
eous hydrocarbons  by  fermentation  in  the  digestive  tract.  The 
respiration  experiments  of  Pettenkofer  &  Voit  on  dogs,  however 
(compare  p.  72),  showed  but  a  small  excretion  of  such  gases,  while 
Tappeiner  ||  denies  the  presence  of  methane  in  any  part  of  the  dog's 
alimentary   canal.      In  the   case   of  carnivora,   then,  the  above 

*  Phil.  Mag.  (4),  32,  182. 

f  Virchow's  Archiv.,  29,  414. 

%  Landw.  Jahrb..  8,  65. 

§  Zeit.  f.  Biol.,  21,  250  and  337. 

||  Quoted  by  Rubner,  ibid.,  19,  318. 


THE  FOOD  AS  A  SOURCE  OF  ENERGY.  273 

assumption  is  at  least  in  harmony  with  current  opinion.  Rubner's 
experiments  were  therefore  directed  to  the  determination  of  the 
metabolizable  energy  of  the  proteids. 

The  earlier  computations  of  the  metabolizable  energy  of  the 
proteids  by  Frankland,  Traube,  Danilewski,  and  others  *  were  af- 
fected by  two  sources  of  error.  First,  the  heats  of  combustion  as 
determined  by  the  imperfect  calorimetric  methods  then  available 
were  seriously  in  error.  Second,  the  manner  of  computing  the 
metabolizable  energy  from  these  data  has  been  shown  by  Rubner 
to  be  incorrect.  Previous  to  his  investigations  the  metabolizable 
energy  of  the  proteids  had  been  very  generally  computed  by  deduct- 
ing from  their  gross  energy  the  energy  of  the  corresponding  amount 
of  urea.  In  other  words,  it  was  assumed  that  all  the  nitrogen  of  the 
proteids  was  split  off  in  the  form  of  urea  and  excreted  in  the  urine, 
which  was  accordingly  regarded  as  being  practically  an  aqueous 
solution  of  urea,  and  that  the  non-nitrogenous  residue  of  the  proteids 
was  completely  oxidized  to  carbon  dioxide  and  water.  Rubner's 
results  show  that  this  assumption  is  seriously  erroneous  and  gives 
too  high  results  for  the  metabolizable  energy. 

In  the  first  place,  it  neglects  entirely  one  of  the  waste  products, 
viz.,  the  feces.  The  latter  are  to  be  regarded  in  the  carnivora, 
especially  on  a  proteid  diet,  as  a  true  excretory  product,  comparable 
to  the  organic  matter  of  the  urine  and  containing  at  most  but  traces 
of  undigested  food.  This  was  early  pointed  out  by  Bischoff  & 
Voit  f  and  is  now  generally  admitted  by  physiologists.  (Compare 
p.  47.)  In  Rubner's  experiments  somewhat  over  3  per  cent,  of 
the  energy  of  the  proteid  food  was  found  in  the  feces. 

In  the  second  place.  Rubner  shows  that  the  urine  is  far  from 
being  a  simple  solution  of  urea. J  His  previous  investigations  §  had 
shown  that  the  extractives  of  lean  meat,  the  form  of  proteid  most 
commonly  used  in  such  experiments,  pass  through  the  system  un- 
changed and  are  excreted  in  the  urine,  thus  increasing  its  content  of 
energy.     By  feeding  meat  previously  treated  with  water  to  remove 

*  Cf.  Rubner,  loc.  tit.,  p.  341. 

f  Ernahrung  des  Fleischfressers,  p.  291 ;  compare  also   Muller,  Zeit    f . 
Biol ,  20,  327;  Rieder,  ibid.,  20,  378;  Tsuboi,  ibid.,  35,  68. 
J  Compare  Chapter  VIII,  p.  241. 
§  Zeit.  f.  Biol.,  20,  265. 


274  PRINCIPLES  OF  ANIMAL   NUTRITION. 

these  extractives,  he  demonstrates  that  in  this  case  also  the  urine 
is  far  from  being  a  simple  solution  of  urea.  With  a  daily  excretion 
of  13.22  grams  of  total  urinary  nitrogen,  there  was  found  in  the  urine 
0.105  gram  of  kreatinin,  0.656  gram  of  cyanuric  acid,  and  an  un- 
determined amount  of  phenol.  The  proportion  of  carbon  to  nitro- 
gen in  the  urine  was  also  notably  higher  than  in  urea,  viz.,  0.523: 1 
in  place  of  0.428: 1,  or  an  excess  of  about  20  per  cent.  Rubner 
concludes  that  the  only  sure  method  of  ascertaining  the  amount  of 
potential  energy  carried  off  in  the  urine  is  the  direct  determination 
of  its  heat  of  combustion.  Accordingly,  in  the  experiments  under 
consideration,  the  urine  was  dried  on  pumice-stone  and  burned  in 
the  calorimeter,  a  correction  being  made  for  the  urea  decomposed 
during  the  drying.  Danilewski,*  about  the  same  time,  also  re- 
ported determinations  of  the  heat  of  combustion  of  the  dry  matter 
of  human  urine  which,  like  Rubner's,  show  an  excess  over  that 
CDmputed  from  the  urea  present. 

The  materials  experimented  on  by  Rubner  were  prepared  lean 
meat,  such  as  has  been  commonly  used  in  feeding  experiments, 
and  meat  with  the  extractives  removed  by  treatment  with  water, 
the  gross  energy  of  each  being  determined  by  burning  the  dried 
material  in  the  calorimeter  after  having  removed  the  fat  by  extrac- 
tion with  alcohol  and  ether,  f  The  prepared  material  (in  the  moist 
state)  was  fed  to  dogs  for  from  five  to  eight  days,  during  all  or  a 
portion  of  which  time  the  feces  and  urine  were  collected  and  their 
content  of  nitrogen  and  energy  determined.  The  amounts  fed  are 
not  stated,  but  the  percentage  of  the  total  nitrogen  fed  which 
reappeared  in  the  feces  is  given.  A  third  experiment  on  a  fasting 
dog  was  added  in  which  the  urine  of  the  second,  third,  and  fourth 
days  was  collected  and  examined. 

So  far  as  the  proteids  are  metabolized  in  the  body  all  their  nitro- 
gen which  does  not  reappear  in  the  feces  will  be  found  in  the  urine. 
On  this  basis  the  nitrogen  per  gram  of  dry  proteids  metabolized  in 
these  < xperiments  was  divided  as  shown  in  the  following  table.  In 
the  case  of  the  fasting  animal,  Rubner  believes  himself  justified,  on  the 
basis  of  other  experiments,  in  assuming  that  the  nitrogenous  tissue 

*  Arch.  ges.  Physiol.,  36,  230. 

t  Subsequent  investigations  have  shown  that  the  material  thus  prepared 
still  contains  traces  of  fat. 


THE  FOOD  AS  A  SOURCE  OF  ENERGY. 


275 


metabolized  had  substantially  the  same  composition  and  heat-value 
as  the  lean  meat  of  the  first  experiment,  and  the  feces  are  also 
assumed  to  be  similar. 


Food. 

Nitrogen  of 
Food, 
Grms. 

Nitrogen  of 
Feces, 
Grms. 

Nitrogen  of 
Urine, 
Grms. 

Lean  meat 

0.1540 
0.1659 
0.1659 

0.0024 
0.0023 
0.0023 

0.1516 
0.1636 
0.1636 

Extracted  lean  meat 

Nothing  (body  tissue) 

The  energy  of  the  excretory  products,  calculated  per  gram  of 
nitrogen,  was  as  follows : 


Food. 

Urine, 
Cals. 

Feces, 
Cals. 

7.450 
6.695 
8.495 

70.290 
81.515 

Extracted  lean  meat 

A  comparison  of  the  above  results  for  the  urine  with  the  energy 
of  urea  (5.41  Cals.  per  gram  of  nitrogen)  fully  confirms  the  conclu- 
sions already  drawn  from  its  chemical  composition. 

From  the  figures  of  the  last  two  tables,  together  with  the  heats 
of  combustion  found  for  the  food  consumed,  viz., 

Lean  meat,  fat  removed 5.345  Gals,  per  gram 

"        "       extractives  and  fat  removed... 5. 754    "       "       " 

we  can  readily  compute  the  energy  of  the  excreta  and  by  difference 
the  metabolizable  energy  of  the  food  per  gram,  as  follows : 


Lean  Meat. 

Extracted 
Lean  Meat. 

Nitrogenous 
Body  Tissue. 

Energy  of  food 

Cals. 

Cals. 
5.3450 

Cals. 

0J854 
1.0945 
4.4741 

Cals. 
5.7540 

Cals. 

0.i683 
1.2878 
3.8889 

Cals. 
5.3450 

,,  &J    .,  r 

0.1683 
1 . 1294 
4.0473 

5.3450 

5.3450 

5.7540 

5.7540 

5.3450 

5.3450 

276 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Rubner  makes  a  slight  correction  in  the  above  figures  for  the 
energy  of  hydration  and  solution.  The  energy  of  the  proteids  was 
determined  in  the  dry  state.  They  were  fed,  however,  moist,  and 
it  is  known  that  an  evolution  of  heat  takes  place  when  dry  proteids 
are  brought  in  contact  with  water.  Consequently  the  potential 
energy  of  the  moist  proteids  is  less  than  that  computed  from  the 
calorimetric  results.  Rubner  estimates  this  loss  (loc.  cit.,  p.  307)  at 
0.5  per  cent.  The  urea  leaves  the  body  in  solution.  Its  solution 
in  water,  however,  causes  an  absorption  of  heat  equal  to  2.4  per 
cent,  of  the  total  energy  of  the  urea,  and  accordingly  (neglecting 
other  organic  matter)  the  heat  value  of  the  urine  is  higher  than  that 
calculated  from  the  calorimetric  results  upon  the  dried  urine.  Both 
these  errors  tend  to  make  the  metabolizable  energy  appear  too 
great.     Rubner's  corrections  are  as  follows: 


Metabolizable  energy  as  above 

Energy  of  hydration 

"        "   solution 

Corrected  metabolizable  energy 


Lean  Meat. 
Cals. 


4.0473 
0.0269 
0.0199 
4.0005 


Extracted 

Lean  Meat. 

Cals. 


4.4741 
0.0288 
0.0215 

4.4238 


Nitrogenous 

Body  Tissue. 

Cals. 


3.8889 
0.0269 
0.0199 
3.8421 


The  energy  lost  in  hydration  is  of  course,  practically  a  diminu- 
tion of  the  gross  energy  of  the  food.  The  energy  absorbed  in  the 
solution  of  the  urea  can  be  regarded  either  as  a  part  of  the  energy  of 
the  excreta  or  as  being  a  part  of  the  general  expenditure  of  energy 
by  the  body  in  internal  work.     (See  the  next  chapter.) 

Rubner  *  has  also  computed  the  metabolizable  energy  of  a  num- 
ber of  proteids  for  which  direct  determinations  are  wanting.  For 
this  purpose  he  uses  the  results  of  Stohmann  f  for  the  gross  energy 
and  assumes,  first,  that  the  nitrogen  will  be  divided  between  feces 
and  urine  in  the  same  ratio  as  in  the  experiment  on  extracted  lean 
meat,  and  second,  that  the  energy  of  these  excretory  products  per 
gram  of  nitrogen  will  be  the  same  as  in  that  experiment.  He  thus 
obtains  the  following  results: 

*  Loc  cit.,  p.  351. 

t  Landw.  Jahrb.,  13,  513. 


THE  FOOD  AS  A  SOURCE  OF  ENERGY. 


277 


Per  Cent. 
Nitrogen. 

Gross 

Energy 

Per  Grm., 

Cals. 

Loss  in 

Excreta, 

etc., 

Cals. 

15.6 

5.634 

1.263 

15.7 

5.577 

1.270 

15.2 

5.715 

1.311 

16.6 

5.754 

1.329 

16.6 

5.508 

1.329 

15.4 

5.345 

1.345 

17.5 

5.359 

1.390 

19.2 

5.595 

1.555 

15.4 

5.345 

1.503 

Metaboliz- 

able  Energy 

Per  Grm., 

Cals. 


Paraglobulin 

Egg  albumin 

Casein 

Syntonin 

Fibrin 

Lean  meat 

Conglutin 

Crystallized  albumin  .  . . 
Nitrogenous  body  tissue 


4.371 
4.307 
4.404 
4.424 
4.179 
4.000 
3.969 
4.090 
3.842 


§  2.  Experiments  on  Man. 

Protein. — Rubner  *  has  also  reported  a  single  experiment  on  a 
man  upon  a  diet  of  meat  with  a  slight  addition  of  fat.  The  results, 
expressed  in  the  same  manner  as  those  given  in  the  preceding  sec- 
tion, that  is,  per  gram  of  dry  matter  of  the  meat,  were — 

Energy  of  food 5.599  Cals. 

"  feces 0.434  Cals. 

"        "  urine 1.027     " 

Metabolizable  energy 4. 138     " 

5.599     "  5.599     " 

Quite  a  number  of  determinations  are  on  record  of  the  ratio  be- 
tween the  nitrogen  and  the  energy  content  of  human  urine.  Rub- 
ner f  reports  the  following  results  upon  various  diets,  including  the 
experiment  on  meat  just  quoted: 

Diet.  Ene^  Per  Grm-> 

.Nitrogen. 

Mother's  milk 12. 10  Cals. 

Cow's  milk — infant 6.93  " 

"    —adult 7.71  " 

Mixed  diet,  poor  in  fat 8 .  57  " 

"    "    " 8.33  " 

"    rich  in  fat 8.87  " 

«         "       "    "    " g  44  " 

"        "      "    "    "  — boy...'.'.............  6.42  " 

Mixeddiet — boy 7.50  " 

Meat 7.69  " 

Potatoes 7.85  " 

*  Zeit.  f.  Biol.,  42,  272.  t  Ibid.,  P-  302. 


278  PRINCIPLES  OF  ANIMAL  NUTRITION. 

With  the  exception  of  the  mother's  milk,  the  results  show  but  a 
slightly  greater  range  than  those  on  the  dog.  The  results  of  At  water 
&  Benedict,*  cited  on  p.  242,  when  computed  per  gram  of  nitrogen, 
give  the  following  results: 

Experiment  No.  5 7.055  Cals. 

"     6 7.839 

"     7 8.060 

"     8 8.447 

"     9 8.326 

"10 7.575 

The  same  authors  report  f  the  average  of  46  determinations  as 
7.9  Cals.  per  gram  of  nitrogen.  Tangl  %  has  reported  materially- 
higher  figures,  especially  for  diets  containing  large  amounts  of 
carbohydrates  and  fat. 

In  the  case  of  a  mixed  diet  more  or  less  of  the  potential  energy 
of  the  feces  may  be  derived  from  the  non-nitrogenous  nutrients 
of  the  food,  and  we  should  hardly  be  justified  in  making  for  these 
experiments  a  computation  like  that  made  for  the  meat  diet.  The 
rather  small  range  of  the  figures  in  most  cases,  however,  would 
seem  to  show  that  the  metabolizable  energy  of  the  proteids  of  ordi- 
nary mixed  dietaries  is  substantially  the  same  as  that  found  by 
Rubner  for  carnivora.  Tangl's  results  perhaps  suggest  the  possi- 
bility of  the  occasional  presence  in  human  urine  of  non-nitrogenous 
matters  similar  to  those  found  so  abundantly  in  that  of  ruminants. 

Rubner's  Computations. — Rubner's  earlier  researches  did  not 
include  experiments  upon  man,  but  from  the  results  given  in  the 
foregoing  section  he  endeavored  to  compute  approximate  factors 
for  the  metabolizable  energy  of  the  mixed  diet  of  man.§  For  this 
purpose  he  estimates  that,  on  the  average,  60  per  cent,  of  the  pro- 
tein of  the  diet  is  derived  from  animal  sources  and  40  per  cent,  from 
vegetable.  For  the  animal  protein  he  uses  the  value  found  above 
for  lean  meat,  and  for  vegetable  protein  the  average  of  the  values 
for  syntonin  and  fibrin  (since  these  have  an  ultimate  composition 

*  U.  S.  Dept.  Agr.,  Office  of  Experiment  Stations,  Bull.  69. 
t  Report  Storrs  Expt.  Station,  1899,  p.  100. 
%  Arch.  f.  (Anat.  u.)  Physiol.,  1899,  261. 
§  Loc.  cit.,  p.  370. 


THE  FOOD  AS  A  SOURCE   OF  ENERGY.  279 

similar  to  that  of  the  proteids  of  the  grains).  Correcting  these 
values  for  the  error  involved  in  the  usual  computation  of  protein 
from  nitrogen,  he  obtains  as  the  average  metabolizable  energy  of 
the  protein  (N  X  6.25)  of  a  mixed  diet  4.1  Cals.  per  gram. 

For  the  fat  and  carbohydrates  it  is  assumed  that  all  their  poten- 
tial energy  is  metabolizable,  but  an  allowance  is  made  in  the  latter 
case  for  the  error  due  to  the  ordinary  computation  of  the  carbo- 
hydrates by  difference  and  for  some  minor  sources  of  uncertainty. 
Rubner's  final  averages  are — 

Protein  (NX6.25) 4.1  Cals.  per  gram. 

Fat 9.3     "      " 

Carbohydrates 4.1     "       "        " 

The  value  for  protein,  by  the  method  of  computation,  includes 
an  allowance  for  the  metabolic  products  contained  in  the  feces,  but 
neither  it  nor  the  values  for  the  other  nutrients  include  any  estimate 
for  the  loss  through  imperfect  digestion.  In  other  words,  they 
refer  to  the  digested  nutrients. 

These  figures  were  designed  expressly  for  computing  the  metab- 
olizable energy  of  human  dietaries,  and  even  for  that  purpose  are 
confessedly  only  approximations.  In  the  absence  of  more  exac 
figures,  however,  they  have  been  somewhat  extensively  used  for 
computing  the  metabolizable  energy  of  the  digested  portion  of  the 
food  of  domestic  animals.  For  purposes  of  approximate  estimates 
such  a  use  of  them  was  perhaps  justifiable,  but  in  too  many  cases 
their  origin  seems  to  have  been  forgotten  and  a  degree  of  accuracy 
ascribed  to  them  which  they  do  not  possess.  As  will  be  shown 
presently,  later  investigations  have  yielded  materially  different 
results  for  the  metabolizable  energy  of  the  several  classes  of  nutri- 
ents in  the  food  of  herbivorous  animals. 

Later  Experiments. — Quite  recently  Rubner  *  has  published  the 
results  of  some  experimental  investigations  into  the  validity  of  the 
averages  or  "standard  figures  "  given  above.  In  these  experiments 
the  weights  and  heats  of  combustion  of  food,  feces,  and  urine  were 
determined  calorimetrically  and  the  metabolizable  energy  as  ob- 
tained from  these  data  was  compared  with  that  computed  by  the  use 
of  the  above  factors.  In  making  the  latter  calculation  an  allowance 
*Zeit.  f.  Biol.,  42,  261. 


28o 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


was  made  for  the  percentage  loss  in  the  feces  equal  to  that  observed 
in  the  actual  experiment.  The  results  for  the  metabolizable  energy 
per  day  were — 


Diet. 

From 

Calorimetric 

Data, 

Cals. 

Computed, 
Cab. 

1911.4 
2060.4 
1773.1 
2400.5 
2698.8 
2574.1 
2549.6 
1746.8 
1765.5 

1911   5 

Rve  bread,  bolted  flour 

2079  3 

"         "      unbolted  flour 

1758.6 

Mixed  diet,  poor  in  fat 

2376  0 

2600  0 

"       "       "       "    "  and  carbohyd's    j  Jj 
Mixed  diet — growing  boys ]  TT 

2608.0 
2610.0 
1724.3 
1737.3 

As  above  noted,  the  computed  results  include  a  deduction  for 
the  energy  of  the  undigested  matter  in  the  feces.  Rubner  finds  that 
the  heat  of  combustion  of  the  organic  matter  of  the  latter  varies 
but  little  even  on  extremes  of  diet,  so  that  the  loss  through  this 
channel  is  approximately  proportional  to  the  amount  of  the  ex- 
cretion. In  the  experiments  on  mixed  diet  the  percentage  loss  of 
energy  in  the  feces  varied  from  4.3  per  cent,  to  7.9  per  cent,  of 
the  energy  of  the  food. 

Atwater's  Investigations. — By  far  the  most  extensive  data 
as  to  the  metabolizable  energy  of  human  foods  and  dietaries  are 
those  derived  from  the  investigations  upon  human  nutrition  made 
under  Atwater's  direction  by  the  United  States  Department  of 
Agriculture  with  the  cooperation  of  Wesleyan  University,  the 
Storrs  Experiment  Station,  and  various  other  experiment  sta- 
tions. Atwater  &  Bryant*  have  summarized  these  results  in  a 
preliminary  report  of  which  the  essential  features  are  given  in 
the  following  paragraphs. 

From  the  best  data  available,  the  heats  of  combustion  of  the 
protein,  carbohydrates,  and  fats  of  various  classes  of  foods  are  esti- 
mated. In  these  estimates  account  is  taken  as  fully  as  possible  of 
the  proportion  of  nitrogen  in  proteid  and  non-proteid  forms,  and 
of  the  varying  percentage  of  nitrogen  in  different  proteids,  the  nitro- 
gen factors  used  being  those  quoted  on  p.  6.      The   accuracy  of 

*  Report  Storrs  Agr.  Expt  Station,  1899,  p.  73. 


THE  FOOD  AS  A  SOURCE  OF  ENERGY.  281 

these  estimates  is  checked  by  a  comparison  of  the  computed  with 
the  actual  heats  of  combustion  of  276  different  samples  of  food,  the 
average  results  showing  a  close  agreement.  Assuming  the  potential 
energy  of  the  urine  to  be  all  derived  from  the  proteids,  the  average 
of  7.9  Cals.  per  gram  nitrogen  given  above  (p.  278)  corresponds  to 
1.25  Cals.  per  gram  of  protein  (NX6.25)  metabolized.  The  loss  of 
energy  in  the  feces  is  estimated  from  a  number  of  digestion  experi- 
ments upon  single  foods,  the  results  being  checked  by  a  comparison 
of  the  actual  and  computed  apparent  digestibility  in  93  digestion 
experiments  on  mixed  diet.  Finally,  the  proportions  of  the  several 
nutrients  which  are  derived  from  different  classes  of  foods  in 
average  mixed  diets  are  computed  from  the  results  of  185  dietary 
studies.  The  final  results  thus  obtained  for  the  metabolizable 
energy  or  "  fuel  value  "  of  the  nutrients  are  shown  in  the  table  on 
page  282. 

The  average  results  for  the  ordinary  mixed  diet  of  man  were — 

Protein 4.0  Cals.  per  gram 

Carbohydrates 4.0      "      "       " 

Fat 8.9      "      " 

These  factors  are  smaller  than  those  proposed  by  Paibner,  largely 
because  they  relate  to  the  total  and  not  to  the  digested  nutrients. 
Comparisons  of  the  computed  with  actual  metabolizable  energy  of 
mixed  dietaries,  using  the  factors  of  the  above  table,  gave  concor- 
dant results. 

§  3.  Experiments  on  Herbivora. 

The  Mockern  Investigations. — The  larger  share  of  our  present 
knowledge  regarding  the  metabolizable  energy  of  the  food  of  her- 
bivora is  due  to  the  investigations  upon  mature  cattle  which  have 
been  made  by  Kellner  *  since  1894  at  the  Mockern  Experiment 
Station.  In  the  earlier  series  of  experiments  (including  those  by 
G.  Kiihn,  reported  by  Kellner  f)  additions  of  commercial  wheat 
gluten  and  of  starch  were  made  to  a  basal  ration  consisting  exclu- 
sively of  coarse  fodder  (hay  or  straw).  In  the  later  series  of  ex- 
periments additions  of  the  same  substances  and  of  oil  and  beet 
molasses  on  the  one  hand,  and  of  coarse  fodders  on  the  other  hand, 
were  made  to  a  mixed  basal  ration. 

*  Landw.  Vers.  Stat.,  47,  275;  50,  245;  58,  1.  f  Ibid.,  44,  257. 


282 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Nutrients 
Furnished 
by  Each 
Group  per 
100  Grms. 
Total. 

Heats  of 
Combus- 
tion per 
Grm. 

Propor- 
tion of 
Total 
Nutrients 
Actually 
Avail- 
able. 

Total 
Energy 
per  Grm. 
in  Avail- 
able Nu- 
trients. 

Fuel  Value. 

Kind  of  Food 
Material. 

Per  Grm. 
Available 
Nutrients. 

Per  Grm. 
Total 
Nutri- 
ents. 

Protein  : 

Meats,  fish,  etc  ... 

Eggs 

Dairy  products  . . . 

Grms. 

43.0 

6.0 

12.0 

Cals. 
5.65 
5.75 
5.65 

Per  Cent. 
97 
97 
97 

Cals. 
5.50 
5.60 
5.50 

Cals. 
4.40 
4.50 
4.40 

Cals. 
4.25 
4.35 
4.25 

Animal  food .... 

61.0 

31.0 
2.0 
5.5 
0.5 

5.65 

5.80 
5.70 
5.00 
5.20 

97 

85 
78 
83 
85 

5.50 

4.95 
4.45 
4.15 
4.40 

4.40 

4.55 
4.45 
3.75 
3.95 

4.25 
3.70 

Legumes 

Vegetables 

Fruits 

3.20 
2.90 
3.15 

Vegetable  food  . 
Total  food 

Fat: 

Meat  and  eggs  .... 
Dairy  products  .  .  . 

39.0 
100.0 

60.0 
32.0 

5.65 
5.65 

9.50 
9.25 

85 
92 

95 
95 

4.80 
5.20 

9.00 
8.80 

4.40 
4.40 

9.50 
9.25 

3.55 
4.00 

9.00 
8.80 

Animal  food .... 
Vegetable  food  . 

92.0 
8.0 

9.40            95 
9.30           90 

8.95 
8.35 

9.40 
9.30 

8.95 
8.35 

Total  food 

Carbohydrates  : 

Animal  food .... 

Cereals 

Legumes 

Vegetables 

100.0 

5.0 
55.0 

1.0 
13.0 

5.0 
21.0 

9.40 

3.90 
4.20 
4.20 
4.20 
4.00 
3.95 

95 

98 
98 
97 
95 
90 
98 

8.90 

3.80 
4.10 
4.05 
4.00 
3.60 
3.85 

9.40 

3.90 
4.20 
4.20 
4.20 
4.00 
3.95 

8.90 

3.80 
4.10 
4.05 
4.00 
3.60 

3.85 

Vegetable  food  . 
Total  food 

95.0 

100.0 

4.15 
4.15 

97 
97 

4.00 
4.00 

4.15 
4.15 

4.00 
4.00 

In  each  experiment  the  digestibility  of  the  ration  was  deter- 
mined in  the  usual  manner,  and  also  the  carbon  of  food,  feces,  urine, 
and  respiration  (including  methane,  etc.),  and  the  nitrogen  and 
heats  of  combustion  of  food,  feces,  and  urine.  The  experiments 
were  made  with  every  precaution  and  extended  over  a  sufficient 
length  of  time  to  ensure  normal  results.  In  each  experiment  the 
respiratory  products  were  determined  in  four  or  five  separate  periods 
of  twenty-four  hours  each.     No  such  complete  experiments  with 


THE   FOOD  AS  A  SOURCE   OF  ENERGY.  283 

other  classes  of  herbivorous  animals  have  been  reported,  although 
partial  data  are  available  from  experiments  on  horses  and  swine. 

Method  of  Stating  Results. — The  determination  of  the 
metabolizable  energy  of  a  given  ration  by  experiments  like  the 
above  is,  in  principle,  very  simple,  although  requiring  many  appli- 
ances and  much  technical  skill.  When,  however,  we  attempt  to 
generalize  the  results  much  greater  difficulties  are  encountered 
than  in  the  cases  previously  considered. 

In  investigations  upon  carnivora  and  upon  man  the  metaboliz- 
able energy,  as  we  have  just  seen,  is  usually  computed  upon  the 
total  nutrients  of  the  food — that  is,  upon  the  total  amounts  of 
protein,  carbohydrates,  and  fat — the  deduction  for  the  loss  of 
energy  in  the  feces  being  included  in  the  factors  employed.  This 
is  possible  because  the  amount  of  potential  energy  thus  removed 
is  small  in  itself  and  subject  to  relatively  small  variations  on  ordi- 
nary diet  and  also  because  the  crude  nutrients  composing  the  food 
are  largely  chemical  compounds  which  are  at  least  fairly  well 
known. 

The  food  of  herbivora,  on  the  contrary,  is  both  more  complex 
and  less  well  known  chemically  and  contains  a  much  larger  and  very 
varying  proportion  of  indigestible  matter.  As  a  consequence  the 
feces,  instead  of  being  chiefly  an  excretory  product,  consist  mainly 
of  undigested  food  residues  with  but  a  small  proportion  of  meta- 
bolic products,  and  contain  a  large  and  variable  part  of  the  total 
potential  energy  of  the  food.  For  all  these  reasons  it  seems  likely 
that  any  attempt  to  compute  general  factors  for  the  metab- 
olizable energy  of  the  crude  nutrients  of  feeding-stuffs  similar  to 
those  of  Rubner  or  Atwater  for  the  nutrients  of  human  foods  would 
be  confronted  by  almost  insuperable  difficulties. 

It  was  natural,  then,  to  attempt  to  eliminate  these  difficulties 
by  computing  the  results  upon  the  digestible  nutrients  of  the  feed- 
ing-stuffs, but  even  here  considerable  difficulties  arise.  The  di- 
gested nutrients,  particularly  in  the  case  of  coarse  fodders,  are  far 
from  being  the  pure  protein,  carbohydrates,  and  fats  which  our 
ordinary  statements  of  composition  and  digestibility  assume  them 
to  be.  Furthermore,  a  considerable  and  a  variable  proportion  of 
the  waste  of  proteid  metabolism  in  the  herbivora  takes  the  form  of 
hippuric  acid,  a  body  less  completely  oxidized  than  urea,  and  ac- 


284  PRINCIPLES  OF  ANIMAL   NUTRITION. 

cordingly  containing  more  potential  energy,  while  the  urine  of 
sheep  and  cattle  also  contains  not  a  little  non-nitrogenous  matter 
of  some  sort.  Finally,  the  slow  and  complicated  process  of  diges- 
tion in  the  herbivora  is  accompanied  by  fermentations  and  the 
evolution  of  gaseous  hydrocarbons  (methane),  and  perhaps  of 
hydrogen,  both  of  which  carry  off  a  more  or  less  variable  propor- 
tion of  the  potential  energy  of  the  food.  By  means  of  experiments 
with  approximately  pure  nutrients  it  is  possible  to  secure  factors 
for  the  metabolizable  energy  of  the  digested  nutrients  of  con- 
centrated feeding-stuffs,  but  in  the  case  of  coarse  fodders  about 
all  that  is  practicable  in  this  direction  is  to  compute  the  results 
of  experiments  upon  the  total  digestible  matter. 

There  is  possible,  however,  a  third  method,  viz.,  to  compute  the 
metabolizable  energy  upon  the  total  organic  matter  of  the  feeding- 
stuff,  expressing  it  either  as  Calories  per  gram  or  pound  of  organic 
matter  or  as  a  percentage  of  the  gross  energy.  In  the  latter  form 
the  result  would  be  analogous  to  a  digestion  coefficient  and  would 
show  what  proportion  of  the  total  energy  of  the  material,  as- deter- 
mined by  combustion  in  the  calorimeter,  was  capable  of  being  met- 
abolized in  the  body.  This  method  of  expressing  the  results  has 
certain  advantages  in  directness  and  simplicity,  and  especially  in 
putting  the  whole  matter  on  the  basis  of  energy  values.  In  the 
succeeding  paragraphs  the  available  data  will  be  considered  from 
both  the  standpoints  last  named. 


METABOLIZABLE  ENERGY  OF  ORGANIC  MATTER. 

For  a  discussion  of  the  matter  from  this  standpoint  we  have  to 
rely  almost  entirely  upon  the  Mockern  investigations  already  men- 
tioned. In  the  case  of  those  earlier  experiments  in  which  the  ration 
consisted  exclusively  of  a  single  coarse  fodder  the  computation  of 
the  metabolizable  energy  of  the  latter  is,  of  course,  readily  made. 
In  the  experiments  in  which  the  food  under  investigation  was  added 
to  a  basal  ration  the  computation  is  somewhat  less  simple,  it  being 
then  necessary  to  compare  the  gross  energy  of  the  added  food  with 
the  increase  in  the  energy  of  the  excreta  in  the  second  period  as 
compared  with  the  first.  The  details  of  both  methods  will  be  best 
explained  by  illustration. 


THE  FOOD  AS  A  SOURCE  OF  ENERGY.  285 

Total  Organic  Matter. 

Coarse  Fodders.  Fed  Alone. — For  Ox  H,  fed  exclusively  on 
meadow  hay,  Kellner  obtained  the  following  results  *  per  day  and 
head: 

Ingesta. 

7,263f  grams  meadow  hay 32,177 . 3  Cals. 

Excreta. 

2,547 f  grams  feces 11,750.3  Cals. 

13,675        "      urine 1,945.0     " 

158.4    "      methane 2,113.7     " 

Total  excreta 15,809.0     " 

Difference 16,368.3     " 

Had  the  ration  exactly  sufficed  for  the  maintenance  of  the  ani- 
mal, the  difference  of  16,368.3  Cals.  would  represent  exactly  its 
metabolizable  energy.  In  reality,  however,  the  nitrogen  and  car- 
bon balance  indicated  a  gain  by  the  animal  of  37.2  grams  of  protein 
(NX 6.00  X)  and  140.8  grams  of  fat,  equivalent  to  1548.8  Cals.,  so 
that  the  amount  of  energy  actually  converted  into  the  kinetic  form 
was  16,368.3  - 1548.8  =  14,819.5  Cals.  The  potential  energy  of  the 
140.8  grams  of  fat,  however,  while  it  was  not  actually  rendered 
kinetic,  might  have  been  had  the  needs  of  the  organism  required  it. 
Its  retention  in  the  potential  form  was,  in  a  sense,  temporary  and 
accidental,  and  its  energy  should  properly  be  considered  as  a  part 
of  the  metabolizable  energy  of  the  food. 

With  the  gain  of  protein,  however,  the  case  is  different.  Its 
total  potential  energy  equals  211.2  Cals.,  but  not  all  of  this  is 
capable  of  conversion  into  kinetic  energy.  According  to  Rubner's 
results  (p.  275)  each  gram  of  urinary  nitrogen  derived  from  the  met- 
abolism of  the  protein  of  lean  meat  corresponds  to  7.45  Cals.  If 
this  result  is  applicable  to  the  forms  of  protein  consumed  by  her- 
bivora  (and  we  shall  see  later  that  there  is  good  reason  to  believe 
that  such  is  approximately  the  case),  then  the  metabolism  of  the 
37.2  grams  of  protein  gained  would  have  added  46.2  Cals.  to  the 
observed  potential  energy  of  the  urine,  while  the  remaining  165 
Cals.  would  have  taken  the  kinetic  form  and  should,  therefore,  be 
regarded  as  part  of  the  metabolizable  energy  of  the  food. 

*  7,0c.  cit.,  53,  9.  f  Dry  matter.  J  Compare  pp.  67,  68. 


286 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


In  other  words,  to  get  at  the  actual  metabolizable  energy  of  the 
ration  in  this  experiment  we  must  add  to  the  observed  potential 
energy  of  the  urine  the  amount  of  46.2  Cals.  by  which  it  would  have 
been  increased  had  all  the  protein  of  the  food  been  metabolized,  or, 
what  is  the  same  thing,  must  subtract  this  amount  from  the  ob- 
served difference  between  food  and  excreta.  This  leaves  16,322.1 
Cals.  as  the  metabolizable  energy  of  7263  grams  of  dry  matter  or 
6750  grams  of  organic  matter  in  meadow  hay,  and  the  metabolizable 
energy  per  gram  of  organic  matter  is  therefore  2.418  Cals. 

Computed  in  the  above  manner,  the  several  experiments  of  this 
category  gave  per  day  and  head  the  following  results : 


! 

Ration. 

8 

W 

ll 

I 

O 

Energy  of 

Metabolizable 
Energy. 

Ani 
mal. 

Food 
Cals. 

Feces. 
Cals. 

Urine 
(Cor- 
rected). 
Cals. 

Meth- 
ane. 
Cals. 

Total, 
Cals. 

Per 

Grm. 
Or- 
ganic 
Mat- 
ter, 
Cals. 

A 
II 
V 

Meadow  hay  I 

"    B.'.'. 

6750 
7816 
7199 
7125 
7809 
6815 

32177.3 

36975 . 1 
34211.5 
33855.4 
37167.3 

32252 . 2 

11750.3 
15524.1 
15312.2 
13765.2 
13880.7 
14669.0 

1991.2 
1925.7* 
1559.3* 
1737.9* 
3224.6 
1686.9 

2113.7 
3137.2 
2268.5 
2480.6 
2646 . 1 
2092.3 

16322.1  2.418 
16388.1  2.097 
15071.5  2.093 

V[ 

15871.7  2.228 

XX 

"   M 

17415.9  ">  230 

I 

"   II.    . 

13804.0 

2.026 

2.182 

B 
III 
IV 

Meadow  hay  and  oat  straw.  .  . 
Clover               •'      "       "      ... 

7107 
7328 
7074 

33794.4 
34603.2 
33405 . 1 

14576.1 
15505.1 
15250.6 

1440.3 
1549.6* 
1481.5* 

2331.2 
2670.1 
2491.3 

15446.8 
14878.4 
14181.7 

2.173 
2.031 
2.004 

*  Energy  of  urine  computed  from  its  carbon  content. 


It  should  be  noted  that  the  figures  for  the  energy  of  the  feces  in 
these  and  in  all  the  succeeding  experiments  include  that  of  the  met- 
abolic products  contained  in  them.  While  the  latter  are  not  derived 
directly  from  the  food  they  are  a  part  of  the  expenditure  made  by 
the  body  in  the  digestion  of  the  food,  and  there  is.  therefore,  the  same 
reason  for  including  their  energy  as  for  including  that  of  the  organic 
matter  of  the  urine. 

Both  contain  a  certain  amount  of  potential  energy,  derived 
ultimately  from  the  food,  which  has  escaped  being  metabolized  in 


THE  FOOD  AS  A   SOURCE  OF  ENERGY.  287 

the  body  and  so  is  to  be  deducted  from  the  total  energy  of  the 
food  to  obtain  its  metabolizable  energy. 

Experiments  on  timothy  hay  made  by  the  writer,*  in  which  the 
amount  of  methane  excreted  was  estimated  from  the  amount  of  non- 
nitrogenous  nutrients  digested,  gave  the  following  results,  the  cor- 
rection for  the  gain  or  loss  of  nitrogen  being  computed  in  a  slightly 
different  way  from  that  explained  above : 


ENERGY  PER  GRAM  ORGANIC  MATTER. 


Experiment  I. 
Cals 

Experiment  II. 
Cals. 

Experiment  VI. 
Cals. 

Steer  1 .... 

2.104 
2.007 
1.904 

1.838 
2.164 

1.824 

2.139 

"     2 

2.175 

"     3 

2.176 

2.005 

1.942 

2.163 

2.037 

It  should  be  noted  that  the  above  figures  are,  as  already  stated, 
approximate  only.  The  energy  of  the  methane  was  estimated,  while 
the  determinations  of  the  energy  of  the  urine  were  not,  in  all  cases, 
satisfactory.  We  are  probably  justified,  however,  in  regarding 
the  results  as  a  close  approximation  to  the  truth. 

Coarse  Fodders  Added  to  Basal  Ration. — As  an  example  of 
this  class  of  experiments  we  may  take  Periods  4  and  7  with  Ox  H.f 
The  rations  in  the  two  periods  were  as  follows : 


Total  Weight. 

Containing  Organic  Matter. 

Period  4. 
Kgs. 

Period  7, 
Kgs. 

Period  4, 
Grms. 

Period  7, 
Grms. 

Difference, 
Grms. 

4 

3                3 

1                1 

3198 

23S6 

818 

6495 

2413 

835 

3297 

Molasses-beet  pulp 

27 
17 

8 

12 

6402 

9743 

3341 

*  Penna   State  Experiment  Station,  Bull   42,  p.  153. 
t  hoc  cit.,  53,  278-335. 


288 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


The  potential  energy  of  food  and  excreta  (that  of  the  urine  cor- 
rected to  nitrogen  equilibrium)  and  by  difference  the  amounts  of 
metabolizable  energy  were: 


Food, 
Cals. 

Feces. 
Cals. 

Urine 

(Corrected), 

Cals. 

Methane, 
Cals. 

Metaboliz- 
able Energy. 
Cals. 

Period  7 

"     4 

46,275.0 
30,338.1 

14,104.8 
8,574.9 

2,593.0 
1,795.0 

3,564.2 
2,579.4 

26,013.0 

17,388.8 

Difference  .... 

15,936.9 

5,529.9 

798.0 

984.8 

8,624.2 

The  metabolizable  energy  of  the  additional  3341  grams  of  or- 
ganic matter  eaten  in  Period  7  was  therefore  8624.2  Cals.  This 
added  food  was  intended  to  consist  of  hay,  but  the  unavoidable 
variations  in  the  moisture  content  of  the  feeding-stuffs  resulted  in  a 
slightly  greater  consumption  of  the  other  ingredients  of  the  ration 
also.  Of  the  3341  grams  of  additional  organic  matter,  3297  grams, 
as  the  previous  table  shows,  were  from  the  hay  and  44  grams  from 
the  basal  ration.  If,  then,  we  would  ascertain  the  metabolizable 
energy  of  the  added  hay  only,  we  must  subtract  from  the  difference 
of  8624.2  Cals.  between  the  two  rations  the  metabolizable  energy  of 
this  44  grams  of  organic  matter  from  the  other  feeding-stuffs. 

But  while  the  gross  energy  of  the  latter  is  known,  its  metabo- 
lizable energy  cannot  be  computed  exactly,  since  it  is  impossible  to 
determine  what  part  of  the  energy  of  the  excreta  was  derived  from 
this  particular  portion  of  the  ration.  By  assuming,  however,  that 
the  same  percentage  of  its  gross  energy  was  metabolizable  as  was 
the  case  with  the  basal  ration,  and  that  its  non-met abolizable  energy 
was  similarly  distributed  between  the  various  excreta,  we  may 
compute  a  correction  which,  although  not  strictly  accurate,  will  not, 
in  view  of  the  small  quantities  involved,  introduce  any  serious  error. 
In  this  case  the  gross  energy  of  the  3297  grams  of  organic  matter  in 
the  added  hay  was  15,728.6  Cals.,  and  the  table  takes  the  form 
shown  on  the  opposite  page. 

As  thus  computed,  the  metabolizable  energy  of  the  3297  grams 
of  organic  matter  added  to  the  basal  ration  in  the  form  of  hay  was 
8504.8  Cals.,  equal  to  2.580  Cals.  per  gram.  The.  total  correction 
amounts  to  119.4  Cals.,  and  even  a  considerable  relative  error  in  it 
would  not  materially  change  the  final  results. 


THE  FOOD  AS  A   SOURCE  OF  ENERGY. 


289 


Food,                 Feces, 
Cals.                  Cals. 

Urine 

(Corrected), 
Cals. 

Methane, 
Cals. 

Metaboliz- 

able  Energy, 

Cals. 

Period  7 

"     4 

46,275.0 
30,338.1 

14,104.8 
8,574.9 

2,593.0 
1,795.0 

3,564.2 
2,579.4 

26,013.0 

17,388.8 

Difference .... 
Correction  .... 

15,936.9         5,529.9 
-208.3           -58.9 

798.0 
-12.3 

984.8 
-17.7 

8,624.2 
-119.4 

Percentages. .  . 

15,728.6 
100.0 

5,471.0 
34.78 

785.7 
5.00 

967.1 
6.15 

8,504.8 
54.07 

In  these  computations  it  is  assumed  that  the  increased  metabo- 
lizable  energy  of  the  ration  is  derived  entirely  from  the  added  feed- 
ing-stuff, or,  in  other  words,  that  the  latter  exerted  no  influence 
either  upon  the  digestibility  of  the  basal  ration  or  upon  the  propor- 
tion of  its  energy  lost  in  urine  and  in  hydrocarbons.  That  such  is 
the  case  we  have  no  means  of  proving,  and  it  is,  indeed,  unlikely 
that  it  is  exactly  true.  The  metabolizable  energy  of  the  added 
feeding-stuff  as  above  computed  includes  any  such  effects — that  is,  it 
represents  the  net  result  to  the  organism  of  the  added  coarse  fodder. 

Table  I  of  the  Appendix  contains  the  results  of  all  the  experi- 
ments of  this  sort,  computed  in  the  manner  illustrated  above.  It 
will  be  noted  that  in  all  but  two  cases  the  correction  is  less  than 
in  the  above  example.  In  each  case  the  table  shows  also  the  per- 
centage of  the  gross  energy  of  the  feeding-stuff  which  was  found  to 
be  metabolizable  and  the  percentage  carried  off  in  each  of  the 
excreta. 

Summary. — The  results  of  the  foregoing  determinations  of  the 
metabolizable  energy  of  the  organic  matter  of  coarse  fodders  are 
summarized  in  the  table  on  page  290,  which  shows  the  gross  and 
metabolizable  energy  per  gram  of  organic  matter  and  also  the 
percentage  of  gross  energy  found  to  be  metabolizable. 

Concentrated  Feeding-stuffs. — The  metabolizable  energy  of 
the  organic  matter  of  a  concentrated  feeding-stuff  when  added  to 
a  basal  ration  can,  of  course,  be  computed  by  the  same  method  as  in 
the  case  of  added  coarse  fodders,  but,  as  we  shall  see,  some  special 
difficulties  arise  in  its  application. 

The  only  commercial  concentrated  feeding-stuff  upon  which 
such  experiments  have  been  reported  is  beet  molasses,  although 


290 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Per  Gram  Organic 
Matter. 


Gross 
Energy, 
Cals. 


Metaboliz- 

able 

Energy, 

Cals. 


Per  Cent. 

Metaboliz- 

able. 


Meadow  Hay  : 
Sample  I.. . , 


A 

B,  OxV. 
B,    "VI. 


B,  average. 

M 

II 

V,  OxF... 
V,    "  G... 


V,  average 

VI,  Ox  H,  Period  2. 


"       VI,  average 

Average  of  seven  samples 

Timothy  Hay  (approximate)  . . 

Oat  Straw  : 

Ox  F 

"    G 


Average.. 

Wheat  Straw  : 

OxH 

"    J 


Average. 


Extracted  Rye  Straw  : 

OxH 

"     J 


Average. 


4.767 
4.731 

4.752 


4.760 
4.734 

4.743 


(4H 


4.751 
4.670 


4.816 


4.743 


4.251  J 


2.418 
2.097 
2.093 
2.228 


2.161 
2.230 
2.026 
1.933 

2.087 


2.010 
2.520 
2.580 
2.540 


2.547 


2.213 
2.037 


1.760 
1.688 


1.724 


1.411 
1.540 


1.475 


3.261 
3.164 


3.213 


50.72 
44.32 
44.06 
46.88 


45.47 

46.86 
42.80 
40.75 
44.00 


42.38 
52.82 
54.07 
53.24 


53.38 


46.56 
43.62 


36.54 
35.05 


35.80 


29.75 
32.47 


31.1] 


76.71 
74.45 


75.58 


experiments  were  also  made  by  Kellner  with  wheat  gluten,  starch, 
oil,  and  extracted  straw,  the  aim  of  which  was  to  determine  the 
metabolizable  energy  of  the  various  digestible  nutrients. 

As  an  illustration  of  this  class  of  experiments  we  may  take  one 
upon  molasses  with  Ox  F,*  comparing  Period  3,  on  the  basal  ration, 
*  hoc.  cit.,  53,  172-227. 


THE  FOOD  AS  A  SOURCE   OF  ENERGY.  291 

with  Period  6,  on  the  same  ration  with  the  addition  of  molasses. 
Comparing,  first,  the  organic  matter  of  the  two  rations  we  have 
the  following: 


Total  Organic 

Matter  Fed, 

Grms. 

Organic  Matter  in 

Molasses, 

Grms. 

Period  6 

8262 
6630 

1632 

1702 

"     3 

0 

1702 

In  the  period  with  molasses  70  grams  less  of  the  basal  ration 
was  consumed  than  in  the  period  without,  and  a  correction  must 
accordingly  be  made  for  this  in  the  way  explained  on  page  288. 
The  energy  of  food  and  excreta  in  the  two  experiments  (that 
of  the  urine  being  corrected  to  nitrogen  equilibrium),  together  with 
the  correction  for  the  70  grams  of  organic  matter,  is  shown  in  the 
following  table : 


Food, 
Cals. 

Feces, 
Cals. 

Urine, 
Cals. 

Methane, 
Cals. 

Metaboliz- 
able  Energy, 

Cals. 

Period  6 

"     3 

37,946.2 
31,327.8 

11,365.8 
9,599.2 

1,786.1 
1,530.0 

2,397.9 
2,560.7 

22,396.4 
17,637.9 

Correction  .... 

6,618.4 
+  330.8 

1,766.6 
+  101.3 

256.1 
+  16.2 

-162.8 
+  27.0 

4,758.5 
+  186.3 

6,949.2 

1,867.9 

272.3 

-135.8 

4,944.8 

Dividing  the  metabolizable  energy  of  the  molasses,  4944.8  Cals., 
by  the  number  of  grams  consumed,  1702,  gives  the  metabolizable 
energy  of  1  gram  of  organic  matter  as  2.905  Cals. 

Real  and  Apparent  Metabolizable  Energy. — The  above 
figures,  however,  demand  more  critical  discussion.  While  the  addi- 
tion of  molasses  to  the  basal  ration  increased  the  amount  of  poten- 
tial energy  carried  off  in  the  feces  and  urine,  it  diminished  that  in 
the  methane;  that  is,  it  acted  in  some  way  to  check  the  fermen- 
tation in  the  digestive  tract  to  which  this  gas  owes  its  origin.  In 
other  words,  under  the  influence  of  the  molasses  the  loss  of  energy 
by  fermentation  of  the  basal  ration  was  diminished  by  135.8  Cals., 
and  this  amount,  by  the  method  of  computation,  is  added  to  the 
metabolizable  energy  of  the  molasses. 


292 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Moreover,  the  loss  of  energy  in  the  feces  is  a  complex  of  sev- 
eral factors.  The  amounts  of  organic  matter  and  of  the  several 
nutrients  excreted  in  the  feces  in  the  two  periods  (not  corrected  for 
the  70  grams  difference  in  organic  matter  consumed)  were  as 
follows : 


Organic 

Matter, 
Grms. 

Protein, 
Grms. 

Crude 
Fiber, 
Grms. 

Nitrogen-       Crude 

free               Fat> 

Extract,         Grms. 

Grms. 

Period  6 

2132 
1797 

403 
284 

595 
527 

1068            66 

"     3 

924            62 

335 

119 

68 

144     i         4 

In  addition  to  protein  and  nitrogen-free  extract,  which  may 
possibly  represent  indigestible  material  in  the  molasses,  the  feces 
contained  68  grams  more  crude  fiber  and  4  grams  more  fat  in  Period 
6  than  in  Period  3.  These  cannot  have  been  derived  from  the 
molasses,  since  the  latter  does  not  contain  these  ingredients.  This 
feeding-stuff,  in  other  words,  diminished  the  apparent  digestibility 
of  the  fiber  and  fat  of  the  basal  ration.  As  a  matter  of  fact,  the 
ingredients  of  molasses  being  practically  all  soluble  in  water,  it  is 
probable  that  nearly  all  the  difference  in  the  amount  digested  is 
due  to  the  diminished  apparent  digestibility  of  the  basal  ration 
under  the  influence  of  the  molasses. 

The  figure  above  given  for  the  metabolizable  energy  includes  all 
these  effects;  that  is,  it  shows  the  net  result  as  regards  energy  ob- 
tained from  molasses  fed  under  the  conditions  of  these  experiments, 
the  nutritive  ratio  of  the  basal  ration  being  1  :  5.8  and  that  of  the 
molasses  ration  1  :  6.4.  To  get  at  the  actual  amount  of  energy  set 
free  from  the  molasses  itself  we  should  need  to  subtract  from  the 
metabolizable  energy  as  calculated  above  the  energy  corresponding 
to  the  decreased  excretion  of  methane  and  to  add  to  it  the  metabo- 
lizable energy  corresponding  to  the  decrease  in  the  amounts  of  crude 
fiber  and  ether  extract  digested,  assuming  that  all  the  excess  of 
protein  and  nitrogen-free  extract  in  the  feces  was  derived  from  the 
molasses.     Computed  in  this  way*  the  real  metabolizable  energy 

*  One  gram  of  crude  fiber  =3.3  Cals.,  and  one  grain  of  ether  extract  = 
8.3Cals.     Seep.  332. 


THE  FOOD  AS  A  SOURCE  OF  ENERGY.  293 

of  the  organic  matter  is  2.977  Cals.  per  gram.  This  would  be  a  mini- 
mum figure,  while  if  we  assume,  as  suggested  above,  that  the  mo- 
lasses is  entirely  digestible,  this  figure  is  still  too  low  and  should  be 
increased  to  equal  the  gross  energy  of  the  organic  matter. 

If,  however,  either  one  of  these  latter  values  were  used  in  com- 
puting the  metabolizable  energy  of  rations,  the  results  would  obvi- 
ously be  too  high  unless  corrections  were  made  for  the  effect  upon 
the  apparent  digestibility  of  the  other  feeding-stuffs  in  the  ration. 
The  figure  first  computed,  while  including  several  different  effects, 
nevertheless  seems  better  adapted  for  use  in  actual  computations 
under  average  conditions,  while  the  second  gives  the  more  accurate 
idea  of  the  store  of  metabolizable  energy  contained  in  the  feeding- 
stuff  regarded  by  itself.  The  distinction  is  analogous  to  that 
between  apparent  and  real  digestibility,  and  we  may  accordingly 
speak  of  the  apparent  and  the  real  metabolizable  energy  of  feeding- 
stuffs. 

The  whole  of  our  present  discussion  of  the  metabolizable 
energy  of  the  organic  matter  (total  or  digestible)  of  food  materials 
relates  to  the  apparent  metabolizable  energy.  This  is  obvious  as 
regards  the  concentrated  feeds  from  the  above  example,  and  logic- 
ally applies  also  to  those  cases  in  which  coarse  fodders  were  added 
to  the  basal  ration,  while  in  the  case  of  the  coarse  fodders  used  alone 
the  distinction  vanishes  or  is  reduced  to  one  between  apparent  and 
real  digestibility.  The  experiment  with  beet  molasses  well  illus- 
trates the  difficulties  in  the  way  of  determining  the  actual  metabo- 
lizable energy  of  feeding-stuffs  which  cannot  be  used  alone. 

Beet  Molasses. — In  two  later  experiments  the  addition  of 
molasses  increased  instead  of  diminishing  the  excretion  of  methane. 
The  results  of  the  three  experiments  upon  molasses,  computed  in 
the  same  manner  as  the  experiments  upon  coarse  fodders,  are  con- 
tained in  Table  II  of  the  Appendix. 

In  the  last  two  experiments  10  to  12  per  cent,  of  the  energy  of 
the  molasses  was  lost  in  the  products  of  intestinal  fermentation, 
but  this  was  more  than  counterbalanced  by  its  less  effect  upon  the 
digestibility  of  the  rations,  so  that  the  final  result  is  a  higher  figure 
for  the  apparently  metabolizable  energy  than  in  the  first  experi- 
ment. Summarizing  the  results  per  gram  as  in  the  case  of  the 
coarse  fodders  we  have: 


294 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Gross 

Energy, 

Cals. 


Apparently 

Metabolizable 

Energy, 

Cals. 


Per  Cent 

Metabolizable. 


Sample  I 

II,  Ox  H 
"     "  J. 


Average,  Sample  II 


4.084 
4.188 


2.905 
3.308 
3.044 


3.176 


71.16 
70.00 
72.70 


75.85 


Starch. — In  a  considerable  number  of  the  trials  commercial 
starch  was  added  to  the  basal  ration.  The  earlier  experiments  by 
Kiihn  were  intended  primarily  to  throw  light  on  the  possible  for- 
mation of  fat  from  carbohydrates  (compare  p.  177).  In  them, 
starch  was  added  to  a  ration  of  coarse  fodder  only  and  the  nutritive 
ratio  was  purposely  made  very  wide,  the  result  being  that  more  or 
less  of  the  starch  escaped  digestion.  In  the  later  experiments  by 
Kellner  the  starch  was  added  to  a  mixed  ration.  Except  in  the 
first  two  experiments  the  nutritive  ratio  was  a  medium  one  and 
but  traces  of  starch  escaped  digestion.  It  will  be  convenient, 
therefore,  to  tabulate  these  two  classes  of  experiments  separately, 
as  has  been  done  in  Tables  III  and  IV  of  the  Appendix,  the  com- 
putations being  made  as  in  the  previous  cases. 

The  same  remarks  which  were  made  on  p.  291  concerning  the 
distinction  between  real  and  apparent  metabolizable  energy  apply 
to  these  results.  As  computed  they  represent  the  net  gain  to  the 
organism  from  the  consumption  of  starch  and  are  the  algebraic  sum 
of  several  factors.  In  particular,  there  was  a  considerable  loss  of 
energy  in  the  feces,  even  in  the  later  experiments  in  which  but 
traces  of  the  starch  itself  escaped  digestion.  In  other  words,  the 
starch  either  lowered  the  digestibility  of  the  basal  ration  or  in- 
creased the  formation  of  fecal  metabolic  products  or  both.  The 
method  of  computation  adopted  virtually  looks  upon  this  as  part 
of  the  necessary  expenditure  in  the  digestion  of  the  starch.  On 
the  other  hand,  there  are  several  cases  in  which  there  was  a  de- 
crease in  the  outgo  of  potential  energy  in  the  urine,  even  after  the 
results  are  corrected  to  nitrogen  equilibrium.  This,  from  our  pres- 
ent point  of  view,  is  credited  to  the  starch  and  increases  its 
apparent  metabolizable  energy. 


THE  FOOD  AS  A  SOURCE  OF  ENERGY. 


295 


The  results  on  starch,  expressed  in  Calories  per  gram  of  organic 
matter,  may  be  summarized  as  follows : 


Gross 

Energy, 

Cals. 


Apparent 

Metaboliz- 

able 

Energy, 

Cals. 


Per  Cent. 

Metaboliz- 

able. 


Kuhn's  Experiments  : 

Sample  I.  Ox  III 

"      "     "    IV 

Average 

Sample  II,  Ox  V,  Period  2a 
"  "  "  "  26 
lt     «     VI)    u        26 

"     "     "      "       3. 

Average 

Average  of  I  and  II 

Kellner's  Experiments  : 

Samples  I  and  II  Ox  B 

"        "    "     "    "   C 

Average 

Sample  III,  Ox  D 

"      "     F 

"      "    G 

Average 

Sample  IV,  Ox  H 

"       "J 

Average 

Average  of  III  and  IV  . . . 


4.249 
4.249 


3.029 
2.705 


4.249 

4.236 
4.236 
4.236 
4.236 


2.867 

3.347 
3.161 
3.018 
2.964 


4.236 
4.243 


4.165 
4.165 


3.123 
2.995 


2.027 
2.028 


4.165 

4.156 
4.156 
4.156 


2.028 

2.792 
2.969 
3.214 


4.151 


4.180 
4.180 


2.992 


3.313 
3.017 


4.180 
4.168 


3.165 
3.079 


71.21 
63.71 


67.46 

78.95 
74.68 
71.26 
69.98 


73.72 
70.59 

48.62 


48.65 

67.20 
71.44 
77.32 


71 


79.22 
72.16 


75.69 
73.84 


Wheat  Gluten. — Seven  experiments  upon  commercial  wheat 
gluten  are  reported,  three  by  Kuhn  and  four  by  Kellner.  The 
chemical  composition  of  the  dry  matter  of  the  three  samples  of 
gluten  employed  is  shown  in  the  first  table  on  the  next  page. 

In  Kuhn's  experiments  the  gluten  caused  a  marked  increase  in 
the  apparent  digestibility  of  the  basal  ration,  which  by  our  method 
of  computation  augments  the  apparent  metabolizable  energy  of 
the  gluten,  so  that  in  one  case  it  amounts  to  over  101  per  cent,  of 
the  gross  energy.     The  correction  for  organic  matter  is  also  rela- 


296 


PRINCIPLES    OF  ANIMAL   NUTRITION. 


Kiihn's 

Experiments, 

Per  Cent. 

Kellner's  Experiments. 

Oxen  B  and  C, 
Per  Cent. 

OxD, 
Per  Cent. 

Ash 

1.36 
87.88 
0.47 
8.07 
2.22 

2.86 
83.45 

0.08 
13.35 

0.26 

2.80 

82  67 

0.43 

13.38 

0.72 

100.00 

100.00 

100.00 

tively  large.  In  Kellner's  experiments  the  variations  are  not  so 
great.  Computed  as  before,  the  results  are  as  shown  in  Table  V  of 
the  Appendix.  Summarizing  Kellner's  figures,  as  probably  the 
more  accurate,  we  have  per  gram  of  organic  matter — 


Gross  Energy 
Cais. 

Apparent 

Metabohzable 

Energy. 

Cals. 

Per  Cent 
Metabolizable. 

Sample  I,  Ox  B,  Period  1 

"       "     "    "       "        3.... 
"       "     "    c 

5.675 
5.675 
5.675 

3.019 
3.719 
4.062 

53.18 
65.55 
71.61 

5.675 
5.808 
5.742 

3.600 
4.061 
3.831 

63.45 

69  90 

Average  of  I  and  II  

66.68 

The  wheat  gluten  was  by  no  means  pure  protein  and  the  above 
figures  of  course  apply  to  the  feeding- stuff  as  a  whole,  including  its 
fat  and  carbohydrates  as  well  as  its  protein.  The  question  of  the 
metabolizable  energy  of  the  latter  will  be  considered  subsequently. 

Peanut  Oil. — Three  experiments  with  this  substance  are  re- 
ported by  Kellner.  In  the  first  the  oil  was  given  in  the  form  of  an 
emulsion,  prepared  by  saponifying  a  small  portion  of  the  oi)  with 
sodium  hydrate,  and  was  completely  digested  In  the  second  and 
third  experiments  it  was  emulsified  with  lime-water.  In  this  form 
it  was  less  well  digested,  and  in  one  case  (Ox  F,  affected  the  digesti- 
bility of  the  basal  ration  unfavorably.  The  results  per  gram  of 
organic  matter,  computed  as  before,  constitute  Table  VI  of  the 
Appendix  and  are  summarized  in  the  following  table: 


THE  FOOD  AS  A  SOURCE   OF  ENERGY. 


297 


Gross  Energy, 
Cals. 

Metabolizable 
Energy, 

Cals. 

Per  Cent. 
Metabolizable. 

Sample  I,  Ox  D 

"     11,     "   F 

"      "      "G 

9.493 
I         9.464        | 

7.382 
4.973 
5.623 

77.76 
52.52 
59.39 

5.298 

55.96 

Summary. — The  foregoing  results  may  be  conveniently  sum- 
marized in  the  table  below,  which  shows  the  average  gross  energy 
per  gram  of  organic  matter,  the  percentage  of  this  gross  energy 
carried  off  unmetabolized  in  the  various  excreta,  and  the  apparent 
metabolizable  energy  expressed  both  per  gram  of  total  organic 
matter  and  as  a  percentage  of  the  gross  energy : 


Meadow  hay 

Timothy  hay 

Oat  straw 

Wheat  straw 

Extracted  rye  straw 

Beet  molasses,  Sample  II 

Starch,  Kiihn's  experiments 

"       Kellner's  experiments: 

Heavy  rations 

Medium  rations 

Wheat    gluten,   Kellner's   experi 

ments 

Peanut  oil,  Ox  D 

"        "       "   F 

"       "      "  G 


Gross 
En'gy 

per 
Grm. 

Or- 
ganic 
Mat- 


4.751 
4.670 
4.816 
4.743 
4.251 
4.188 
4.243 


16.5 
168 


5.742 

9.493 

.464 

.464 


Percentage  Loss  in 


40.96 
47.27 
56.80 
58.22 
12.75 
9.93 
19.59 

55.91 
17.61 

20.16 
24.34 
64.77 
41.00 


5.71 
2.61 
2.08 
2.37 

-0.79 
2.91 

-0.92 

-2.07 
-0.66 

13.08 

-1.08 

-1.19 

1.37 


6.77 
6.50* 
5.32 
8.30 
12.46 
11.31 
10.74 

-2.49 
9.21 

0.08 
-1.02 
■16.10 
-1.76 


Apparent 

Metabolizable- 

Energy. 


Per 
Grm. 
Or- 
ganic 
Mat- 
ter, 
Cals. 


2.213 
2.037 
1.724 
1.475 
3.213 
3.174 
2.995 

2.028 
3.079 

3.831 
7.382 
4.973 
5.623 


*  Estimated. 


Digestible  Organic  Matter. 
As  appears  especially  from  the  figures  of  the  last  table,  the  loss 
of  energy  in  the  feces  is  the  one  which  is  subject  to  the  greatest  vari- 
ation.    In  other  words,  the  digestibility  of  a  feeding-stuff  is  the 


198 


PRINCIPLES    OF  ANIMAL    NUTRITION. 


most  important  single  factor  in  determining  its  content  of  metabo- 
lizable  energy.  We  may  eliminate  this  factor  by  computing,  on 
the  basis  of  the  determinations  of  digestibility,  the  energy  of  the 
digested  organic  matter  and  the  proportion  of  this  energy  which 
was  lost  in  urine  and  methane  or  was  metabolizable.  In  this  way 
we  may  secure  figures  which  will  be  useful'as  a  basis  for  estimat- 
ing the  energy  values  of  rations  in  experiments  in  which  it  has 
not  been  determined,  and  which  will  also  afford,  from  some  points 
of  view,  a  better  idea  of  the  relative  extent  of  the  losses  other  than 
those  in  the  feces. 

Coarse  Fodders  Alone. — In  the  cases  in  which  coarse  fodder 
constituted  the  exclusive  ration  the  computation  from  the  data 
given  on  p.  286  and  the  amounts  of  organic  matter  apparently 
digested  in  the  several  experiments  is  very  simple  and  yields  the 
following  results  per  gram  digested  organic  matter: 


Gross 
Energy. 


Urine, 
Per 
Cent. 


Methane 
Per 
Cent. 


Metabolizable 
Energy. 


Per 
Cent. 


Per 
Grm. 
Cals. 


A 

II 

V 

VI 

XX 

I 


Meadow  hay  I  . 
"  A. 
"  B. 
"  B. 
"  M. 
"     II. 


4.509 
4.408 
4.317 
4.398 
4.452 
4.371 


9.75 
8.98 
8.25 
8.65 
13.85 
9.59 


10.35 
14.62 
12.00 
12.35 
11.36 
11.90 


79.90 
76.40 
79.75 
79.00 
74.79 
78.51 


Average 

Average  for  timothy  hay  . 


4.409 
4.377 


9.85 
4.95 


12.09 
12.33 


78.06 
82.72 


3.603 
3.368 
3.443 
3.474 
3.330 
3.432 

3.442 
3.620 


Coarse  Fodders  Added  to  Basal  Ration. — From  the  re- 
sults contained  in  Table  I  of  the  Appendix  we  may  compute  in  sub- 
stantially the  same  manner  the  total  and  metabolizable  energy  of 
the  digestible  organic  matter  of  the  coarse  fodders  which  were 
added  to  the  basal  rations.  In  the  table  referred  to,  a  correction 
was  introduced  for  the  small  differences  in  the  amount  of  the  basal 
rations  consumed  in  the  periods  compared.  In  the  present  com- 
putations it  has  been  assumed  that  the  organic  matter  of  these 
small  differences  possessed  the  same  digestibility  as  the  total  organic 
matter  of  the  basal  ration.     For  example,  in  the  case  of  Ox  H, 


THE  FOOD  AS  A  SOURCE  OF  ENERGY.  299 

Periods  4  and  7,  the  amounts  of  digestible  organic  matter  in  the 

two  rations  were : 

Period  7 7106  grams 

Period  4 4845      " 

Difference 2261      " 

The  table  shows,  however,  that  in  Period  7  the  animal  received  44 
grams  more  of  total  organic  matter  in  the  basal  ration  than  in 
Period  4.  In  the  latter  period  the  digestibility  of  the  organic 
matter  was  found  to  be  75.7  per  cent.  Consequently,  of  the 
excess  of  2261  grams  of  digestible  organic  matter  in  Period  7 
44X0.757  =  33  grams  maybe  regarded  as  derived  from  the  basal 
ration  and  2261  —  33  =  2228  grams  from  the  meadow  hay  added. 
The  corresponding  corrected  amounts  of  energy  as  given  in  the 
same  table  are — 


Per  Grm.  Digested 

Organic  Matter 

Cals. 


Energy  of  added  hay  (corrected) . 
"        "  corresponding  feces.  .  . 


"        "  digested  matter. 
Metabolizable  energy 


15728.6 
5471.0 


10257.6 
8504.8 


4.604 
3.817 


The  table  on  the  next  page  contains  the  results  of  these  com- 
putations expressed  per  gram  of  digested  organic  matter.  Kell- 
ner*  has  made  the  same  comparison  in  a  slightly  different  man- 
ner. His  results  for  the  gross  energy  of  the  digested  matter  are 
given  subsequently  (p.  310).  Those  for  metabolizable  energy  do 
not  differ  materially  from  those  here  given. 

Concentrated  Feeding-stuffs. — The  results  of  experiments 
upon  concentrated  feeding-stuffs  may  of  course  be  computed  in  the 
same  manner  as  those  upon  coarse  fodders  just  considered.  In  the 
case  of  materials  like  starch,  oil,  and  gluten,  however,  which  differ 
widely  from  ordinary  feeding-stuffs  and  which  produce  material 
and  readily  traceable  effects  upon  the  digestibility  of  the  basal 
ration,  relatively  little  value  attaches  to  computations  of  the  appar- 
ent metabolizable  energy,  and  only  the  average  results  with  these 
materials  have  been  included  in  the  summary  on  page  301  for  the 
*  Loc.  tit.,  53,414  and  447. 


3°° 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


1 

2 

2 
7 
2 

2 

1 

I 

I 

5 

5 

Total 

Energy, 

Cals. 

Loss  in 

Apparent 

Metabolizable 

Energy. 

I 
q 
< 

Urine. 
Per  Cent. 

Methane, 
Per  Cent. 

Per  Cent. 

Per  Grm.. 
Cals. 

F 
G 

Meadow  Hay  : 

Sample  V 

"       v 

Average 

Sample  VI 

"       VI 

"      VI 

Average 

Oat  Straw  : 

Sample  II 

"       II 

Average 

Wheat  Straw  : 
Sample  I 

Average 

Extracted  Straw  : 
Sample  I 

Average 

4.356 
4.496 

8.61 
7.72 

10.20 
12.58 

81.19 
79.70 

3.537 
3.583 

II 
H 
J 

4.426 

4.531 
4.604 
4.506 

8.17 

8.32 
7.66 
9.64 

11.39 

7.74 
9.43 
9.33 

80.44 

83.94 
82.91 
81.03 

3.560 

3.803 
3.817 
3.651 

F 
G 

4.547 

4.441 
4.586 

8.54 

5.30 
4.32 

8.83 

10.17 
14.42 

82.63 

84.53 
81.26 

3.757 

3.754 
3.726 

H 
J 

4.514 

4.488 
4.397 

4.81 

4.75 
6.49 

12.30 

20.11 
19.67 

82.89 

75.14 

73.84 

3.740 

3.373 
3.247 

H 
J 

4.443 

4.240 
4.164 

5.62 

-0.52 
-1.29 

19.89 

13.99 
14.58 

74.49 

86.53 
86.71 

3.310 

3.668 
3.611 

4.202 

-0.91 

14.29 

86.62 

3.640 

• 

sake  of  completeness.  Those  upon  peanut  oil  have  been  omitted, 
since  the  varying  effect  upon  digestibility  and  upon  the  methane 
fermentation  makes  the  results  as  computed  in  this  way  appear 
of  questionable  significance. 

Summary. — The  average  results  upon  the  various  materials 
experimented  with  are  summarized  on  the  opposite  page. 

As  appears  from  the  figures  of  the  table,  the  apparent  metabo- 
lizable energy  of  the  digestible  organic  matter  of  the  different  coarse 
fodders  is  quite  uniform.  At  first  sight  it  appears  somewhat  sur- 
prising that  oat  straw  should  show  more  favorable  results  than  hay, 
but  the  reason  is  readily  seen  in  the  smaller  loss  which  takes  place 
in  the  urine ;  in  wheat  straw  this  loss  is  somewhat  larger,  while  that 


THE  FOOD  AS  A  SOURCE  OF  ENERGY. 
ENERGY  OF    DIGESTED  ORGANIC   MATTER. 


3OI 


Total 

Energy. 

Cals. 

Loss  in 

Apparent 

Metabolizable 

Energy. 

Urine, 
Per 
Cent. 

Methane, 
Per 
Cent. 

Per 
Cent. 

Per 
Grm., 

Cals. 

Meadow  hay  (seven  samples) 

4.439 
4.377 
4.514 
4.443 
4.202 
4.124 
4.192 
4.012 
5.749 

9.62 

4.95 

4.81 

5.62 

-0.91 

3.24 

-1.19 

-0.92 

16.59 

11.52 
12.33 
12.30 
19.89 
14.29 
12.52 
13.42 
11.12 
0.02 

78.86 
82.72 
82.89 
74.49 
86.62 
84.24 
87.77 
89.80 
83.39 

3.501 
3  620 

3  740 

Wheat  straw 

3  310 

3  640 

3  473 

Starch,  Klihn's  experiments 

"      Kellner's  experiments  * 

Wheat  gluten,  Kellner's  experiments.. 

3.679 
3.603 

4.792 

Average  of  Samples  III  and  IV. 
in  the  methane  is  considerably  larger,  resulting   in  a  materially 
lower  figure  for  metabolizable  energy. 

The  results  summarized  in  the  two  preceding  tables,  it  should 
be  remembered,  include,  as  already  pointed  out,  all  the  effects  pro- 
duced by  the  addition  of  the  material  under  experiment  to  the 
basal  ration ;  that  is,  they  give  the  apparent  metabolizable  energy. 
In  the  case  of  the  coarse  fodders  no  other  method  of  computation 
is  practicable,  and  the  same  would  be  true  in  most  instances  of 
ordinary  concentrated  commercial  feeding-stuffs.  In  such  cases  it 
is  rarely  possible  to  distinguish  with  accuracy  between  the  energy 
derived  from  the  material  experimented  with  and  the  subsidiary 
effects  of  the  latter  upon  the  digestibility  of  the  several  in- 
gredients of  the  ration  or  upon  the  losses  of  energy  in  urine  and 
methane.  We  may  anticipate,  therefore,  that  the  results  of  future 
determinations  of  the  metabolizable  energy  of  ordinary  feeding- 
stuffs  will  of  necessity  be  expressed  substantially  in  the  summary 
manner  here  employed. 

With  the  nearly  pure  nutrients  used  in  many  of  Kellner's  ex- 
periments the  case  is  different.  Here  it  is  possible  to  take  account, 
to  a  large  degree,  of  the  secondary  effects,  such  as  those,  for  exam- 
ple, which  in  the  case  of  wheat  gluten  result  in  figures  exceeding 
100  per  cent,  for  the  apparent  metabolizable  energy,  and  to  compute 
results  which  represent  more  nearly  the  actual  metabolizable  energy 
contained  in  the  substances  themselves.     In  these  cases,  therefore, 


3°- 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


the  averages  of  the  tables  are  of  less  significance  than  the  results 
given  in  the  following  pages,  where  the  digestible  nutrients  are 
made  the  basis  of  the  computation. 

ENERGY    OF    DIGESTIBLE    NUTRIENTS. 

The  foregoing  paragraphs  have  dealt  with  the  apparent 
metabolizable  energy  of  feeding-stuffs,  and  the  results  have 
been  expressed  in  terms  of  total  or  of  digestible  organic  matter, 
or  as  percentages  of  gross  energy.  We  now  turn  to  a  con- 
sideration of  such  data  as  are  available  regarding  the  several  con- 
ventional groups  of  nutrients  into  which  the  food  of  herbivorous 
animals  is  ordinarily  divided  and  inquire  whether  it  is  possible  to 
compute  average  factors  for  their  metabolizable  energy  which 
shall  be  useful  in  themselves  and  be  of  value  particularly  for  pur- 
poses of  comparison  with  earlier  experiments.  This  was  the  special 
purpose  of  Kellner's  investigations,  and  his  experiments  supply 
valuable  data  on  these  points  as  regards  cattle  and  presumably 
other  ruminants,  which  may  be  supplemented  to  a  certain  extent 
from  experiments  by  other  investigators  upon  horses  and  swine. 
In  considering  the  experiments  from  this  standpoint,  Kellner's 
discussion  and  methods  of  computation  have  been  closely  followed, 
the  attempt  being  made  to  compute  as  accurately  as  possible  the 
real  metabolizable  energy  of  the  several  nutrients. 

Gross  Energy. 
If  it  were  possible  to  add  pure  nutrients  to  a  basal  ration  and 
be  sure  that  they  would  have  no  effect  upon  the  utilization  of  the 
latter,  it  would  be  a  comparatively  simple  matter  to  determine  their 
real  metabolizable  energy.  As  a  matter  of  fact,  however,  as  has 
been  seen,  this  is  not  possible.  Not  only  is  it  impracticable  to  secure 
large  quantities  of  pure  nutrients,  but  each  such  addition  to  the  basal 
ration  is  liable  to  affect  especially  the  digestibility  of  the  latter. 
Consequently  the  difference  in  metabolizable  energy  between  the 
two  rations  fails  to  represent  correctly  the  real  metabolizable  energy 
of  the  nutrient  added.  In  order  to  compute  the  latter  we  must 
have  a  basis  for  correcting  the  results  for  the  small  variations  in  the 
amounts  of  other  nutrients  digested,  and  for  this  purpose  we  need 
to  know  the  total  or  gross  energy  of  the  digested  matters. 


THE  FOOD  AS  A  SOURCE   OF  ENERGY. 


3°3 


Crude  Fiber. — In  four  of  his  experiments  on  hay  fed  alone, 
Kellner  *  determined  the  heats  of  combustion  of  the  crude  fiber  of 
the  food  and  of  the  feces  with  the  following  results  per  gram : 


Crude  Fiber  of 
Hay,  Cals. 

Crude  Fiber  of 
Feces,  Cals. 

I.... 

II.... 
III.... 

IV.... 

4.4350 
4.3907 
4.4548 
4 . 4230 

4 . 7378 
4.7423 
4.9037 
4.7426 

It  appears  from  these  figures  that  the  crude  fiber  of  meadow 
hay  has  a  higher  heat  value  than  pure  cellulose  (4.1854  Cals.  accord- 
ing to  Stohmann),  obviously  due  to  the  admixture  of  compounds 
richer  in  carbon,  while  the  indigestible  crude  fiber  of  the  feces  has 
a  still  higher  heat  value.  Merrill  f  has  also  reported  similar  results 
for  the  crude  fiber  of  oat  hay,  clover  silage,  and  oat  and  pea  silage, 
as  follows: 


Crude  Fiber  of  Fodder. 
Cals.  per  Grm. 

("rude  Fiber  of  Feces. 
Cals.  per  Grm. 

4 .  405 
4.610 
4.667 

4.662 
5.215 
4.820 

Oat  and  pea  silage .... 

It  follows  that  the  digested  portions  of  the  crude  fiber  must 
contain  less  potential  energy  than  the  crude  fiber  of  the  feed,  and 
from  the  known  digestibility  of  the  latter  it  is  easy  to  calculate 
what  the  heat  of  combustion  of  the  digested  portion  must  be. 
Kellner 's  results,  after  deducting  5.711  Cals.  per  gram  for  the  slight 
amounts  of  nitrogenous  matter  still  contained  in  the  crude  fiber, 
were  as  shown  on  the  next  page. 

The  average  result  shows  that  not  only  the  chemical  com- 
position but  likewise  the  heat  of  combustion  of  the  digested  crude 
fiber  varies  but  little  from  that  of  pure  cellulose.  Merrill's  figures, 
computed  in  the  same  manner  from  the  data  of  the  digestion 
experiments  reported  by  Bartlett,J  but  without  the  correction  for 

*  Loc.cit.,  47,299. 

t  Maine  Expt.  Station,   Bull.  67,  p.  170. 

J  Ibid.,  pp.  140  and  150,  and  Report,  1898,  p.  87. 


3°4 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Crude 
Fiber, 
Grms. 


Equivalent 

Energy, 

Cals. 


In  hay.. 
"  feces. 


II 


III 


IV 


Digested  fiber , 

Heat  of  combustion  per  gram  . 


In  hay  . 
"    feces. 


Digested  fiber 

Heat  of  combustion  per  gram . 


In  hay . 

"  feces 


Digested  fiber 

Heat  of  combustion  per  gram . 


In  hay. . 
"  feces. 


Digested  fiber 

Heat  of  combustion  per  gram . 


Average  heat  of  combustion  per  gram. 


2832 
1034 


1798 


2394 

822 


1572 


2329 
769 


1560 


1978 
716 


1262 


12532.8 
4869.2 


7663.6 
4.2623 


10503.0 
3878.1 


6624.9 
4.2143 


10367.7 
3754.0 


6613.7 
4.2396 


8732.0 
3479.2 


5252.8 
4.1623 


4.2196 


nitrogenous   matter,  give  the  following  results  per  gram  for  the 
digested  crude  fiber : 

Oat  hay 4. 161  Cals. 

Clover  silage 4. 123     " 

Oat  and  pea  silage 4 .  584     " 

Ether  Extract. — Similar  determinations  by  Kellner  *  on  the 
ether  extract  of  hay  and  feces  yielded  the  following  results  per  gram : 


Ether  Extract 
of  Hay,  Cals. 

Ether  Extract 
of  Feces.  Cals. 

I 

II 

Ill 

IV 

V 

Average . . 

9.1604 

I    9.3240    \ 

9.0554 
9.1062 

9.7690 
9.8923 
9.8646 
9.8314 
9.7640 

9.8243 

9.1940 

*  Loc.  cit.,  47,  301. 


THE  FOOD  AS  A   SOURCE  OF  ENERGY.  305 

A  calculation  similar  to  that  made  for  the  crude  fiber  yielded  the 
following  figures  for  the  heat  of  combustion  of  the  digested  portion : 

1 8.239Cals. 

II 7.802     " 

III 8.185     " 

IV 8.267     " 

V 8.685     " 

That  these  results  are  more  or  less  discordant  is  not  surprising 
in  view  of  the  uncertain  elements  involved  in  the  determinations. 
Applying  the  average  figures  for  the  energy  per  gram  of  the  ether  ex- 
tracts to  the  total  amounts  eaten  and  excreted  in  the  five  experiments 
taken  together,  we  have  for  the  average  energy  of  the  apparently 
digested  ether  extract  8.322  Cals.  per  gram,  a  figure  considerably 
below  the  results  recorded  on  p.  238  for  either  animal  or  vegetable 
fats.  It  must  be  remembered,  however,  that  the  ether  extract  of 
the  feces  contains  more  or  less  metabolic  products,  so  that  the 
above  result  does  not  represent  the  actual  energy  of  the  digested 
ether  extract.  It  does,  however,  represent  the  energy  correspond- 
ing to  the  difference  between  food  and  feces  with  which  we  reckon 
in  computing  rations,  and  from  this  point  of  view  it  is  of  value. 

Nitrogen-free  Extract. — The  nitrogen-free  extract  cannot 
be  separated  and  examined  like  the  crude  fiber  and  the  ether  ex- 
tract, but  it  is  possible  to  arrive  at  an  estimate  of  its  heat  of  com- 
bustion indirectly.  For  this  purpose  Kellner  assumes  the  average 
heat  of  combustion  of  the  proteids  (proteid  nitrogen  X  6.25)  as 
5.711  Cals.  per  gram  and  that  of  the  non-proteids  as  equal  to  that 
of  asparagin,  viz.,  3.511  Cals.  per  gram.  By  subtracting  from  the 
gross  energy  of  food  or  feces  as  directly  determined  the  energy  of 
the  amounts  of  proteids,  non-proteids,  crude  fiber,  and  ether, ex- 
tract shown  by  analysis  to  be  present,  he  computes  the  heat  of 
combustion  of  the  nitrogen-free  extract.  Furthermore,  by  compar- 
ing the  results  on  food  and  feces  as  in  the  case  of  the  crude  fiber  the 
heat  of  combustion  of  the  digested  portion  may  be  computed. 
The  results  per  gram  of  such  a  computation  for  the  same  four  ex- 
periments were :  * 

*  hoc.  cU.,  47,  303-306. 


3°<5 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


N.-fr.  Extract 

of  Hay, 
Cals.  per  Gram 

N  -fr.  Extract 

of  Feces, 
Cals.  per  Gram. 

Digested  N  -fr. 

Extract. 
Cals.  per  Gram. 

I 

II 

4.5713 
4.6547 
4.5029 
4.6081 

5.2834 
5.4212 
5.1058 
5.2484 

4.203 
4.146 

Ill 

4.246 

IV 

4  335 

4.584 

5.265 

4.232 

In  view  of  the  indirect  nature  of  the  computation  the  results 
agree  as  well  as  could  be  expected  and  show  that,  as  might  be 
anticipated  from  its  chemical  composition,  the  heat  of  combustion 
of  the  digested  portion  of  the  nitrogen-free  extract  did  not  vary- 
widely  from  that  of  starch. 

Digested  Matter  of  Mixed  Rations. — The  Mockern  experi- 
ments afford  accurate  data  as  to  the  energy  of  the  total  digested 
matter  of  a  large  number  of  mixed  rations.  Kellner  *  has  com- 
pared this  with  the  computed  energy  of  the  same  material.  For 
this  computation  the  factors  used  were :  for  fat,  8.322  Cals.  per  gram ; 
for  crude  fiber  and  nitrogen-free  extract,  the  average  of  Stohmann's 
figures  for  starch  and  cellulose,  4.184  Cals.  per  gram;  for  protein 
provisionally,  5.711  Cals.  per  gram.  Of  the  fifty-nine  experiments, 
twelve,  in  which  large  amounts  of  wheat  gluten  or  oil  were  fed, 
showed  sufficient  differences  to  indicate  that  the  figures  assumed 
for  protein  and  fat  were  too  low  as  applied  to  these  two  materials. 
In  the  other  forty-seven  cases  the  differences  were  nearly  all  less 
than  2  per  cent,  of  the  total  amount  and  were  in  both  directions. 

The  special  interest  of  these  results  lies  in  the  fact  that  they 
show  that  we  may  safely  use  the  above  figures  as  indicated  on  p. 
302  to  correct  the  results  reached  from  a  comparison  of  two  rations. 

Nitrogen- free  Extract  of  Starch. — As  an  example  of  Kell- 
ner's  method  of  computation  we  may  compare  the  results  for  Ox  H 
in  Period  3,  with  starch,  and  in  Period  4,  on  the  basal  ration.  The 
total  energy  of  the  apparently  digested  matter  (compare  Table 
IV  of  the  Appendix)  was — 

Period  3,  with  starch 28,718  Cals. 

Period  4,  without  starch 2]  ,763      " 

Difference 6,955      " 

*  hoc.  cit.,  53,  407. 


THE  FOOD  AS  A  SOURCE  OF  ENERGY. 


3°7 


A  slightly  less  amount  of  the  basal  ration  was  eaten  in  Period  3 
than  in  Period  4.  The  difference  in  crude  nutrients  and  in  esti- 
mated digestible  nutrients  was  as  follows : 


Total, 
Grms. 

Estimated  Digestible. 

Grms. 

Equivalent 
Energy,  Cals. 

4 
13) 
23  ) 

2 
24 

11.4 
100.4 

111.8 

Nitrogen-free  extract. .  . . 

This  amount  of  112  Cals.  should  be  added  to  the  energy  of 
the  digested  matter  of  Period  3  or  subtracted  from  that  of  Period  4 
in  order  to  render  them  comparable,  thus  making  the  real  difference 
due  to  the  starch  7067  Cals.  Still  further,  the  starch  diminished 
the  digestibility  of  the  other  nutrients  of  the  ration  by  the  following 
amounts : 


Grms. 

Equivalent 
Energy,  Cals. 

118 

17 
9 

673.8 
71.1 
74.9 

Crude  fiber 

Ether  extract 

819.8 

Had  these  amounts  been  digested  in  Period  3  as  in  Period  4,  the 
energy  of  the  digested  matter  of  the  ration  would  have  been  820 
Cals.  greater,  and  the  difference  between  the  two  periods  would 
have  been  7887  Cals.  The  digestible  nitrogen-free  extract  was 
1876  grams  more  in  Period  3  than  in  Period  4.  Assuming  all  of 
this  to  be  derived  from  the  starch,  we  have  for  the  energy  of  each 
gram  of  digested  nitrogen-free  extract  7887  ~  1876  =  4.204  Cals. 

The  following  table*  contains  the  results  of  all  the  starch 
experiments  computed  in  the  manner  just  outlined: 


*Loc.  cit.,  53,  412. 


308  PRINCIPLES   OF  ANIMAL   NUTRITION. 

ENERGY  OF  DIGESTED  NITROGEN-FREE  EXTRACT  OF  STARCH. 

Ox  III 4.283  Cals. 

Ox  IV 4.202  " 

Ox  V  (Period  2a) 4.380  " 

Ox  V  (Period  26) 4.324  " 

Ox  VI  (Period  26) 4.159  " 

Ox  B 4.050  " 

Ox  C 4.000  " 

OxD 4.099  " 

Ox  F 4.219  " 

OxG 4.213  " 

OxH 4.204  " 

Ox  J 4.095  " 

Average 4 .  185     " 

Carbohydrates  of  Extracted  Straw. — Computed  in  the 
same  manner  as  the  experiments  upon  starch,  the  two  experiments 
upon  this  substance  gave  the  following  results :  * 

OxH 4.278  Cals. 

OxJ 4.216     " 

Average 4 .  247     " 

This  average  is  slightly  higher  than  would  be  computed  on  the 
assumption  that  the  digested  crude  fiber  and  nitrogen-free  extract 
had  the  heat  values  respectively  of  the  digested  crude  fiber  of  hay 
and  the  digested  nitrogen-free  extract  of  starch. 

Peanut  Oil. — Four  experiments  upon  this  substance  similarly 
computed  give  the  following  results;  * 

OxD 8  508  Cals. 

OxE 8  845     " 

OxF 8  820     " 

OxG 9.112     " 

Average 8 .821     " 

*Loc  tit,  63   413  and  414 


THE  FOOD  AS  A  SOURCE  OF  ENERGY.  309 

As  in  the  case  of  the  ether  extract  of  hay,  the  energy  of  the 
digested  fat  is  less  than  that  of  the  original  material,  which  was 
9.478  Cals.  per  gram. 

Protein  of  Wheat  Gluten. — Comparing  the  experiments  with 
and  without  this  material  exactly  as  in  the  case  of  the  starch,  we 
have  the  following  results  *  for  the  energy  of  the  digested  protein : 

Ox  B  (Period  1) 5.728  Cals. 

Ox  B  (Period  3) 5.817  " 

Ox  C  (Period  3 5.712  " 

Ox  D  (Period  4) 6.040  " 

Ox  E  (Period  4) 6.009  " 

Ox  III  (Period  3) 6.166  " 

Ox  III  (Period  4) 6.277  " 

Ox  IV  (Period  3) 6.061  " 

Average 5 .  976    " 

In  these  trials  three  different  kinds  of  gluten  were  used  which 
were  prepared  by  somewhat  different  processes.  The  averages  for 
the  three  sorts  separately  were  as  follows  : 

No.  1 5.732  Cals. 

"   2 6.025     " 

"   3   6.168     " 

5.975     " 

The  above  figures  refer  to  the  so-called  crude  protein,  that  is,  to 
nitrogen  X  6.25.  The  proteins  of  wheat,  however,  contain  con- 
siderably over  16  per  cent,  of  nitrogen.  Using  Ritthausen  's  factor, 
namely,  5.7,  for  the  computation  of  protein  from  nitrogen  reduces 
the  amount  of  protein  in  the  gluten  and  increases  that  of  the 
nitrogen-free  extract  by  the  same  amount.  The  energy  of  the 
digested  protein  when  computed  on  this  basis  equals  6.148  Cals. 
per  gram. 

Organic  Matter  of  Coarse  Fodders. — For  the  total  digested 
organic  matter  of  hay  and  straw  the  following  heat  values  per  gram 
were  computed :  * 

*  Loc.  cit.,  53,  412  and  414. 


3io 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


4317  Cals. 
4398     " 


4357 


4427 


Meadow  hay  I,  Ox  A 4509  Cals. 

A,  "II 4408    " 

B,  "    V.... 
B,  "VI... 

M,  "    XX 4452 

II,  "     1 4371 

V,  "    F 4355  Cals. 

V,  "    G 4495 

VI,  "    H 4534 

VI,  "    H 4601 

VI,  "    J 4502 

Average  of  7  kinds 4437    " 

Oat  straw,  Ox  F 4443  Cals. 

"       OxG 4584     " 


•a,,  | 


Average 


4513 


Wheat  straw,  Ox  H 4553  Cals. 

"       Ox  J 4387     " 

Average 4470     " 

The  digestible  matter  of  the  straw  has  apparently  about  the  same 
heat  value  as  that  of  hay. 

Metabolizable  Energy. 

Protein. — A  portion  of  the  gross  energy  of  the  digested  protein 
is  removed  in  the  urea  and  other  nitrogenous  products  of  metabo- 
lism, and  in  addition  to  this  there  is  to  be  considered  the  possibility 
of  a  loss  of  energy  by  fermentation  in  the  digestive  tract. 

Losses  in  Methane. — In  nine  of  the  Mockern  experiments  in 
which  wheat  gluten  or  flesh-meal  was  added  to  the  basal  ration,  the 
amount  of  carbon  excreted  in  the  form  of  hydrocarbons  per  day 
and  head  was  as  tabulated  on  the  opposite  page. 

The  differences  between  the  excretion  with  and  without  gluten 
are  small  in  amount  and  are  sometimes  positive  and  sometimes 
negative,  the  averages  being  probably  within  the  limit  of  experi- 
mental error.     The  percentage  losses  of  energy  in   methane  as 


THE  f-OOD  AS  A  SOURCE  OF  ENERGY. 


3ii 


Period. 

Gluten 
Added, 
Grms. 

Carbon  in  Form  of  Hydrocarbons. 

From  Basal 
Ration, 
Grms. 

With 

Addition  of 

Gluten, 

Grms. 

Differences, 
Grms. 

Kichn  : 

Ox  III 

3 

4 
3 

2a 
26 

1 
3 
3 

4 

680 
1360 

680 
1000* 
1000* 

1700 
1700 
1700 
1600 

186.4 
186.4 

187.7 
148*7 
148.7 

205.7 
207.6 
187.6 
162.9 
157.4 

+  19.3 

"    III 

+  21.2 

"     IV... 

-  0.1 

"    XX 

+  14.2 

"   XX 

+  8.7 

Average 

Kellner  : 

Ox  B 

171.6 

208.9 
208.9 
183.0 
166.1 

184.2 

211.0 
200.9 
167.1 
170.7 

+  12.6 
+  2.1 

"   B 

-  8.0 

"   C 

-15.9 

"    D 

+   4,6 

Average 

191.7 

187.4 

-  4.3 

*  Flesh-meal. 

computed  in  Table  V  of  the  Appendix,  like  the  figures  just  given 
for  the  carbon  of  the  methane,  lead  to  the  conclusion  that  the  pro- 
tein of  the  food  does  not  participate  in  the  methane  fermentation. 
Those  figures  were : 

Ox  III,  Period  3 , 10.81  per  cent. 


Ill, 
IV, 
B, 
B, 

C, 
D, 


Average 


5.08 

-1.26 

0.08 

-1.62 

-3.69 

1.91 

0.83 


Kellner  *  reaches  the  same  conclusion  by  comparing  the  ratio 
of  the  methane  carbon  to  the  amount  of  digested  carbohydrates 
(nitrogen-free  extract  +  crude  fiber)  in  the  several  periods.  The 
former  amounted  to  the  following  per  cent,  of  the  latter  in  his 
experiments : 

*Loc.  cit.,  53,  420. 


312 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Basal  Ration, 
Per  Cent. 

Basal  Ration 
+  Gluten, 
Per  Cent. 

Ox  B 

2.94 
2.94 
2.71 
2.75 
2.87 

2.84 

2.96 
2.82 
2.41 
2.71 
3.19 

2.82 

"    B 

"    c 

"    D 

"    E 

Had  the  large  quantities  of  digestible  protein  added  to  the  basal 
rations  produced  any  material  amount  of  methane,  that  fact  must 
have  been  reflected  in  the  above  percentages.  This  method  of 
comparison  takes  into  account  the  probable  effect  of  the  carbo- 
hydrates of  the  wheat  gluten  in  increasing  the  production  of 
methane,  and  the  substantial  agreement  of  the  results  with  and 
without  protein  leads  to  the  same  conclusion  as  the  preceding 
data.  It  seems  fair  to  presume  that  this  conclusion  applies  to 
protein  in  general,  although  a  strict  demonstration  of  it,  especially 
for  coarse  fodders,  would  have  its  difficulties. 

Losses  in  Urine. — While  the  assumption  that  the  urine  is 
essentially  an  aqueous  solution  of  urea  leads  to  grave  errors  in  the 
case  of  the  carnivora,  this  is  still  more  emphatically  true  of  the  urine 
of  herbivora,  particularly  of  ruminants.  The  presence  in  the  urine 
of  herbivora  of  hippuric  acid  and  other  nitrogenous  compounds  less 
highly  oxidized  than  urea  has  of  course  long  been  known,  while, 
as  stated  on  p.  27,  the  presence  of  considerable  amounts  of  non- 
nitrogenous  organic  matter  was  subsequently  demonstrated  by 
Henneberg  and  by  G.  Kiihn  in  the  urine  of  ruminants. 

It  follows  from  these  facts  that  the  energy  content  of  the  urine 
of  these  animals  must  be  higher  in  proportion  to  its  nitrogen  than 
is  the  case  with  carnivora  or  with  man,  but  the  experimental  dem- 
onstration of  this  fact  and  the  realization  of  the  extent  and  im- 
portance of  the  difference  are  of  comparatively  recent  date. 

Cattle. — It  is  to  Kellner  *  that  we  owe  the  first  direct  determi- 
nations of  the  potential  energy  of  the  urine  of  cattle.  The  two 
animals  used  in  the  experiment  were  fed,  the  one  (A)  on  meadow 
hay,  and  the  other  (B)  on  meadow  hay  and  oat  straw.  The  results 
as  regards  the  urine  were  as  follows,  per  day  and  head: 
*  Loc.  cit.,  47,  275. 


THE  FOOD  AS  A  SOURCE   OF  ENERGY. 


313 


Total  nitrogen 

"    carbon  . 

Hippunc  acid 

Total  energy  . 


Ox  A. 


61.28  grams. 
203.20       " 
145.00       " 
1945.00  Cals 


Ox  B 


46.63  grams 
161.30       " 
126.40       " 
1549.40  Cals. 


Assuming  all  the  nitrogen  not  contained  in  the  hippuric  acid  to 
have  been  in  the  form  of  urea,  we  have  the  following  as  the  distri- 
bution of  the  carbon  and  of  the  energy  of  the  urine : 


Ox  A. 

Ox  B 

Amount. 

Per  Cent. 

Amount 

Per  Cent 

Carbon  : 

In  hippuric  acid 

Grms. 
87.48 
21.40 
94.32 

43.05 
10.53 
46.42 

Grms 
76.26 
15.75 
69.29 

47.28 
9.76 

"  other  compounds.  .  .  . 

42.96 

Total    

203.20 

Cals. 
821.30 
271.40 
852.30 

100.00 

42.23 
13.95 
43.82 

161.30 

Cals. 
715.90 
199.60 
633.90 

100.00 

Energy  • 

In  hippuric  acid 

46.20 
12.88 

"  other  compounds.  .  .  . 

40.92 

Total 

1945.00 

100.00 

1549.40 

100.00 

While  the  assumption  that  all  the  nitrogen  was  present  either 
as  hippuric  acid  or  urea  is  not  strictly  correct,  still  the  figures  suffice 
to  show,  first,  that  a  considerable  proportion  of  the  energy  of  the 
proteids  of  the  food  may  be  removed  in  the  hippuric  acid,  and 
second,  that  the  urine  contains  relatively  considerable  amounts  of 
non-nitrogenous  organic  matter.  Had  the  energy  of  the  urine 
been  computed  from  its  nitrogen  reckoned  simply  as  urea,  the 
results  would  have  been  as  follows: 


Ox  A. 

OxB. 

331.6  Cals 
1945.0      " 

252.3  Cals 

1549.4      " 

In  experiments  by  the  writer  on  the  maintenance  ration  of 
cattle,*  determinations  of  the  total  energy  of  the  urine  of  steers 
*  Penna.  Experiment  Station,  Bull  42,  p.  150. 


3J4 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


were  likewise  made, 
were  as  follows: 


Calculated  per  gram  of  nitrogen  the  results 


Feed. 


Timothy  hay  and  corn  meal 

Cotton-seed  feed 

Timothy  hay 

"  "    and  starch 

Wheat  straw,  corn  meal,  and  Unseed  meal 


Steer  No.  1.     Steer  No.  2.     Steer  No. 


37.79  Cals. 
40.64    " 
19.29   " 
25.02    " 
11.24    " 


28.35  Cals. 
34.25   " 
18.01    " 

10.77    " 


28.82  Cals. 
12.47    " 


10.95 


The  methods  employed  to  prepare  the  urine  for  combustion 
were  not  altogether  satisfactory,  and  the  range  of  possible  error 
is  rather  large.  In  but  two  cases,  however,  was  the  energy  of  the 
urine  less  than  twice  that  corresponding  to  its  nitrogen  calculated 
as  urea  (5.434  Cals.),  while  in  one  case  it  reached  over  seven  times 
that  amount.  Neither  carbon  nor  hippuric  acid  having  been  deter- 
mined, no  computations  can  be  made  as  to  the  amount  of  non- 
nitrogenous  matter  present. 

Jordan  *  has  reached  similar  results  on  the  urine  of  cows,  the 
average  energy  content  per  gram  of  nitrogen  being  as  follows : 


Total  Nitrogen, 
Grms. 

Potential  Energy, 
Cals. 

Energy  per  Grm. 
Nitrogen,  Cals. 

Cow  No.  12: 

Period  1 

"      2 

87.0 

78.8 
42.8 
65.5 

1658.3 
1547.2 
1323.5 
1452.5 

19.06 
19.63 

"      3 " 

30.93 

Cow  No.  10 

22.18 

As  in  the  writer's  experiments,  the  energy  per  gram  of  nitrogen 
varies  within  wide  limits,  being  greatest  when  the  total  nitrogen 
of  the  urine  is  least.  In  other  words,  it  would  appear  that  the 
non-nitrogenous  ingredients  of  the  urine  of  cattle  are  subject  to 
less  fluctuation  than  the  nitrogenous  ingredients. 

Kellner's  later  experiments  have  fully  confirmed  his  earlier 
results,  as  will  appear  in  greater  detail  in  subsequent  paragraphs. 
He  finds  that  the  carbon  rather  than  the  nitrogen  of  the  urine  is 
the  measure  of  its  potential  energy,  and  that  an  estimate  of  10 
Cals.  per  gram  of  carbon  gave  for  his  experiments  results  closely 
approximating  the  truth. f 

*  New  York  State  Experiment  Station,  Bull.  197,  p.  28. 
t  hoc.  cit.,  53,  437. 


THE  FOOD  AS  A  SOURCE  OF  ENERGY.  315 

Other  Species. — We  may  probably  assume  without  serious  error 
that  the  results  obtained  with  cattle  apply  in  general  to  sheep  and 
other  ruminants.  No  direct  determinations  of  the  energy  of  the 
urine  of  the  horse  or  the  hog  have  yet  been  reported,  but  Zuntz  & 
Hagemann  *  have  made  some  estimates  of  it  in  the  case  of  the 
horse  on  a  mixed  ration  of  hay,  oats,  and  straw.  They  determined 
the  total  carbon  and  total  nitrogen  of  the  urine  and,  on  the  assump- 
tion that  only  urea  and  hippuric  acid  are  present,  compute  the 
proportion  of  each  of  these,  and  thence  the  energy  of  the  urine. 
They  thus  find  the  potential  energy  of  the  latter,  per  gram  of  nitro- 
gen, equal  to  15.521  Cals.  Neither  hippuric  acid  nor  energy  having 
been  determined  directly,  it  is  impossible  to  check  the  above  com- 
putation or  to  ascertain  whether  any  non-nitrogenous  organic 
matter  was  present.  It  is  to  be  noted,  however,  that  the  ratio  of 
carbon  to  nitrogen  in  the  urine  was  much  lower  than  in  Kellner's 
experiments  on  cattle,  viz.: 

Zuntz  &  Hagemann 1.526  : 1 

Kellner,  Ox  A 3.S15  : 1 

Ox  B 3.458  : 1 

This  fact  clearly  indicates  that  at  least  there  was  very  much  less 
non-nitrogenous  matter  present  in  the  former  case. 

Meissl,  Strohmer  &  Lorenz  f  in  their  respiration  experiments 
on  swine  likewise  determined  carbon  and  nitrogen  in  the  urine. 
Computed  by  the  method  of  Zuntz  &  Hagemann  the  energy  of  the 
urine  averaged  9.55  Cals.  per  gram  of  nitrogen,  while  the  average 
ratio  of  carbon  to  nitrogen  was  0.745  : 1.  These  results  would 
seem  to  indicate  that  the  loss  of  energy  in  the  urine  of  the  hog 
is  not  very  much  greater  than  in  that  of  the  carnivora. 

Metabolizable  Energy  of  Protein  of  Concentrated  Feeds. 
— Accepting  it  as  demonstrated  that  there  is  no  material  loss  of 
potential  energy  in  the  form  of  fermentation  products  of  protein, 
the  data  regarding  the  energy  of  the  urine  just  considered  afford 
the  basis  for  an  approximate  estimate  of  the  metabolizable  energy 
of  the  digested  protein. 

Cattle. — Kellner's  experiments  upon  cattle  afford  data  for  com- 
puting the  metabolizable  energy  of  the  digested  protein  of  wheat 

*  Landw.  Jahrb.,  27,  Supp.  Ill,  239.  f  Zeit.  f.  Biol.,  22,  63. 


316 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


gluten  and  of  beet  molasses.  The  method  of  computation  is  pre- 
cisely similar  to  that  already  employed  for  calculating  the  metabo- 
lizable  energy  of  the  total  organic  matter;  that  is,  the  results  upon 
the  basal  ration  are  subtracted  from  those  upon  the  ration  con- 
taining the  material  under  experiment. 

Taking  as  an  example  the  results  upon  wheat  gluten  with  Ox  C 
in  Periods  1  and  3  we  have  the  following  comparison : 


Digested. 

Energy 

of  Urine, 

Cals. 

Gain  of 
Nitrogen 

by 
Animal, 
Grms. 

Protein, 
Grms. 

Crude 
Fiber, 
Grms. 

Nitrogen- 

Free 
Extract, 
Grms. 

Ether 
Extract. 
Grms. 

Period  3    . 

1694 
59S 

1279 
1289 

5648 
5464 

34 

40 

2592  8   l     20  31 

"      1 

1666.4        16.01 

Difference 

1096 

-10 

184 

-6 

926.4 

4.30 

The  difference  of  4.3  grams  in  the  amount  of  nitrogen  gained 
by  the  animal  is  equivalent  to  32  Cals.  which  would  otherwise  have 
appeared  in  the  urine.  This  added  to  the  926.4  Cals.  actually 
found  makes  a  total  of  958.4  Cals.  for  the  increase  in  the  potential 
energy  of  the  urine  due  to  the  1096  grams  of  protein  digested. 
There  are  also  differences  in  the  amount  of  non-nitrogenous  matters 
digested,  particularly  of  the  nitrogen-free  extract.  As  Tables  I,  III 
and  IV  of  the  Appendix  show,  both  starch  and  crude  fiber,  as  repre  • 
sented  by  the  extracted  straw,  tend  to  diminish  the  amount  of  energy 
carried  off  in.  the  urine.  These  differences  were  observed  when  t'n  mi 
2  to  2.5  kilograms  of  these  substances  were  added  to  the  basal 
ration.  If  the  differences  are  proportional  to  the  amount  fed,  the 
energy  corresponding  to  the  small  difference  observed  in  this  ex- 
periment would  not  exceed  15  or  20  Cals.,  and  may  be  neglected, 
while  the  maximum  difference  in  any  experiment  of  the  series 
would  probably  not  exceed  70  to  75  Cals.  Assuming  that  all  the 
additional  protein  digested  came  from  the  wheat  gluten,  we  have 
for  the  corresponding  energy  of  the  urine 

958 . 4  -7- 1096  =  0 .  874  Cals.  per  gram  protein  digested. 

Subtracting  this  from  the  total  energy  of  the  digested  protein  as 
found  on  p.  309,  viz.,  5.975  Cals.,  we  have  5.101  Cals.  as  the  metabo- 


THE  FOOD  AS  A  SOURCE   OF  ENERGY. 


317 


lizable  energy  of  one  gram  of  digested  protein  of  wheat  gluten  in 
this  experiment. 

For  the  four  experiments  upon  this  substance,  computed  as  in 
the  above  example,  the  results  were  as  follows: 


Protein 
digested 

from 
Gluten, 
Grms. 

Difference  in 
Energy  of  Urine.* 

Total, 
Cals. 

Per  Grm.  of 

Protein, 

Cals. 

Ox  B,  Periods  1  and  3 

2185 
1096 
1056 
1148 

2547.3 
958.4 
1061.1 
1362.1 

1  166 

"    C,  Period  3 

0  874 

"    D,       "      4 

1  005 

"    E,       "      4 

1   186 

1371 

1482.2 

1  081 

*  Corrected  to  nitrogen  equilibrium. 


Subtracting  from  the  total  energy  of  the  digested  protein  the 
potential  energy  carried  off  in  the  urine  we  have  for  the  metab- 
olizable  energy  of  one  gram  of  protein 

5 .  975  Cals.  - 1 .  081  Cals.  =  4 .  894  Cals. 

If  we  use  Ritthausen's  factor,  5.7,  for  proteids,  the  average 
digested  protein  becomes  1250  grams  and  the  loss  of  energy  in  the 
urine  1.190  Cals.  per  gram  of  protein.  Subtracting  this  from  6.148 
Cals.,  the  gross  energy  of  one  gram  of  NX5.7  (p.  309),  we  have  for 
the  metabolizable  energy  of  the  latter  4.958  Cals.  per  gram. 

The  average  increase  in  the  energy  of  the  urine  for  each  addi- 
tional gram  of  nitrogen  excreted  in  these  experiments  (6.756  Cals.) 
was  almost  exactly  the  same  as  Rubner  found  in  his  experiment 
on  extracted  lean  meat  (6.695  Cals.).  This  may  be  taken  as  indi- 
cating that  the  process  of  proteid  metabolism  is  substantially  the 
same  in  both  classes  of  animals,  while  the  fact  that  the  result  is 
notably  greater  than  the  energy  of  urea  shows  that  in  the  herbivora 
as  in  the  carnivora  other  waste  products  than  urea  result  from  the 
proteid  metabolism. 

In  three  other  experiments  beet  molasses  was  added  to  the 
basal  ration,  resulting  in  the  digestion  of  an  increased  amount  of 
nitrogenous  matter.     Computing  the  results  as  in  the  case  of  the 


3i8 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


wheat  gluten,  and  assuming  that  the  large  amounts  of  soluble 
carbohydrates  digested  had  no  effect  on  the  potential  energy  of  the 
urine,  the  results  were  as  follows: 


Protein  Digested 

from  Molasses, 

Grms. 

Difference  in  Energy  of  Urine.* 

Total,  Cais. 

Per  Grm.  Protein, 
Cals. 

Ox  F 

117 
160 
122 

256.1 
240.3 
192.6 

2.189 

"    H 

"    J 

Average 

1.502 
1.579 

133 

229.7 

1.727 

*  Corrected  to  nitrogen  equilibrium. 

It  will  be  seen  that  the  loss  of  energy  in  the  urine  is  much 
greater  than  in  the  case  of  the  gluten  or  than  in  Rubner's  experi- 
ments with  carnivora.  Since  it  is  improbable  that  the  soluble 
carbohydrates  of  the  molasses  escape  oxidation,  it  would  appear 
that  some  of  the  nitrogenous  material  of  the  latter  must  have 
passed  through  the  system  unmetabolized.  Kellner  suspects  that 
it  is  made  up  in  part  at  least  of  xanthin  bases. 

If  we  consider  the  nitrogen  of  the  molasses  to  represent  crude 
protein  (NX6.25)  with  a  heat  value  of  5.711  Cals.  per  gram,  the 
metabolizable  energy  per  gram  would  be  3.984  Cals.  In  view, 
however,  of  the  fact  that  only  a  very  small  proportion  of  the  nitro- 
gen of  the  molasses  is  in  the  proteid  form,  such  a  calculation  seems 
of  doubtful  value. 

Swine. — In  the  investigations  of  Meissl,  Strohmer  and  Lorenz  * 
upon  the  production  of  fat  from  carbohydrates  (p.  176)  the  carbon 
and  nitrogen  of  the  urine  were  determined  in  six  experiments. 

Applying  to  the  results  Zuntz  &  Hagemann's  method  of 
computation  (p.  315)  we  obtain  the  following  estimates  for  the 
energy  per  gram  of  nitrogen  in  the  urine  of  the  hog  in  these 
experiments  and  for  the  corresponding  metabolizable  energy  of  the 
digested  protein: 


*  Zeit.  f.  Biol.,  22,  63. 


THE  FOOD   AS  A  SOURCE   OF  ENERGY. 


3X9 


Experi- 
ment 
No. 


Feed. 


Nitrogen 
as  Urea, 
Grms. 


Nitrogen 

as  Hip- 

puric 

Acid, 

Grms. 


Total 
Energy 
of  Urine 


Energy 

per  Grm 

of 


Pals        Nitrogen, 
Cals. 


Metabo- 
lizable 
Energy 

per 

Grm. 

Protein, 

Cals. 


Rice 

Barley 

Whey,  rice,  and  flesh  meal 
Nothing 


9.58 
9.22 
13.04 
59.89 
9.35 
6.48 


0.88 
1.04 
1.04 
1.17 
0.45 
0.29 


115.7 
125.6 
146.5 
410.0 
83.7 
56.4 


11.06 

12.24 

10.40 

6.72 

8.54 

8.33 


3.941 
3.753 
4.048 
4.636 
4.344 
4.379 


Kornauth  &  Arche  *  report  the  following  results  on  the  urine  of 
swine  fed  chiefly  upon  cockle : 


Experiment 
No. 

Nitrogen, 
Grms. 

Carbon, 
Grms. 

Ratio, 
C  :N. 

1 

10.56 
10.30 
10.41 

10.30 
9.53 
9.96 

0.975  :  1 
0.926  :  1 
0.957  :  1 

2 

3 

Average 

10.42 

9.93 

0.953  :  1 

The  results,  computed  as  in  the  previous  case,  make  the  average 
energy  content  of  the  urine  10.27  Cals.  per  gram  of  nitrogen, 
equivalent  to  a  metabolizable  energy  of  4.067  Cals.  per  gram  of 
protein. 

In  the  two  fasting  experiments  of  Meissl,  Strohmer  &  Lorenz 
the  ratios  of  carbon  to  nitrogen  and  of  computed  energy  to  nitro- 
gen are  similar  to  those  obtained  with  fasting  carnivora.  The 
abundant  supply  of  proteids  in  the  diet  in  the  fourth  experiment 
seems  to  have  had  the  effect  of  reducing  these  ratios  to  values 
comparable  with  those  obtained  by  Rubner  for  extracted  meat 
and  by  Kellner  for  the  digested  protein  of  wheat  gluten.  These 
facts  seem  to  indicate  clearly  that  the  nature  of  the  proteid  meta- 
bolism in  all  these  animals  is  substantially  the  same.  In  the  ex- 
periments in  which  ordinary  grains  were  used,  the  computed  energy 
content  of  the  urine  is  notably  greater  relatively  to  its  nitrogen. 
How  far  the  excess  of  carbon  found  in  these  cases  was  due  to  an 
*  Landw.  Vers.  Stat.,  40,  177. 


320  PRINCIPLES   OF  ANIMAL   NUTRITION. 

increased  formation  of  hippuric  acid  and  what  part  of  it,  if  any,  is 
to  be  ascribed  to  the  presence  of  non-nitrogenous  matter  in  the 
urine,  the  experiments  afford  no  means  of  estimating. 

The  Horse. — Zuntz  &  Hagemann's  results  on  the  horse,  p.  315, 
although  the  result  of  feeding  mixed  rations,  may  be  conveniently 
considered  here.  The  computed  energy  of  the  urine  was  15.521 
Cals.  per  gram  of  nitrogen,  equivalent  to  2.483  Cals.  per  gram  of 
protein.  Assuming  for  the  latter,  as  before,  a  value  of  5.711  Cals., 
there  remains  for  the  metabolizable  energy  3.228  Cals.  per  gram. 

Protein  of  Coarse  Fodders. — Almost  the  only  data  on  this 
point  are  those  afforded  by  Kellner's  experiments  upon  cattle.  In 
those  in  which  coarse  fodders  were  used  alone  we  can  of  course 
compute  the  metabolizable  energy  of  the  protein  directly  from  the 
amount  digested  and  from  the  energy  of  the  urine.  In  those 
experiments  in  which  coarse  fodders  were  added  to  a  basal  ration 
we  can  compare  the  two  experiments  in  the  same  manner  as  those 
upon  gluten,  neglecting,  as  in  that  case,  the  differences  in  the  non- 
nitrogenous  nutrients  digested. 

Passing  over  the  details  of  the  computation,  the  final  results, 
including  the  metabolizable  energy  of  the  digested  protein  com- 
puted upon  the  assumption  that  its  gross  energy  equals  5.711 
Cals.  per  gram,  are  as  given  in  the  table  on  the  opposite  page.* 

The  writer's  experiments  on  timothy  hay,  the  results  of  which 
as  regards  the  energy  of  the  urine  have  already  been  given  on  p.  314, 
when  computed  in  the  same  manner  as  the  above  experiments  give 
the  following  results  for  the  metabolizable  energy  of  the  digested 
protein : 

Steer  1 2.625  Cals. 

"     2 2.830     " 

"     3 3.716     " 

Average 3.057     " 

Influence  of  Non-nitrogenous  Matter  of  Urine. — In  the  previous 
paragraphs  there  appeared  reasons  for  supposing  that  the  processes 
of  proteid  metabolism  are  essentially  the  same  in  all  domestic 

*  The  figures  given  in  this  table  for  digested  protein,  energy,  etc.,  refer 
solely  to  that  derived  from  the  coarse  fodder  and  not  to  that  of  the  total 
ration. 


THE  FOOD  AS  A  SOURCE   OF  ENERGY. 


321 


Protein 

(NX6.25) 

Digested, 

Grms. 

Difference  in 
Energy  of  Urine.* 

Metaboliz- 

able  Energy 

per  Grm. 

Digestible 

Protein, 

Cals. 

Total, 
Cals. 

Per  Grm.  of 

Protein 

Digested, 

Cals. 

Meadow  Hay  : 

No.    I,  Ox   A 

440 
342 
137 
146 
193 
220 
213 
413 
451 
458 
540 

1991.3 

1686.9 

583.2 

556.5 

781.4 

798.0 

930.5 

1925.7 

1559.3 

1737.9 

3224 . 6 

4.526 
4.933 
4.257 
3.812 
4.049 
3.632 
4.368 
4.662 
3.456 
3.794 
5.973 

1    185 

"     II,    "      I 

0  778 

"     V,    "      F 

1  454 

"     V,   "     G 

1  899 

"    VI,    "     H,  Period  1 .  .  . 
"   VI,    "     H,      "       7... 
"  VI,   "      J 

1.662 
2.079 
1  343 

"     A,    "     II 

1  049 

"     B,   "     V 

2  255 

"     B,    "    VI 

1  917 

"     M,   "XX 

0  262 

323 

35 

48 

1434.1 

354.2 
274.0 

4.439 

10.120 
5.710 

1  272 

Oat  Straw  : 

No.  II,  Ox  F 

—4  409 

"     II,    "    G 

-0  001 

42 

-11 
14 

314.1 

289.7 
413.2 

7.478 

(?) 
29.520 

-1  767 

Wheat  Straw  : 

No.  I,  Ox  H 

(?) 
-23.809 

"     I,  "      J 

2 

351.5 

(?) 

(?) 

*  Corrected  to  nitrogen  equilibrium. 


animals  and  consequently  that  the  metabolizable  energy  of  the 
proteids  cannot  be  widely  different.  In  these  results  upon  coarse 
fodders  we  meet  an  apparent  contradiction  of  this  conclusion,  the 
metabolizable  energy  of  the  digestible  protein  as  above  computed 
being  quite  variable  and  much  lower  than  the  values  found  for  pure 
proteids,  while  in  the  straw  we  get  large  negative  values. 

These  latter  results,  however,  while  appearing  at  first  sight  para- 
doxical, furnish  the  clue  to  the  apparent  contradiction.  In  the 
case  of  the  straws  it  is  evident  that  a  very  considerable  part  of  the 
potential  energy  of  the  urine  must  have  been  contained  in  non- 
nitrogenous  substances,  and  that  the  latter  must  have  been  derived 
largely  from  the  non-nitrogenous  matter  of  the  food.  We  have 
already  seen,  however,  that  these  non-nitrogenous  excretory  prod- 


322  PRINCIPLES  OF  ANIMAL   NUTRITION. 

ucts  are  a  normal  constituent  of  the  urine  of  cattle  both  on  hay  and 
on  mixed  rations.  Their  effect  on  the  computation  becomes  more 
obvious  in  the  case  of  the  straws,  simply  because  of  the  relatively 
small  amount  of  protein  in  the  latter  feeding-stuffs.  In  these  cases 
we  get  impossible  results  when  we  assume  that  all  the  potential 
energy  of  the  urine  is  derived  from  the  proteids  metabolized,  but  it 
is  clear  that  the  results  on  the  hays  must  be  affected  by  the  same 
error,  and  there  is  little  question  that  the  low  and  variable  results 
noted  in  the  table  are  to  be  explained  in  part  in  this  way.  We 
know  no  essential  difference  between  the  real  proteids  of  the  differ- 
ent coarse  fodders,  nor  between  those  of  coarse  fodders  and  grain, 
nor  any  reason  why  they  should  not  be  metabolized  in  substantially 
the  same  way  in  the  body  and  possess  approximately  the  same 
metabolizable  energy.  It  would  seem  more  reasonable,  then,  to 
assume  that  the  proteids  of  coarse  fodders  are  metabolized  sub- 
stantially like  those  of  concentrated  fodders,  and  to  take  provision- 
ally the  results  obtained  for  the  protein  of  wheat  gluten  as  repre- 
senting approximately  the  metabolizable  energy  of  the  digested 
protein  of  the  total  ration,  while  we  regard  the  remaining  energy 
of  the  urine  as  derived  largely  from  the  non-nitrogenous  nutrients 
of  the  food. 

Hippuric  Acid. — The  statement  last  made,  however,  requires 
some  modification.  Not  a  little  of  the  potential  energy  of  the  urine 
of  cattle  is  contained  in  the  hippuric  acid  which  these  animals 
excrete  so  abundantly.  This  being  a  nitrogenous  product,  it  is 
natural  to  look  upon  it  as  derived  from  the  proteids  of  the  food, 
but  it  must  not  be  forgotten  that  this  is  only  partially  true.  Its 
glycocol  portion  originates  in  the  proteids,  but  its  phenyl  radicle 
appears  to  be  derived  in  these  animals  largely,  if  not  wholly  from 
the  non-nitrogenous  ingredients  of  the  food  (compare  p.  45).  If 
the  metabolism  of  one  gram  of  protein  is  arrested  at  the  glycocol 
stage  by  the  presence  in  the  organism  of  benzoic  acid,  there  has 
already  been  liberated  from  it  about  3  Cals.  of  energy,  while  about  2.7 
Cals.  remain  in  the  glycocol.  The  resulting  hippuric  acid,  however, 
contains  about  11.6  Cals.  of  potential  energy,  or  more  than  the 
original  protein.  In  this  case,  then,  the  larger  share  of  the  energy 
of  the  excretory  product  (8.9  Cals.  out  of  11.6  Cals.),  although  con- 
tained in  a  nitrogenous  substance,  is  derived  ultimately  from  the 


THE  FOOD  AS  A  SOURCE  OF  ENERGY.  323 

non-nitrogenous  matter  of  the  food.  It  is  clear,  then,  that  the 
non-nitrogenous  moiety  of  the  hippuric  acid  and  the  non-nitrogen- 
ous organic  matter  of  the  urine  together  represent  a  large  share 
of  the  potential  energy  of  the  latter,  and  that  it  is  quite  as  in- 
correct to  compute  the  metabolizable  energy  of  the  protein  on  the 
assumption  that  all  the  energy  of  the  urine  is  derived  from  it  as  it 
is,  on  the  other  hand,  to  simply  deduct  from  its  gross  energy  the 
energy  of  the  equivalent  amount  of  urea. 

Ether  Extract. — Our  only  data  upon  this  ingredient  are  fur- 
nished by  the  four  experiments  upon  steers  by  Kellner  in  which 
peanut  oil  was  added  to  the  ration.  In  the  first  two  experiments 
this  oil  was  emulsified  by  means  of  a  small  quantity  of  soap  made 
from  the  same  oil.  The  result  was  a  milky  fluid  which  was  readily 
digestible  and  which  caused  no  considerable  decrease  in  the  digesti- 
bility of  the  basal  ration.  In  the  second  two  experiments  the  oil 
was  emulsified  with  lime-water,  giving  a  thickish  mass  which  was 
not  very  well  digested  and  which,  in  the  case  of  Ox  F  particularly, 
caused  a  considerable  decrease  in  the  digestibility  of  the  crude  fiber 
and  nitrogen-free  extract  of  the  basal  ration.  It  should  be  noted 
that  in  the  experiment  with  Ox  E  the  oil  was  not  added  to  a  basal 
ration,  but  was  substituted  for  a  part  of  the  bran.  From  Table  VI 
of  the  Appendix  we  obtain  the  summary  tabulated  on  the  next 
page,  showing  the  effects  of  the  oil  upon  the  loss  of  energy  in  the 
gaseous  hydrocarbons  and  in  the  urine,  the  results  of  the  experi- 
ment on  Ox  E  being  included. 

Upon  the  evidence  of  these  four  experiments,  bearing  in  mind 
that  the  one  with  Ox  E  was  upon  the  substitution  of  oil  for  bran, 
we  should  not  be  inclined  to  ascribe  to  the  fat  of  the  food  any  con- 
siderable effect  either  upon  the  formation  of  hydrocarbons  or  upon 
the  amount  of  potential  energy  carried  off  in  the  urine.  As  regards 
the  hydrocarbons,  the  differences  in  the  cases  of  Oxen  D  and  G  are 
insignificant.  In  the  case  of  Ox  F,  on  the  contrary,  the  production 
of  hydrocarbons  was  reduced  nearly  one  half;  this  it  may  be  noted 
was  the  case  in  which  there  was  a  considerable  effect  upon  the 
digestibility  of  the  basal  ration.  As  regards  the  energy  of  the  urine, 
the  differences,  except  in  the  case  of  Ox  E,  are  relatively  small  and 
are  in  both  directions. 

Provisionally,  therefore,  we  are  probably  justified  in  assuming 


324 


PRINCIPLES    OF  ANIMAL    NUTRITION. 


Ani- 
mal. 

Period. 

Energy  of  Urine 
(Corrected).  Cals. 

Energy  of 
Methane,  Cals. 

D 

3 

1 

3 

1 

5 
3 

5 
3 

With  oil 

2851.2 
2407.0 

2909  0 

D 

2957  0 

-55.8 

2026.2 
2312.9 

—48  0 

E 

With  oil 

2640  8 

E 

Basal  ration 

2950  4 

-286.7 

1455.0 
1530.0 

-309.6 

F 

With  oiJ 

1369  1 

F 

2560  7 

-75.0 

1452.1 
1359.6 

-1191  6 

G 
G 

With  oil 

Basal  ration 

2371.2 
2524.7 

92.5 

-153.5 

as  Kellner  does  that  none  of  the  energy  of  the  fat  was  lost  either  in 
the  hydrocarbons  or  in  the  urine,  and  that  consequently  the  metab- 
olizable  energy  of  the  digested  fat  was  the  same  as  its  gross  energy, 
namely,  8.821  Cals.  per  gram,  as  given  on  p.  308.  If  we  assume  that 
the  ether  extract  of  hay  behaves  like  the  peanut  oil,  taking  no  part 
either  in  the  production  of  methane  or  in  the  loss  of  energy  through 
the  urine,  its  metabolizable  energy  would  likewise  be  the  same  as 
its  gross  energy ,  namely, 8.322  Cals.  per  gram,  as  computed  on  p.  305. 
No  results  upon  the  metabolizable  energy  of  the  ether  extract  are 
available  in  the  case  of  other  species  of  herbivorous  animals. 

Carbohydrates.— Those  of  Kellner's  experiments  in  which 
starch,  as  a  representative  of  the  readily  digestible  carbohydrates, 
and  extracted  straw,  consisting  largely  of  "crude  fiber,"  were  added 
to  the  basal  ration  afford  data  for  an  approximate  computation  of 
the  metabolizable  energy  of  this  group  of  nutrients  in  the  ox,  and 
experiments  by  Lehmann,  Hagemann  &  Zuntz  afford  partial  data 
for  the  horse. 

Starch. — The  results  of  the  Mockern  experiments,  as  recorded 
in  Tables  III  and  IV  of  the  Appendix,  show  that  the  starch  had 
but  a  slight  effect  upon  the  amount  of  potential  energy  carried  off 
in   the  urine  of   the  ox,  although  the  general  tendency  was  to 


THE  FOOD  AS  A  SOURCE   OF  ENERGY. 


325 


diminish  it  slightly.  On  the  other  hand,  the  formation  of  hydro- 
carbons was  markedly  increased  except  in  two  cases.  It  has  al- 
ready been  shown  that  the  proteids  of  the  food  do  not  take  part  in 
the  production  of  these  gases,  and  that  the  same  is  probably  true 
of  the  fat  under  normal  conditions.  Neglecting  the  small  effects 
upon  the  urine,  therefore,  we  may  compare  directly  the  increase  in 
the  digested  carbohydrates  with  the  increase  in  the  gaseous  hydro- 
carbons, using  for  this  purpose  the  differences  between  the  two 
rations  uncorrected  for  the  slight  variations  in  the  consumption  of 
dry  matter. 

Taking  first  the  last  five  of  Kellner's  experiments.*  which  seem 
to  represent  the  most  normal  conditions,  we  have  the  following : 


Difference  in  Carbohydrates 
Digested. 

Difference  in 

Crude  Fiber, 
Grms. 

Nitrogen-free 

Extract, 

Grms. 

Methane, 
Cals. 

Ox  D,  Period  2 

-64 
-64 
-50 
-26 
-  9 

+  1388 
+  1609 
+  1598 
+  1861 
+  1501 

+  424  4 

"    F        "       4 

"    G        "       4 

"    H,      "      3 

+  822.0 
+  645.8 
+  604  5 

"J        "      3 

+  769  9 

Totals 

-213 

+  7957 

3266  6 

Assuming  that  the  same  proportion  of  hydrocarbons  is  pro- 
duced in  the  fermentation  of  crude  fiber  as.  in  that  of  starch,  we 
may  compare  the  algebraic  sum  of  the  two  with  the  energy  of  the 
methane  as  follows: 

3266 . 6  Cals.  -  (7957  - 213)  =  0 .  422  Cals.  per  gram. 
Subtracting  the  latter  result  from  the  gross  energy  of  the  digested 
nitrogen-free   extract   of  starch,   we   have   for  the   metabolizable 
energy  of  the  latter 

4 .  185  Cals.  -  0 .  422  Cals.  -  3 .  763  Cals.  per  gram. 
In  the  experiments  on  Oxen  B  and  C   the  basal  ration  was  a 
heavy  one,  with  a  rather  wide  nutritive  ratio,  and  already  con- 
tained large  amounts  of  digestible  carbohydrates.    Under  these  cir- 
cumstances the  added  starch  was  very  imperfectly  digested,  while 
*Loc   cit.,  53.422. 


326 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


the  production  of  hydrocarbons  was  diminished.  Kcllner  suggests 
that  the  latter  effect  may  have  been  due  to  a  partial  suppression 
of  the  organisms  causing  the  methane  fermentation  by  other  species, 
and  suspects  that  the  presence  of  large  amounts  of  carbohydrates 
along  with  little  protein  favors  this  result.  At  any  rate,  the  con- 
ditions are  evidently  unusual  if  not  abnormal. 

In  Kuhn's  experiments  the  starch  was  added  to  a  ration  of 
coarse  fodder.  The  nutritive  ratio  was  wide,  but  the  absolute 
amount  of  carbohydrates  was  much  less  than  in  the  two  experiments 
by  Kellner  just  mentioned,  less  starch  appeared  to  escape  diges- 
tion, and  the  production  of  hydrocarbons  was  increased  in  every 
case.     The  following  are  Kuhn's*  results: 


Difference  in  Carbohydrates 
Digested. 


Crude  Fiber, 
Grms. 


Nitrogen-free 
Extract,  Grms. 


Difference  in 

Energy  of 

Methane, 

Cals. 


Ox  III,  Period  2 


IV,      ' 

2 

V,      ' 

2a 

V,      ' 

26 

V,      ' 

3 

VI,       ' 

2a 

VI,      ' 

26 

VI,      ' 

3 

Totals  . . 

-220 
-180 
-195 
-130 
-176 
-146 
-  88 
-156 


1529 
1408 
1537 
1539 
2619 
1468 
1554 
2587 


706.2 
856.7 
752.6 
665.5 

1181.0 
729.5 
649.9 

1407.0 


14241 


6948.4 


Assuming  as  before  the  equivalence  of  crude  fiber  and  nitrogen- 
free  extract  as  regards  the  production  of  hydrocarbons  we  have 

6948.4  Cals. -(14241  - 1291)  =  0.537  Cals.  per  gram, 
4 .  185  Cals.  -  0 .  537  Cals.  =  3 .  648  Cals.  per  gram. 

Determinations  by  Lehmann,  Hagemann  &  Zuntz  f  of  the 
amount  of  methane  produced  by  the  horse  will  be  considered  in 
connection  with  the  metabolizable  energy  of  crude  fiber.  Zuntz  \ 
has  pointed  out  that  the  fermentation  of  the  food  in  the  horse  takes 
place  largely  in  the  ccecum  and  after  the  more  digestible  carbo- 
hydrates have  been  resorbed.     Accordingly  he  regards  the  metabo- 

*  hoc.  tit.,  44,  570. 

t  Landw.  Jahrb.,  23,  125. 

t  Arch.  ges.  Physiol.,  49,  477. 


THE  FOOD  AS  A  SOURCE   OF  ENERGY. 


327 


lizable  energy  of  starch  and  similar  bodies  in  this  animal  as  equal 
to  their  gross  energy,  viz.,  4.185  Cals.  per  gram  in  the  case  of 
starch. 

Extracted  Straw. — The  two  experiments  in  which  extracted 
straw  was  added  to  the  basal  ration,  when  computed  as  in  the  case 
of  the  starch  experiments,  give  the  following  results : 


Difference  in  Carbohydrates 
Digested. 

Difference  in 

Crude  Fiber, 
Grms. 

Nitrogen-free 
Extract, 
Grms. 

Methane, 
Cals. 

Ox  H,  Period  5.  .' 

2046 
1987 

439 
449 

1425  1 

"    J,       "       5 

1425  2 

Totals 

4033 

888 

2850.3 

The  loss  of  energy  in  the  hydrocarbons  equals  0.579  Cals.  per 
gram  of  total  digestible  carbohydrates  (of  which  82  per  cent,  was 
crude  fiber),  and  the  corresponding  metabolizable  energy  of  the 
carbohydrates  is  3.668  Cals.  per  gram.  This  is  a  materially  lower 
figure  than  Kellner  found  for  starch  and  indicates  that  the  loss  of 
energy  in  the  gaseous  products  of  fermentation  is  greater  in  the 
case  of  crude  fiber  than  in  that  of  the  more  soluble  carbohydrates, 
an  indication  which,  as  we  shall  see,  is  confirmed  by  the  results  of 
other  experiments. 

Carbohydrates  of  Coarse  Fodders. — Upon  the  same  two 
assumptions,  viz.,  that  the  carbohydrates  are  the  sole  source  of  the 
gaseous  hydrocarbons,  and  that  the  latter  represent  the  entire  loss 
of  energy  from  the  digested  carbohydrates,  we  may  compute  the 
metabolizable  energy  of  the  total  digestible  carbohydrates  of  the 
various  coarse  fodders  exactly  as  in  the  case  of  the  extracted  straw. 
the  results  being  tabulated  on  the  next  page. 

If  we  average  the  results  for  each  feeding-stuff  and  compute 
them  as  in  the  foregoing  cases,  our  findings  are  as  given  on  p.  329, 
where  the  rations  are  arranged  in  the  order  of  their  crude  fiber 
content.  In  computing  the  metabolizable  energy,  the  gross  energy 
of  the  digested  carbohydrates  has  been  assumed  to  be  the  average 


328 


PRINCIPLES  OF  ANIMAL   NUTRITION. 
COARSE   FODDERS  ALONE. 


Digested  Carbohydrates. 

Animal. 

Crude  Fiber, 

Grms. 

Nitrogen- 
free 
Extract, 
Grms. 

Energy  of 

Methane, 

Grms. 

A 

Meadow  hav  I 

1262 
1765 
1572 
1642 
1560 
1266 
1702 
1676 
1565 

2713 
2610 
2315 
2420 
2999 
2348 
2357 
2226 
2L45 

2113.7 

II 

"           "     A 

3137  2 

V 

"     B 

2268  5 

VI 

"     B 

2480 . 6 

XX 

"     M 

2646 . 1 

I 

"    II 

2092  3 

B 
III 
IV 

"           "     and  oat  straw ...  . 
Clover        "       "     "         "    .... 

2331.2 
2670.1 
2491.3 

COARSE  FODDER  ADDED  TO  BASAL  RATION. 


Period. 

Difference  in 
Carbohydrates  Digested. 

Energy  of 

Methane, 

Cals. 

Animal. 

Crude  Fiber, 
Grms. 

Nitrogen- 
free 
Extract, 
Grms. 

F 
G 
H 
H 
J 
F 
G 

1 
2 
2 
7 
2 
2 
1 
1 
1 

Meadow  hay    V 

"      V 

"    VI 

"    VI 

"    VI 

Oat  straw  II 

"      II 

546 
538 
703 
739 
683 
694 
595 
821 
829 

836 

886 

1129 

1236 

1213 

721 

684 

524 

616 

689.9 
907.4 
727.2 
898.0 
984.8 
679.2 
923.4 

H 
J 

Wheat  straw  I 

"      I 

1213.0 
1281.0 

of  the  results  given  on  pp.  304  and  306  for  the  digested  crude  fiber 
and  nitrogen-free  extract  of  coarse  fodders,  viz.,  4.226  Cals.  per 
gram. 

As  a  whole,  the  figures  given  on  p.  329  show  a  tendency 
toward  an  increased  production  of  methane  with  an  increase  in 
the  proportion  of  crude  fiber,  but  considerable  variations  are 
found  in  individual  cases.  It  is  evident,  therefore,  from  these 
results,  as  well  as  from  those  already  cited  in  connection  with  the 
experiments  upon  starch  and  upon  molasses,  that  a  variety  of 
factors  influence  the  extent  of  this  fermentation. 


THE  FOOD  AS  A  SOURCE  OF  ENERGY. 


329 


100  Parts  Digested 

Carbohydrates 

Contain 

Energy  of 
Methane 
per  Grm. 
Digested 
Carbo- 
hydrates, 
Cals. 

Metaboliz- 
able  Energy 

Crude 
Fiber. 

Nitrogen- 
free 
Extract. 

Digested 
Carbo- 
hydrates 
per  Grm., 
Cals. 

31.7 
34.2 
35.0 
37.3 
38.6 
40.4 
41.9 
42.6 
47.8 
59.1 

68.3 
65.8 
65.0 
62.7 
61.4 
59.6 
58.1 
57.4 
52.2 
40.9 

0.532 
0.580 
0.579 
0.458 
0.569 
0.597 
0.574 
0.678 
0.595 
0.894 

3.694 

"    M 

3.646 

"     II 

3.647 

"  VI 

3.768 

"     V 

3.629 

"        "     B 

"         "  and  oat  straw 

Clover       "    "      "        "     

3.657 
3.652 
3.548 
3.631 

Wheat  straw  V 

3.332 

A  comparison  of  the  methane  production  with  the  digestibility 
of  the  feeding-stuffs  shows  in  general  that  the  former  is  greatest 
when  the  latter  is  least,  that  is,  with  the  feeding-stuffs  which  tend 
to  remain  longest  in  the  digestive  tract.  Here  too,  however,  excep- 
tions occur,  and  it  would  appear  that  the  physical  condition  of  the 
feeding-stuff  is  not  without  its  influence.  The  exceedingly  com- 
plicated nature  of  digestion  in  ruminants,  and  the  fact  that  it  is  a 
chemical  rather  than  a  physiological  process,  and  is  therefore  sub- 
ject to  considerable  variations  according  to  the  nature  and  amount 
of  the  food,  render  it  difficult,  if  not  impossible,  with  our  present 
knowledge  to  compute  very  trustworthy  averages  for  the  amount 
of  energy  carried  off  in  this  way. 

Crude  Fiber.  Ruminants. — Both  the  ultimate  composition  and 
the  heat  of  combustion  of  the  digested  nitrogen-free  extract  have 
been  shown  to  agree  quite  closely  with  those  of  starch,  and  the 
nutritive  value  of  the  former  has  commonly  been  assumed  to  be 
the  same  as  that,  of  the  latter.  If  we  are  justified  in  somewhat 
extending  this,  and  assuming  that  the  nitrogen-free  extract  of 
coarse  fodders  suffers  the  same  loss  by  the  methane  fermentation 
as  does  starch,  the  figures  of  the  preceding  paragraphs  supply 
data  for  computing  the  corresponding  loss  suffered  by  the 
crude  fiber. 


33° 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


In  the  case  of  the  extracted  straw,  for  example,  there  was 
digested  in  the  total  of  the  two  experiments : 

Crude  fiber 4033  grams 

Nitrogen-free  extract 888     " 

Assuming  the  loss  of  energy  in  the  methane  to  have  been  0.422 
Cal.  per  gram  of  nitrogen-free  extract  digested  (the  same  as  that 
found  by  Kellner  for  starch,  p.  325),  the  888  grams  of  these  sub- 
stances correspond  to  a  loss  of  374.7  Cals.  Subtracting  this  from 
the  total  loss  of  2850.2  Cals.,  we  have  2475.5  Cals.  as  the  energy  of 
the  methane  produced  from  4033  grams  of  crude  fiber,  which  is  equal 
to  0.614  Cal.  per  gram.  The  total  energy  of  the  digested  crude  fiber 
was  shown  on  p.  304  to  be  approximately  4.220  Cals.  per  gram. 
Subtracting  the  loss  in  the  methane,  0.614  Cal.,  leaves  3.606  Cals. 
as  the  metabolizable  energy  of  one  gram  of  digested  crude  fiber. 
A  similar  computation  of  the  average  results  upon  the  other  coarse 
fodders  affords  the  figures  of  the  following  table  for  the  metabo- 
lizable energy  of  one  gram  of  digestible  crude  fiber: 


Digestible  Crude  Fiber  of 


Metabolizable 

Energy, 

Cals. 


Extracted  straw 

Hay  fed  alone 

"     added  to  basal  ration 

Oat  straw  added  to  basal  ration  .  . 
Wheat  straw  added  to  basal  ration 


3.606 
3.311 
3.606 
3.437 
3.001 


The  loss  of  energy  in  methane,  as  thus  computed,  is  in  all 
instances  greater  than  in  the  case  of  starch.  Owing,  however,  to 
the  slightly  higher  value  obtained  for  the  gross  energy  of  the 
digested  crude  fiber,  the  difference  in  metabolizable  energy  between 
starch  and  crude  fiber  is  somewhat  less  marked,  and  is  hardly 
sufficient  of  itself  to  justify  assigning  a  materially  lower  nutritive 
value  to  the  latter. 

It  is  worthy  of  note  also  that  the  loss  in  the  methane  appears  to 
be  a  very  variable  one,  justifying  the  conclusion  already  reached 
that  other  factors  than  the  proximate  composition  of  the  food  ma- 
terially affect  the  extent  of  the  methane  fermentation. 

The  Horse. — The  production  of  methane  by  the  horse  appears 
to  be  much  less  copious  than  that  by  ruminants.     Lehmann,  Hage- 


THE  FOOD  AS   A  SOURCE   OF  ENERGY. 


33* 


mann  &  Zuntz  *  in  eight  respiration  experiments  obtained    the 
following  results,  the  hydrocarbons  being  computed  as  methane: 


Crude  Fiber  Digested. 

Methane  Excreted. 

698.5 

grams 

26.8  grams 

538.9 

" 

33.4      " 

451.7 

" 

13.0      " 

<< 

11 

20.0      " 

u 

It 

(t 
« 

tt 

16.4      " 

31.0  " 

22.1  " 
23.0      " 

As  already  noted  on  p.  326,  Zuntz  f  has  pointed  out  that  the 
fermentation  of  the  food  in  the  horse  takes  place  largely  in  the 
ccecum  and  after  the  more  digestible  carbohydrates  have  been 
resorbed.  The  authors  consequently  compute  the  excretion  of 
methane  entirely  upon  the  crude  fiber  of  the  food.  On  the  average 
of  the  eight  somewhat  discordant  experiments,  in  which  the  food 
consisted  of  oats,  hay,  and  cut  straw,  100  grams  of  digested  crude 
fiber  yielded  4.7  grams  of  methane,  which  corresponds  exactly  with 
the  results  reported  by  Tappeiner  \  f°r  the  artificial  fermenta- 
tion of  cellulose.  In  the  same  experiments  an  excretion  of  approxi- 
mately 0.203  gram  of  hydrogen  per  100  grams  digested  crude  fiber 
was  observed.  Deducting  the  corresponding  amounts  of  energy 
from  the  energy  of  the  apparently  digested  cellulose  we  have — 

Total  energy  of  1  gram 4.220  Cals. 

Energy  of  CH4  (0.047  gram).  0.627  Cal. 

Energy  of  H (0.002  gram)...  0.070    "  0.697  Cal. 

Metabolizable  energy  of  1  gram 3 .  523  Cals.§ 

While  less  methane  is  apparently  produced  by  the  horse  than 
by  the  ox,  the  assumption  that  it  all  arises  from  the  fermentation 
of  the  crude  fiber  gives  the  latter  a  metabolizable  energy  not  greatly 
different  from  that  found  in  the  case  of  the  ox.     It  is  of  course 

*  Landw.  Jahrb.,  23,  125. 
t  Arch.  ges.  Physiol,  49,  477. 
%  Zeit.  f.  Biol.,  20,  88. 

§  As  computed  by  the  authors,  3.487  Cals.  on  the  basis  of  4.185  Cals. 
total  energy  per  gram  of  crude  fiber. 


33* 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


implied  in  this  that  the  metabolizable  energy  of  the  digested  nitro- 
gen-free extract  is  the  same  as  its  gross  energy. 

Summary. — The  results  recorded  in  the  preceding  paragraphs 
regarding  the  metabolizable  energy  of  the  several  classes  of  digesti- 
ble nutrients  are  summarized  in  the  following  table : 


METABOLIZABLE   ENERGY   OF   DIGESTIBLE  NUTRIENTS. 


Cattle, 

Cals.  per 

Grm. 


Horse, 

Cals.  per 

Grm. 


Swine, 

Cals.  per 

Grm. 


Proton  (NX 6.25): 

From  wheat  gluten 

"     (NX5.7) 

"      beet  molasses 

"      mixed  grain 

"  "       ration  of  oats,  hay,  and  straw 

"      meadow  hay 

"     timothy  hay  

"     straw 


4.894 
4.958 
3.984 


Fat: 

From  peanut  oil 

"      hay  (ether  extract) 


Carbohydrates  : 

Starch,  Kellner's  experiments  .  . 

"       Klihn's 
Nitrogen-free  extract  (assumed) 
Crude  fiber,  of  extracted  straw  . 
"  "      "   hay  fed  alone. 


1.272 
3.057(?) 
(?) 


8.821 
8.322 


3.763 
3.648 


added  to  basal  ration 

oat  straw 

wheat  straw 

mixed  ration 


3.606 
3.311 
3.606 
3.437 
3.001 


4.083 


3.228 


4.185 


3.523 


Perhaps  the  most  striking  thing  about  these  figures  is  the  wide 
range  of  the  results  upon  the  same  class  of  nutrients.  For  reasons 
already  stated,  this  is  most  noticeable  with  the  protein,  but  it  is 
sufficiently  marked  also  with  the  other  two  groups.  Moreover, 
such  meager  data  as  we  possess  regarding  other  animals  than  the  ox 
indicate  that  the  results  vary  with  the  species  of  animal,  a  fact 
which  should  not  surprise  us,  but  which,  nevertheless,  adds  mate- 
rially to  the  complexity  of  the  subject  and  greatly  widens  the  range 
of  necessary  investigation.  It  is  obvious,  therefore,  that  at  present 
our  knowledge  is  too  imperfect  to  allow  of  the  assignment  of  average 
values  for   the  metabolizable  energy  of  the  different  classes  of 


THE  FOOD  AS  A  SOURCE   OF  ENERGY.  333 

nutrients  (as  ordinarily  determined)  even  for  a  single  species  of 
animal. 

The  results  tabulated  above,  however,  are  amply  sufficient  to 
justify  the  statement  on  p.  279  that  Rubner's  averages  are  not  appli- 
cable to  herbivorous  animals,  and  that  the  metabolizable  energy 
as  computed  with  their  aid  is  likely  to  vary  widely  from  the  truth. 
Indeed,  since  Rubner's  factor  for  fat  (9.3  Cals.  per  gram)  is  2.27 
times  that  for  carbohydrates  and  protein  (4.1  Cals.  per  gram)  a 
computation  of  the  metabolizable  energy  of  feeding-stuffs  or  rations 
as  it  has  not  uncommonly  been  made  simply  gives  a  series  of  figures 
about  4.1  times  as  great  as  that  obtained  for  total  digestible  matter 
when  the  digestible  fat  is  reduced  to  its  starch  equivalent  by  multi- 
plication by  2\.  So  far,  then,  as  a  comparison  of  one  feeding-stuff 
or  ration  with  another  is  concerned,  this  process  adds  no  whit  to  our 
knowledege.  It  does,  it  is  true,  give  some  idea,  albeit  an  inade- 
quate one,  of  the  total  amount  of  metabolizable  energy  present.  As 
yet,  however,  our  accurate  knowledge  of  the  energy  requirements  of 
domestic  animals  for  various  purposes  is  comparatively  meager. 
If  we  base  our  computations  on  the  feeding  standards  now  current, 
we  simply  repeat  with  them  the  useless  multiplication  performed 
on  the  feeding-stuffs.  On  the  other  hand,  if  we  take  the  results  of 
such  exact  experiments  on  the  metabolism  of  energy  as  are  available, 
then,  as  the  above  results  show,  we  shall  be  computing  the  energy 
requirements  upon  one  basis  and  the  energy  supply  upon  a  mate- 
rially different  one. 

Significance  of  the  Results. — A  much  more  fundamental  prob- 
lem than  that  raised  in  the  foregoing  paragraph  confronts  us  when 
we  come  to  reflect  upon  the  general  method  by  which  it  has  been 
attempted  to  compute  the  metabolizable  energy  of  nutrients,  and 
to  consider  the  real  significance  of  the  results.  In  so  doing  we  may 
properly  confine  ourselves  to  the  results  upon  cattle,  those  for  horses 
and  for  swine  being  more  or  less  fragmentary  and  uncertain.  By 
far  the  larger  proportion  of  the  results  above  tabulated,  as  well 
as  the  most  important  of  them,  are  based  on  experiments  in 
which  additions  were  made  to  a  basal  ration,  the  computation  being 
by  difference.  As  was  pointed  out  in  discussing  the  apparent 
metabolizable  energy  of  the  organic  matter  on  previous  pages, 
and  as  was  specifically  illustrated  in  the  case  of  one  experiment  on 


334  PRINCIPLES   OF  ANIMAL   NUTRITION. 

molasses  (p.  291),  the  difference  in  the  metabolizable  energy  of  the 
excreta  is  the  algebraic  sum  of  the  differences  in  the  energy  of 
methane,  urine,  and  the  several  proximate  ingredients  of  the  feces, 
and  some  of  these  differences  may  be  positive  and  others  negative. 
The  computations  of  the  metabolizable  energy  of  the  organic  matter 
as  made  in  the  earlier  paragraphs  give  the  net  result  to  the  animal 
under  the  condition  of  the  experiment  and  include  all  the  secondary 
effects  upon  digestion,  etc. 

In  the  computations  here  considered  Kellner's  methods  have 
been  followed.  In  the  first  place  the  influence  of  the  added  feed 
upon  the  digestibility  of  the  basal  ration  has  been  eliminated  by 
basing  the  computation  upon  the  digested  matter.  Still  further, 
such  effects  as  the  decrease  of  the  methane  excretion  in  certain  of 
the  experiments  with  molasses,  oil,  and  starch,  and  the  diminished 
export  of  energy  in  the  urine  under  the  influence  of  starch  and  ex- 
tracted straw,  have  not  entered  into  the  computation.  In  other 
words,  the  endeavor  has  been  to  determine  the  actual  amount  of 
energy  liberated  by  the  breaking  down  of  the  molecules  of  the  di- 
gested starch  or  protein  or  fat  in  the  organism  without  regard  to 
these  various  incidental  effects;  that  is,  to  determine  the  real  and 
not  the  apparent  metabolizable  energy. 

Either  method  of  computation  would  seem  to  be  entirely  defensi- 
ble, and  our  choice  between  them  will  be  largely  determined  by 
the  point  of  view.  For  the  purposes  of  the  physiologist,  desirous 
of  tracing  the  details  of  the  chemistry  and  physics  of  metabolism, 
the  results  obtained  by  the  latter  method  will  be  of  more  interest. 
On  the  other  hand,  the  student  of  nutrition  who  is  especially  in- 
terested in  the  problems  of  feeding  will  not  fail  to  note  that  the 
results  thus  reached  represent,  from  his  standpoint,  only  a  part  of 
the  truth.  They  show  (barring  errors  of  detail)  how  much  energy 
is  liberated  in  the  body  from  the  several  nutrients,  but  the  loss  or 
saving  of  energy  in  the  incidental  processes  constitutes  just  as  real 
a  part  of  the  balance  of  energy  which  he  wishes  to  determine  as  the 
energy  liberated  from  the  nutrients  themselves,  and  must  be  taken 
account  of  in  his  calculations.  Whether  this  can  best  be  done  by 
using  some  such  factors  as  those  just  tabulated  and  then  making 
a  correction  for  these  incidental  gains  and  losses,  or  whether  the 
method  followed  in  the  earlier  paragraphs  is  to  be  preferred,  it 


THE  FOOD  AS  A  SOURCE  OF  ENERGY.  335 

would  probably  be  premature  to  attempt  to  decide  at  present. 
Pending  further  investigation  and  experience,  however,  it  should 
be  remembered  that  the  figures  on  p.  332  will  give,  in  most  cases, 
too  high  results  for  the  metabolizable  energy  of  mixed  rations,  while 
the  same  thing  is  still  more  emphatically  true  of  Rubner's  factors. 

One  additional  point  requires  mention.  In  discussing  the 
metabolizable  energy  of  protein  it  was  pointed  out  (p.  320)  that 
it  is  at  least  a  plausible  hypothesis  that  the  proteids  are  metabo- 
lized in  the  herbivora  substantially  as  in  carnivora,  and  that  the 
excess  of  energy  in  the  urine  is  derived  from  the  non-nitrogenous 
ingredients  of  the  food.  If  we  accept  this  hypothesis,  however, 
and  assume  the  metabolizable  energy  of  protein  (N  X  6.25)  to 
be  in  the  neighborhood  of  4.9  Cals.  per  gram,  then  the  figures 
for  the  non-nitrogenous  nutrients  are  subject  to  a  still  further 
deduction,  especially  in  the  case  of  coarse  fodders.  If  we  were  to 
assign  to  the  fat  its  full  value  as  given,  it  would  not  be  difficult  to 
compute  the  metabolizable  energy  of  the  carbohydrates  on  this 
basis,  and  probably  a  set  of  factors  could  be  worked  out  which 
would  correspond  to  the  actual  results  obtained  with  mixed  rations. 
These,  however,  if  successfully  obtained,  would  be  substantially 
identical  with  the  results  given  on  previous  pages  for  the  apparent 
metabolizable  energy  of  total  or  of  digestible  organic  matter,  and 
it  does  not  appear  that  the  former  would  offer  sufficient  advantages 
over  the  latter  to  justify  the  labor  involved  in  their  computation. 


CHAPTER  XI. 

INTERNAL   WORK. 

§  i.  The  Expenditure  of  Energy  by  the  Body. 

Having  considered  the  food  in  the  light  of  a  supply  of  energy 
to  the  animal,  it  now  becomes  desirable  to  take  a  more  general 
view  of  the  subject  and  inquire  into  the  uses  to  which  the  energy 
of  the  food  is  applied  in  the  organism. 

We  have  already  distinguished  between  that  portion  of  the 
potential  energy  of  the  food  which  is  convertible  into  kinetic  energy 
in  the  body,  and  which  we  have  here  called  metabolizable  energy, 
and  that  portion  of  it  which  is  rejected  for  one  reason  or  another 
in  the  potential  form  in  the  various  excreta.  This  latter  portion 
we  may  dismiss  from  consideration  for  the  present.  The  former 
portion — the  metabolizable  energy — as  common  experience  informs 
us,  is  applied  to  two  main  purposes.  First,  it  supplies  the  energy 
for  carrying  on  the  various  activities  of  the  body.  Second,  if  the 
supply  is  in  excess  of  the  requirements  of  the  body  a  portion  of  it 
may  temporarily  escape  conversion  into  the  kinetic  form  and  be 
stored  up  as  gain  of  tissue,  notably  of  fat.  We  may  say  briefly, 
then,  that  the  metabolizable  energy  of  the  food  is  used,  first,  for 
the  production  of  "  physiological  work  "  and  second,  for  the  storage 
of  energy. 

Physiological  Work. — The  term  "  physiological  work "  in  the 
previous  sentence  is  employed  in  a  somewhat  loose  and  general 
sense  to  designate  all  those  activities  in  the  body  which  are  sus- 
tained by  the  metabolizable  energy  of  the  food  and  whose  ultimate 
result  is  the  production  of  heat  or  motion.  A  more  definite  idea 
of  what  the  term  includes  may  be  gained  by  a  consideration  of  the 
chief  factors  which  go  to  make  up  the  physiological  work  of  the 
body. 

336 


INTERNAL   WORK.  337 

Work  of  the  Voluntary  Muscles. — The  most  obvious  form 
of  physiological  work  is  that  performed  by  the  contraction  of  the 
voluntary  muscles,  either  in  the  performance  of  useful  work  or  in 
the  various  incidental  movements  made  during  the  waking  hours. 
In  a  sense  the  production  of  muscular  work  may  be  said  to  be  the 
chief  end  of  the  metabolizable  energy  of  the  food,  inasmuch  as 
all  the  other  activities  of  the  body  (apart  from  the  reproductive 
functions  and,  of  course,  from  mental  activities)  are  accessory  to 
this.  In  amount,  however,  the  energy  of  muscular  work  is  much 
less  than  the  energy  expended  in  other  forms  of  physiological  work 
and  consumes  a  comparatively  small  percentage  of  the  metaboliz- 
able energy  of  the  food. 

Internal  Work. — The  body  of  an  animal  in  what  we  commonly 
speak  of  as  a  state  of  rest  is  still  performing  a  large  amount  of  work. 
The  most  evident  forms  of  this  are  the  work  of  circulation  and  res- 
piration. In  addition  to  these,  however,  there  are  less  obvious  kinds 
of  work  whose  total  is  probably  very  considerable.  The  body  is  an 
aggregate  of  living  cells.  The  living  cell,  however,  is  constantly 
carrying  on  activities  of  various  sorts,  and  these  activities  require 
a  supply  of  energy,  although  how  much  of  the  energy  of  the  food 
is  consumed  in  the  various  processes  of  secretion,  osmosis,  karyoki- 
nesis,  etc.,  it  is  difficult  to  say. 

In  the  numerous  varieties  of  internal  work  the  energy  involved 
passes  through  various  forms.  Ultimately,  however,  since  it 
accomplishes  no  work  upon  the  surroundings  of  the  animal,  it 
is  converted  into  heat  and  leaves  the  body  either  by  radiation  and 
conduction,  as  the  latent  heat  of  water  vapor  or  as  the  sensible 
heat  of  the  excreta. 

Work  of  Digestion  and  Assimilatipn. — Logically  the  work 
of  digestion  and  assimilation  would  be  classed  as  part  of  the  internal 
work  of  the  body,  but  motives  of  convenience  make  it  desirable  to 
consider  it  separately. 

In  a  fasting  animal,  with  the  digestive  tract  empty,  the  various 
forms  of  internal  work  indicated  above  go  on  with  a  considerable 
degree  of  constancy,  and  the  resulting  heat  production  is  quite 
uniform  under  like  conditions.  If  food  be  given  to  such  an  animal 
there  results  very  promptly  an  increase  in  the  excretion  of  carbon 


338  PRINCIPLES   OF  ANIMAL   NUTRITION. 

dioxide  and  the  absorption  of  oxygen  and  in  the  amount  of  heat 
produced.      In  general  terms  this  is  brought  about  in  four  ways: 

First,  the  muscular  work  required  for  masticating  and  swallow- 
ing the  food  and  moving  it  through  the  digestive  apparatus  involves 
an  expenditure  of  energy  which  finally  gives  rise  to  the  evolution 
of  heat. 

Second,  the  activity  of  the  various  secreting  glands  of  the  diges- 
tive tract  is  stimulated,  again  making  a  demand  for  energy  and 
giving  rise  to  an  increased  heat  production. 

Third,  the  work  of  the  resorbing  cells  likewise  makes  demand 
for  energy. 

Fourth,  the  various  fermentations,  cleavages,  hydrations,  and 
syntheses  which  the  food  ingredients  undergo  in  the  course  of  diges- 
tion, resorption,  and  assimilation  may  occasion  in  individual  cases 
either  an  evolution  or  an  absorption  of  energy,  but  taken  as  a  whole 
result  in  the  production  of  a  greater  or  less  amount  of  heat  and  con- 
sume a  corresponding  amount  of  the  metabolizable  energy  of  the  food. 

Production  of  Heat. — The  body  temperature  of  the  healthy 
warm-blooded  animal  is  practically  constant,  any  considerable 
variation  from  the  average  indicating  some  serious  disturbance  of 
the  animal  economy.  Since  this  temperature  is  ordinarily  higher 
than  that  of  the  environment,  a  continual  production  of  heat  is 
necessary  to  maintain  it. 

As  stated  above,  the  various  forms  of  internal  work,  including 
the  work  of  digestion  and  assimilation,  give  rise  in  the  aggregate 
to  the  evolution  of  a  large  amount  of  heat,  and  this  heat  is  of  course 
available  for  the  maintenance  of  the  body  temperature. 

Whether  its  amount  is  sufficient  for  this  purpose,  or  whether 
under  any  or  all  circumstances  there  is  a  production  of  heat  for  its 
own  sake,  simply  to  keep  the  animal  warm,  is  still  a  debatable 
question.  Many  eminent  physiologists,  notably  Chauveau  and 
his  associates,  hold  that  the  primary  function  of  metabolism  is  to 
furnish  energy  for  the  physiological  processes  going  on  in  the  body. 
They  hold  that  the  potential  energy  of  the  food  is  converted  imme- 
diately into  some  form  of  physiological  energy,  which  in  its  turn, 
in  fulfilling  its  functions,  is  converted  into  heat  which  serves  inci- 
dentally to  maintain  the  body  temperature.  In  other  words,  they 
regard  heat  as  substantially  an  excretion  and  would  consider  that 


INTERNAL    WORK.  339 

in  the  course  of  organic  evolution  those  forms  have  survived  in 
which  the  incidental  heat  production  was  sufficient  to  meet  the 
demand  of  the  environment. 

Other  physiologists  no  less  eminent  hold  that  at  least  an  ex- 
ceptional demand  for  heat  (low  external  temperature)  may  be  met 
by  a  direct  combustion  of  food  or  body  material  for  that  purpose. 
We  shall  have  occasion  later  to  give  further  consideration  to  these 
divergent  views. 

Summary. — The  following  scheme  may  serve  to  summarize 
what  has  been  said  above  regarding  the  uses  to  which  the  energy  of 
the  food  is  put  in  the  body,  the  possible  direct  heat  production  being 
considered,  for  convenience,  as  part  of  the  physiological  work  of 
the  body  in  order  to  include  it  among  the  other  forms  of  the 
expenditure  of  energy : 

r  Energy  of  excreta. 


Gross  energy    1  r  Physiological 


Metabolizable 
energy 


work 


Work    of   voluntary 
muscles. 

Internal  work. 

Work  of  digestion  and 
assimilation. 


^  Heat  production. 
Storage  of  energy. 

For  the  sake  of  directness  of  statement,  language  has  been  used 
above  which  seems  to  imply  that  the  food  is  directly  pxidized  some- 
what like  the  fuel  in  a  locomotive.  While  statistically  the  effect  is 
the  same  as  if  this  were  the  case,  it  must  not  be  forgotten  that  the 
body  itself  constitutes  '  a  reservoir  of  potential  energy  and  that 
the  energy  liberated  in  its  various  activities  comes  primarily  from 
the  potential  energy  stored  up  in  its  various  tissues,  while  the  func- 
tion of  the  food  is  to  make  good  the  loss  this  occasioned. 

The  metabolism  of  matter  and  energy  in  the  body  might  be 
compared  to  the  exchange  of  water  in  a  mill-pond.  The  water  in 
the  pond  may  represent  the  materials  of  the  body  itself,  while  the 
water  running  in  at  the  upper  end  represents  the  supply  of  matter 
and  energy  in  the  food,  and  that  going  down  the  flume  to  the  mill- 
wheel  the  metabolism  required  for  the  production  of  physiological 
work  as  above  defined.  The  water  flowing  into  the  pond  does  not 
immediately  turn  the  wheel,  but  becomes  part  of  the  pond  and 
loses  its  identity.      Part  of  it  may  be  drawn  into  the  main  current 


34Q  PRINCIPLES   OF  ANIMAL   NUTRITION. 

and  enter  the  flume  comparatively  soon,  while  another  part  may 
remain  in  the  pond  for  a  long  time.  Pursuing  the  comparison  still 
further,  as  but  a  small  proportion  of  the  energy  liberated  in  the  de- 
scent of  the  water  in  the  flume  takes  the  form  of  mechanical  energy, 
most  of  it  being  converted  into  heat,  so  in  the  body  but  a  small 
proportion  of  the  energy  expended  in  physiological  work  takes 
ultimately  the  form  of  mechanical  energy.  Finally,  if  we  compare 
the  flow  of  water  in  the  stream  below  the  dam  to  the  heat  produc- 
tion of  the  body,  that  flow  may  be  increased,  in  case  of  need,  in  two 
ways,  viz.,  by  opening  the  gate  wider  and  letting  more  water  pass 
through  the  flume  (increase  of  physiological  work)  or  by  lowering 
the  dam  and  allowing  more  water  to  flow  over,  corresponding  to  a 
heat  production  for  its  own  sake,  if  such  takes  place. 

The  succeeding  sections  of  this  chapter  will  be  devoted  to  a  con- 
sideration of  the  expenditure  of  energy  in  the  various  forms  of  in- 
ternal work,  including  that  of  digestion  and  assimilation,  while  the 
subjects  of  the  production  of  external  work  and  of]  the  storage  of 
energy  may  be  more  appropriately  considered  in  subsequent  chap- 
ters. 

§  2.  The  Fasting  Metabolism. 

If  an  animal  be  deprived  of  food  for  a  sufficient  length  of  time 
to  empty  the  digestive  tract,  and  kept  in  a  state  of  rest  as  regards 
muscular  exertion,  the  expenditure  of  energy  in  external  work  and 
in  the  work  of  digestion  and  assimilation  are  both  eliminated,  while 
there  can  be,  of  course,  no  storage  of  energy.  Under  these  condi- 
tions the  metabolism  of  energy  in  the  organism  is  confined  to  the 
maintenance  of  those  essential  vital  activities  which  were  grouped 
above  under  the  term  "  internal  work "  in  the  narrower  sense,  to- 
gether with  any  direct  production  of  heat  for  its  own  sake.  The 
fasting  animal,  then,  affords  the  most  favorable  opportunity  to 
study  the  laws  governing  the  expenditure  of  energy  for  the  internal 
work  of  the  body.  The  fasting  metabolism  has  already  been  con- 
sidered in  Part  I  from  the  side  of  the  matter  involved;  here  we  are 
concerned  with  its  energy  relations. 

Nature  of  Demands  for  Energy. 
Without  attempting  to  enter  into  details,  it  may  be  said  that 
the  internal  work  of  the  fasting  organism  may  be  roughly  classified 


INTERNAL   WORK.  341 

as  muscular,  glandular,  and  cellular.  To  the  demand  for  energy 
for  these  purposes  we  have  probably  to  add,  at  least  in  some  cases, 
a  direct  demand  for  heat  production. 

Muscular  Work. — The  more  obvious  forms  of  muscular  work 
in  the  quiescent  animal  are  circulation  and  respiration.  To  these 
are  to  be  added  as  minor  factors  any  movements  of  other  in- 
ternal organs,  and  especially  the  general  tonus  of  the  muscular 
system,  while  finally,  the  various  incidental  movements  made  by 
such  an  animal,  although  not  logically  belonging  in  the  category 
of  internal  work,  practically  have  to  be  classed  there  in  actual 
experimentation.  It  would  be  aside  from  the  purpose  of  this 
volume  to  enter  into  any  detailed  consideration  of  these  forms  of 
internal  work,  but  a  few  general  statements  regarding  their  amount 
may  be  of  interest. 

Circulation.. — The  work  performed  by  the  heart  is  determined 
by  two  factors,  viz.,  the  weight  of  the  blood  moved  and  the  mean 
arterial  pressure  overcome.  Quite  divergent  results  have  been  ob- 
tained by  various  investigators  for  the  former  factor,  while  the 
latter  is  more  readily  determinable.  Zuntz  &  Hagemann  *  estimate 
the  output  of  blood  by  the  heart  of  the  horse  from  a  comparison 
between  the  blood  gases  and  the  respiratory  exchange,  and  compute 
the  expenditure  of  energy  in  circulation  to  be  5.01  per  cent,  of  the 
total  metabolism  of  the  horse  in  a  state  of  rest  and  3.77  per  cent, 
during  moderate  work.  Hill  f  estimates  the  average  work  of  the 
heart  in  man  at  about  24,000  kilogram-meters  in  twenty-four  hours. 
As  the  velocity  of  the  circulation  increases,  the  friction  in  the  pe- 
ripheral blood-vessels,  and  consequently  the  arterial  pressure,  rap- 
idly augments,  so  that  in  case  of  severe  muscular  exertion,  for  ex- 
ample, the  work  of  the  heart  may  readily  become  excessive. 

Respiration. — The  work  of  respiration  consists  essentially  of 
an  expansion  of  the  thorax  against  the  resistance  caused  by  the 
atmospheric  pressure  and  the  elasticity  of  the  lungs  and  the  rib 
cartilages.  Zuntz  &  Hagemann  J  estimate  its  amount  in  the 
horse  at  about  4.7  per  cent,  of  the  total  metabolism. 

Muscular  Tonus. — As  was  pointed  out  in  Chapter  VI,  the  living 

*  Landw.  Jahrb.,  27,  Supp.  Ill,  371. 

t  Schaffer's  Text-book  of  Physiology,  II,  43. 

X  hoc.  cit. 


34= 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


muscle  is  in  a  constant  state  of  slight  tension  or  tonus,  and  is  con- 
stantly the  seat  of  metabolic  activities  which  we  may  presume 
serve,  in  part  at  least,  to  maintain  that  tonus.  This  is,  of  course, 
equivalent  to  saying  that  there  is  a  continual  liberation  of  kinetic 
energy  in  the  resting  muscle,  which  temporarily  takes  the  form  of 
muscular  elasticity  but  ultimately  appears  as  heat.  As  to  the 
amount  of  energy  thus  liberated  exact  information  seems  to  be  lack- 
ing, but  in  view  of  the  relatively  large  mass  of  the  muscles  as  com- 
pared with  that  of  the  other  active  tissues  we  may  assume  that  it 
is  not  inconsiderable.  The  same  thing  would  seem  to  be  indicated 
also,  as  noted  in  Chapter  VI  (p.  191),  by  the  great  decrease  in  the 
metabolism  and  heat  production  ordinarily  observed  as  the  result 
of  paralysis  of  the  motor  nerves  by  curari. 

Incidental  Muscular  Work. — It  is  rare  that  an  animal,  even 
when  at  rest  in  the  ordinary  sense,  does  not  execute  more  or  less 
motions  of  various  parts  of  the  body,  all  of  which  involve  a  conver- 
sion of  potential  energy  into  the  kinetic  form.  Even  apparently 
insignificant  movements  may  materially  increase  the  amount  of 
metabolism.  Zuntz  &  Hagemann,*  for  example,  report  a  respira- 
tion experiment  upon  a  horse  in  which  the  uneasiness  caused  by  the 
presence  of  a  few  flies  in  the  chamber  of  the  apparatus  caused  an 
increase  of  over  10  per  cent,  in  the  metabolism.  Johanson,  Lan- 
dergren,  Sonden  &  Tigerstedt,f  in  two-hour  periods,  found  the  fol- 
lowing average  and  minimum  values  per  day  and  kilogram  weight 
for  the  excretion  of  carbon  dioxide  by  a  fasting  man  during  sleep, 
the  results  plainly  showing  the  increased  metabolism  due  to  rest- 
lessness : 


Third  day  (first  day  of  fasting) 

Fourth  "     

Fifth      "    (very  restless) 

Sixth      "    

Seventh "    


Minimum, 
Grms. 


6.744 
6.768 
7.524 
6.684 
6.564 


Subsequently  Johanson  %   compared  the  excretion   of  carbon 
dioxide  by  a  fasting  man  when  simply  lying  in  bed  (awake)  with 

*  Landw.  Jahrb.,  23,  161. 

t  Skand.  Arch.  f.  Physiol.,  7,  29. 

%  Ibid.,  8,  85. 


INTERNAL    WORK. 


343 


•that  obtained  when  all  the  muscles  were  as  perfectly  relaxed  as 
possible.     The  results  per  hour  were : 

Lying  in  bed  24 .  94  grams 

Complete  muscular  relaxation 20.72       " 

Furthermore,  there  is  more  or  less  muscular  exertion  involved 
during  the  waking  hours  in  maintaining  the  relative  position  of  the 
different  members  of  the  body.  This  is  notably  true  of  the  effort 
of  standing.  In  experiments  with  the  respiration-calorimeter 
under  the  writer's  direction*  the  heat  production  of  a  steer  per 
minute  while  standing  and  lying  was  found  to  be  approximately 
as  follows: 


Lying, 
Cals. 

Standing 
Cals. 

Ratio,  Lying  to 
Standing. 

5.322 
5.781 
6.310 
6.605 

7.031 
7.700 
8.177 
8.495 

1  •  1  321 

"      B 

1  •  1  332 

"      C 

1  •  1  296 

"      D 

1  :  1  286 

Zuntz  f  found  an  even  greater  difference  in  the  case  of  the  dog, 
the  average  oxygen  consumption  per  minute  being — 

Lying 174 . 3  c.c. 

Standing 245.6    " 

In  experiments  of  any  considerable  duration  on  normal  animals 
it  is  impossible  to  avoid  more  or  less  expenditure  of  energy  in  this 
incidental  muscular  work,  while  it  is  often  a  matter  of  difficulty 
to  make  the  different  periods  of  an  experiment  comparable  in  this 
respect. 

Glandular  Work. — The  activity  of  the  various  secretory,  ab- 
sorptive, and  excretory  organs  may  be  conveniently  summarized 
under  this  head.  While  the  purpose  of  the  glandular  metabolism 
is,  in  the  majority  of  cases,  primarily  a  chemical  one,  the  accom- 
plishment of  this  purpose  involves  an  expenditure  of  energy  which, 

*  Proc   Soc   Prom.  Agr  Sci  ,  1902. 
t  Arch.  ges.  Physiol  ,  68,  191. 


344  PRINCIPLES   OF  ANIMAL   NUTRITION. 

so  far  as  it  is  not  removed  from  the  body  in  the  potential  form  in 
the  secretions  or  excretions,  ultimately  takes  the  form  of  heat. 

Moreover,  the  fundamental  features  of  glandular  metabolism 
appear  to  be  indentical  with  those  of  muscular  metabolism.  Thus 
Henderson  *  has  shown  that  the  active  submaxillary  gland  of  the 
dog  does  not  lose  nitrogen  as  compared  with  the  inactive  gland, 
but  does  lose  weight,  evidently  from  the  metabolism  of  non- 
nitrogenous  matter.  Similarly,  Bancroft  f  found  the  respiratory 
exchange  of  the  same  gland  during  activity  to  be  three  or  four  times 
that  during  rest.  If  we  may  accept  these  results  as  typical,  we 
must  conclude  that  glandular,  like  muscular  metabolism  is  largely 
at  the  expense  of  non-nitrogenous  matter,  and  shall  not  hesitate  to 
summarize  the  two  together  as  parts  of  the  internal  work  of  the 
body. 

Cellular  Work. — While  both  muscular  and  glandular  work 
are  forms  of  cell  activity,  a  passing  mention  may  be  made  for  the 
sake  of  completeness  of  such  processes  as  imbibition,  filtration, 
osmosis,  protoplasmic  motion,  karyokinesis,  etc.,  which,  while 
taking  place  in  the  various  organs,  are  so  general  in  their  nature  and 
form  so  essential  a  part  of  our  conception  of  cell  life  that  it  seems 
proper  to  speak  of  them  collectively  as  cellular  work.  As  to  the 
quantitative  importance  of  these  activities,  so  far  as  they  can  be 
differentiated  from  the  special  functions  of  the  various  organs,  we 
lack  the  data  for  forming  any  definite  conception,  although  it 
would  appear  that  it  must  be  small. 

Heat  Production. 

As  we  have  just  seen,  the  forms  of  internal  work  are  numerous 
and  some  of  them  are  not  readily  accessible  to  measurement.  All 
of  them,  however,  have  this  in  common,  that  the  energy  used  in 
their  performance  ultimately  assumes  the  form  of  heat. 

This  being  the  case,  while  the  single  factors  making  up  the 
internal  work  are  not  readily  determined,  a  determination  of  the 
total  heat  produced  by  a  fasting  animal  in  a  state  of  rest  (either 
directly  or  by  computation  from  the  amount  and  kind  of  matter 
metabolized)  will  show  the  total  amount  of  energy  consumed  in  the 

*  Am.  Jour.  Physiol.,  3,  19.  t  Journal  of  Physiol  ,  27,  31. 


INTERNAL    WORK. 


345 


performance  of  the  internal  work  and  how  it  varies  under  varying 
conditions.  Carnivorous  animals,  with  their  short  and  relatively 
simply  digestive  canal,  lend  themselves  most  readily  to  experiments 
of  this  sort  although  rabbits  and  guinea-pigs  have  been  employed 
to  some  extent,  as  well  as,  for  short  periods,  men. 

Constancy  Under  Uniform  Conditions.  —  Attention  has  al- 
ready been  called  in  Chapter  IV  to  the  relative  constancy  of  the 
total  metabolism  of  the  fasting  animal,  particularly  as  compared 
with  the  total  mass  of  active  tissue  in  the  body.  This  constancy 
has  been  especially  emphasized  by  Rubner,*  and  forms  the  basis 
of  his  determinations  of  the  replacement  values  of  the  several 
nutrients  which  will  be  considered  in  the  following  chapter. 
With  a  rabbit  the  following  daily  averages,  computed  per  100 
parts  of  nitrogen  in  the  body,  were  obtained: 


Day  of  Experiment. 

Nitrogen  in 
Urine. 

Fat 
Metabolized. 

Third  to  eighth 

Ninth  to  fifteenth 

2.16 
2.19 

16.2 
13.8 

Since  the  ratio  of  proteids  to  fat  metabolized  did  not  vary 
greatly  in  these  trials,  the  total  amount  of  carbon  dioxide  ex- 
creted may  be  taken  as  an  approximately  accurate  measure  of 
the  total  metabolism.  For  the  several  days  of  the  experiment, 
this  was  as  follows: 


Average  Live 

Weight, 

Grms. 

Carbon  Dioxide  Excreted. 

Day. 

Per  Head, 

Grms. 

Per  Kg. 

Live  Weight, 
Grms. 

Fifth 

2091 
2002 
1907 
1864 
1764 
1731 
1716 
1697 

36.1 
31.8 
30.3 
29.2 
30.2 
27.4 
27.4 
25.5 

17.26 

15  90 

Ninth 

15.90 

Tenth 

15.65 

Twelfth 

17.18 

Thirteenth 

15.81 

15.95 

Fifteenth 

15.90 

*  Zeit.  f.  Biol.,  17,  214;  19,  312. 


346  PRINCIPLES   OF  ANIMAL   NUTRITION. 

With  a  dog  the  following  results  were  obtained : 


Live 

Weight, 
Grms. 

Nitrogen 

in  Urine, 

Grms. 

Fat 

Metab- 
olized, 
Grms. 

Carbon  Dioxide 
Excreted. 

Day. 

Per  Head, 
Grms. 

Per  Kg. 

Live 
Weight, 
Grms. 

First 

9190 
8920 
8620 
8190 
8030 
7890 
7970 
7830 

4.23 
2.89 
3.65 
2.59 
2.41 
2.53 
2.98 
3.02 

51.74 
45.94 
42.90 
45.55 
41.83 
36.48 
37.45 
33.80 

187.4 
157.5 
146.9 
151.7 
140.4 
127.9 
134.8 
125.0 

20.70 

17.83 

Fourth 

17.99 

Tenth   

18.70 

17.86 

Twelfth            

16.13 

Thirteenth           

17.06 

16.12 

Rubner  also  quotes  the  following  results  by  Kuckein  on  a  cockj 

~  Carbon  Dioxide  per 

D&y-  Kg.  Live  Weight. 

Third 21 .  73  grams. 

Fifth 21.47       " 

Seventh 21.43       " 

Rubner 's  experiments  on  a  guinea-pig  *  show  a  similar  constancy, 
the  heat  production  being  computed  from  the  total  metabolism: 

n  Heat  Production 

L,ay-  per  Kilogram. 

First 149.9  Cals. 

Second 162.6  " 

Third 156.5  " 

Fourth 140.5  " 

Fifth 137.3  " 

Sixth 150.6  " 

Seventh 157.4  " 

Eighth 155.6  " 

Ninth 162.6  " 

Concerning  this  point  Rubner  says :  f  "  The  uniformity  of  the 
fasting  metabolism  proves  that,  in  spite  of  the  undoubted  limita- 
tation  of  all  the  voluntary  functions  which  can  cause  a  consump- 
tion of  matter,  no  further  reduction  of  the  metabolism  is  possible, 
*  Biologische  Gesetze,  p.  15.  t  Loc.  cit.,  19,  326. 


INTERNAL    WORK.  347 

and  we  recognize  from  this  that  we  have  to  do  here  with  a  constant 
metabolism  which  is  indissolubly  connected  with  life  itself.  The 
animal  in  the  fasting  state  adjusts  itself  to  the  minimum  metabolism. " 

In  other  words,  the  metabolism  and  consequent  heat  production 
of  the  fasting,  quiescent  animal  speedily  reaches  a  minimum  which 
represents  the  aggregate  demands  of  the  vital  activities  of  the 
organism  for  energy ;  that  is,  which  represents  the  internal  work  of 
the  body  in  the  sense  in  which  the  words  are  here  used,  plus  the 
metabolism  required  for  any  direct  production  of  heat  which  may 
be  necessary  to  maintain  the  normal  temperature  of  the  animal. 

The  relative  importance  of  the  internal  work  in  the  narrower 
sense  and  of  the  direct  heat  production  as  regards  their  demands  for 
a  supply  of  energy  will  appear  more  clearly  when  we  consider,  in 
the  following  paragraphs,  the  effects  of  varying  conditions,  and 
particularly  of  the  thermal  environment,  upon  the  heat  produc- 
tion of  the  fasting  animal. 

Influence  of  Thermal  Environment  on  Heat  Production.* — An 
animal,  particularly  in  the  temperate  zones,  is  subject  to  consider- 
able variations  of  external  conditions,  particularly  of  temperature, 
which,  in  the  first  place,  tend  to  affect  the  rate  at  which  it  emits 
heat,  and  secondarily,  within  certain  limits  to  modify  the  amount 
of  heat  produced  in  the  body. 

Body  Temperature. — As  regards  their  body  temperature, 
animals  have  been  divided  into  two  great  classes :  the  cold-blooded 
(poikilothermic),  whose  temperature  as  a  rule  differs  but  slightly 
from  that  of  their  surroundings,  and  the  warm-blooded  (homoio- 
thermic),  whose  temperature  remains  approximately  constant  dur- 
ing health  whatever  be  that  of  their  surroundings.  Since  all  our 
domestic  animals,  as  well  as  man  himself,  belong  to  the  second 
group,  it  alone  will  be  considered  in  the  following  paragraphs. 

Since  the  animal  is  constantly  producing  heat  in  the  various 
ways  already  indicated,  it  is  obvious  that  in  order  to  maintain  a 
constant  body  temperature  it  must  be  able  to  give  off  this  heat  at 
the  same  average  rate  at  which  it  is  produced.  Ranke  illustrates 
this   necessity  in   a   striking   manner   by  computing  that  if  the 

*  The  discussion  of  this  subject  follows  to  a  considerable  extent  that  of 
Ranke  in  the  introduction  to  his  "Einwirkung  des  Tropenklimas  auf  die 
Ernahrung  des  Menschen,"  Berlin,  1900. 


348  PRINCIPLES  OF  ANIMAL   NUTRITION. 

body  of  a  man  were  unable  to  give  off  the  heat  which  it  pro- 
duces, a  single  day  would  suffice  to  raise  it  to  a  pasteurizing 
temperature,  while  in  the  course  of  a  year,  at  the  same  rate,  a 
temperature  of  over  17,000°  C.  would  be  reached. 

Furthermore,  since  the  external  conditions  of  temperature  are 
subject  to  frequent  and  sudden  changes,  it  is  obvious  that  the 
balance  between  heat  production  and  emission  must  be  capable  of 
prompt  adjustment  to  varying  circumstances. 

Thermic  Range. — The  ability  of  the  animal  body  to  adapt 
itself  to  changes  of  temperature  has,  however,  often  been  ex- 
aggerated. As  a  matter  of  fact  this  adaptation  is  possible  only 
within  a  comparatively  narrow  range,  and  unless  we  hold  fast  to 
this  fundamental  idea  we  are  in  danger  of  reaching  fallacious 
and  absurd  conclusions.  Man  has  considerably  extended  the  range 
of  climate  within  which  he  can  exist  by  means  of  clothing,  shelter, 
artificial  heat,  and  even  to  a  slight  extent  artificial  refrigeration, 
and  this  fact  often  leads  unconsciously  to  an  overestimate  of  the 
possible  thermic  range.  These  means  of  artificial  protection  re- 
sult essentially  in  modifying  the  temperature  to  which  the  body  is 
actually  exposed,  and  the  same  is  true  in  a  less  degree  of  the  differ- 
ences in  the  summer  and  winter  coats  of  animals.  The  fact  still 
remains  that  the  actual  thermic  range  of  any  species  is  and  must  be 
strictly  limited.  All  life  implies  a  certain  amount  of  metabolism, 
and  consequently  of  heat  production.  With  rising  temperature  a 
point  must  sooner  or  later  be  reached  at  which  the  animal  is  unable 
to  impart  this  heat  to  its  surroundings  as  fast  as  it  is  produced,  and 
in  which  the  rise  in  temperature  necessarily  resulting  will  prove 
fatal.  With  falling  temperature  a  point  will  be  reached  at  which 
the  greatest  possible  amount  of  metabolism  in  the  body  will  be 
unable  to  equal  the  rate  at  which  heat  is  lost  to  the  surroundings 
and  the  animal  will  perish  from  cold.  Both  the  maximum  ami 
minimum  points  and  the  extent  of  the  thermic  range  will  vary  for 
different  species  and  varieties  of  animals,  but  at  best  the  range  is 
relatively  small. 

Means  of  Regulation. — Within  the  thermic  range  of  a  given 
animal  the  adjustment  to  its  thermal  environment  may  be  effected 
in  one  or  both  of  two  ways,  viz.,  by  a  regulation  of  the  rate  of  emis- 
sion of  heat  or  by  a  variation  in  the  heat  production. 


INTERNAL    WORK.  349 

Regulation  of  Rate  of  Emission. — Heat  is  given  off  by  the  body 
in  four  principal  ways:  (1)  by  conduction;  (2)  by  radiation;  (3) 
by  evaporation  of  water;  (4)  as  the  sensible  heat  of  the  excreta. 

By  conduction,  heat  is  transferred  directly  from  the  body  to 
its  surroundings,  including  such  solid  objects  as  it  may  be  in  con- 
tact with  and  particularly  the  air.  The  rate  of  loss  in  this  way 
will  depend  upon  the  relative  temperature  and  conductivity  of 
the  surface  of  the  body  and  of  the  substances  with  which  it  is  in 
contact,  and  in  case  of  the  air  will  be  also  influenced  by  the  rate 
of  motion  of  the  latter  relatively  to  that  of  the  body. 

By  radiation,  a  constant  exchange  of  heat  goes  on  between  the 
body  and  objects  not  in  immediate  contact  with  it.  Since  the  body 
is  usually  warmer  than  its  surroundings,  the  net  result  of  this  ex- 
change is  a  loss  of  heat  by  the  body,  the  amount  of  which  depends 
upon  the  specific  radiating  power  of  the  surface  of  the  body  and 
upon  the  difference  in  temperature  between  the  latter  and  sur- 
rounding objects. 

By  evaporation  of  water  from  the  skin,  and  to  a  less  degree 
from  the  mucous  membrane  of  the  air-passages,  a  large  amount 
of  heat  may  be  removed  as  latent  heat  of  vaporization.  The 
amount  of  water  evaporated  from  the  skin,  and  consequently  the 
rate  at  which  heat  is  carried  off,  will  depend  in  part  on  the 
amount  transpired  by  the  skin,  but  when  this  is  abundant, 
chiefly  upon  the  relative  humidity  of  the  air  and  upon  its  rate  of 
movement. 

Finally,  the  heat  removed  in  the  excreta  is  relatively  small,  and 
in  the  case  of  the  fasting  animal  in  particular  is  insignificant  as 
compared  with  the  losses  through  the  other  three  channels. 

In  general  we  may  say  that  the  rate  of  emission  of  heat  in  all 
of  the  first  three  ways  named  is  determined  by  two  sets  of  condi- 
tions, viz.,  those  relating  to  the  environment  of  the  animal  (tem- 
perature, relative  humidity,  movement  of  air)  and  those  relating 
to  the  animal  itself  and  particularly  to  its  surface. 

The  conditions  of  the  first  set,  of  course,  are  beyond  the  control 
of  the  organism.  Their  tendency  is  to  produce  the  same  effect  upon 
the  rate  of  emission  of  heat  that  they  would  upon  that  of  a  lifeless 
body,  viz.,  to  increase  it  as  the  temperature  of  the  surroundings  is 
lowered  and  their  conducting  power  increased.     In  the  case  of  the 


35°  PRINCIPLES  OF  ANIMAL   NUTRITION. 

living  animal  this  tendency  is  offset  by  the  regulative  mechanism 
acting  upon  the  second  set  of  conditions,  so  that,  e.g.,  a  fall  in 
the  temperature  of  its  surroundings  within  certain  limits  instead 
of  increasing  the  rate  of  emission,  as  in  the  case  of  a  lifeless  body, 
has  no  effect  upon  it.  This  regulation  of  the  rate  of  emission  is 
effected  chiefly  by  means  of  changes  in  the  temperature  and  state 
of  moisture  of  the  skin,  brought  about  on  the  one  hand  through  the 
vaso-motor  mechanism  and  on  the  other  through  the  special  nerves 
of  perspiration. 

Variations  of  external  temperature  acting  upon  the  peripheral 
nerves  influence  b}r  reflex  action  the  activity  of  the  vaso-motor 
nerves  which  regulate  the  caliber  of  the  minute  blood-vessels. 
Exposure  to  cold  causes  a  contraction  of  the  capillaries  of  the 
skin  and  a  relaxation  of  those  of  the  viscera.  As  a  result  more 
blood  passes  through  the  latter,  while  the  flow  through  the  skin 
is  diminished,  the  latter  becomes  paler,  and  since  the  heat  given 
off  is  not  fully  replaced  by  the  blood  current,  its  temperature  falls. 
Exposure  to  heat  has  the  contrary  effect.  The  capillaries  of  the 
skin  relax,  more  blood  flows  through  them,  the  skin  becomes  flushed 
and  its  temperature  rises,  while  the  flow  of  blood  to  the  viscera  is 
checked.  A  fall  in  the  temperature  of  the  skin,  however,  tends  to 
diminish  the  rate  of  emission  of  heat  both  by  conduction  and  radia- 
tion, while  a  rise  in  its  temperature  has  the  opposite  effect,  thus 
counteracting  the  tendency  of  changes  of  external  temperature. 
In  other  words,  the  "emission  constant"  of  the  skin  changes  to 
meet  changes  in  external  conditions.  So  exactly  are  these  mech- 
anisms adjusted  in  health  that  within  certain  rather  narrow  limits 
they  maintain  the  rate  of  emission  of  heat,  and  consequently  the 
average  temperature  of  the  body,  very  nearly  constant. 

Obviously,  however,  there  must  be  a  limit  above  which  the 
temperature  and  radiating  power  of  the  skin  cannot  be  increased 
to  compensate  for  a  rise  in  external  temperature.  The  second 
method  of  regulation  then  comes  more  markedly  into  play  through 
the  familiar  act  of  perspiration,  or  sweating.  At  high  temperatures 
the  activity  of  the  sweat-glands  is  greatly  stimulated,  in  part 
doubtless  by  the  more  abundant  supply  of  blood  to  the  skin,  but 
chiefly  by  reflex  stimulation  of  the  special  nerves  which  control  the 
secretion  of  sweat.     The  evaporation  of  the  relatively  large  amount 


INTERNAL    WORK. 


351 


of  water  thus  supplied  to  the  surface  of  the  skin  is  a  powerful  means 
of  refrigeration,  as  we  know  no  less  from  common  experience  than 
from  scientific  determinations,  the  evaporation  of  a  single  gram  of 
water  requiring  approximately  0.592  Cal.  of  heat.  With  very 
high  temperatures,  especially  in  a  humid  atmosphere,  however, 
even  this  method  of  disposing  of  the  heat  becomes  insufficient  and 
the  extreme  upper  limit  of  the  thermal  range  is  passed. 

These  two  methods  of  regulation  of  the  body  temperature  are 
often  spoken  of  collectively  as  "  physical "  regulation. 

Variations  in  Amount  of  Heat  Produced. — Just  as  there  is  a 
superior  limit  beyond  which  the  regulation  of  the  body  tempera- 
ture by  the  means  above  described  cannot  be  carried,  so  it  is  obvious 
that  there  must  be  a  lower  limit  of  regulation.  However  much  the 
cutaneous  circulation  may  be  reduced,  the  skin  will  always  lose  heat 
to  a  sufficiently  cold  environment  faster  than  it  is  being  generated 
by  the  internal  work  of  the  body.  Under  these  circumstances  the 
only  method  by  which  the  temperature  of  the  animal  can  be  main- 
tained is  an  increase  in  the  rate  of  generation  of  heat. 

That  changes  of  external  temperature  affect  the  amount  of  heat 
generated  was  shown  by  the  experiments  of  Lavoisier  and  the 
observations  of  Liebig,  but  Liebermeister  *  appears  to  have  been 
the  first  to  clearly  enunciate  the  theory  of  regulation  by  variations 
in  the  rate  of  production.  The  fact  of  such  regulation  has  been 
fully  demonstrated  by  numerous  subsequent  investigators.  As  a 
typical  example  we  may  take  the  well-known  experiments  of  Theo- 
dor  f  on  a  cat,  some  of  the  results  of  which  are  as  follows: 


Temperature, 
Deg. 
Cent. 

CarbonDioxide 

Excreted, 

Grms. 

Oxygen 

Taken  Up, 

Grms. 

Temperature, 
Deg. 
Cent. 

CarbonDioxide 

Excreted, 

Grms. 

Oxygen 

Taken  Up, 

Grms. 

-5.5 

-3.0 

0.2 

5.0 

19.83 
18.42 
18.24 
17.90 

17.48 
18.26 
19.95 
14.82 

12.3 
16.3 
20.1 
29.6 

17.63 
15.73 
14.34 
13.12 

17.71 
14.74 
12.78 
10.87 

Numerous  other  investigators  have  obtained  similar  results, 
but  the  effect  of  low  temperature  in  stimulating  the  heat  produc- 
tion of  warm-blooded  animals  is  too  well  established  to  require  an 

*  Arch  f.  (Anat.  u.)  Physiol.,  1860,  pp.  520  and  589;  1861,  p.  661. 
t  Zeit.  f.  Biol.,  14,  51. 


352  PRINCIPLES   OF  ANIMAL   NUTRITION. 

extended  citation  of  authorities  here.     Some  of  Rubner's  *  more 

recent  results,  however,  are  of  interest  as  showing  the  delicacy  of 

the  reaction.     The  experiments  were  made  on  fasting  dogs  in  a 

state  of  complete  rest,  the  heat  production  being  computed  from 

the  total  metabolism  of  carbon  and  nitrogen : 

Tempera-  Heat  Production  per 

ture,  Deg.  C.  Kg.  in  24  Hours. 

(13.8 78.68  Cals. 

J  14.9 74.74    " 

I]  17.3 69.78     " 

Ll8.0 67.06    " 

(11.8 40.60  " 

I  12.9 39.13  " 

11 1  15.9 35.99  " 

ll7.5 35.22  ° 

t  13.4 39.65     " 

III]  19.5 35.10     " 

(27.4 30.82     " 

This  method  of  regulation  of  the  body  temperature  is  often  briefly 
designated  as  "  chemical "  regulation. 

Just  how  the  additional  generation  of  heat  is  effected  is  not  so 
clear.  From  the  fact  that  the  muscles  are  the  seat  of  a  very  large 
part  of  the  heat  production  of  the  body  we  should  naturally  be 
inclined  to  look  to  them  as  the  source  of  the  increase.  In  quite  a 
number  of  experiments  on  man,  of  which  those  of  A.  Loewy  f  and 
of  Johansson  \  may  be  especially  mentioned,  a  stimulation  of  the 
heat  production  with  falling  temperature  was  only  observed  when 
there  was  visible  muscular  action,  such  as  shivering,  while  in  the 
other  cases  only  the  "  physical "  regulation  occurred.  Any  contrac- 
tion of  the  muscles  would  of  course  be  a  source  of  heat,  but  the  in- 
crease with  falling  external  temperature  has  been  repeatedly  observed 
with  animals  in  the  absence  of  this  obvious  cause.  Whether  in 
such  cases  there  is  an  increase  in  the  tonus  of  the  muscles,  involv- 
ing an  increase  in  their  metabolism,  or  whether,  through  some 
form  of  reflex  stimulation,  the  rate  of  oxidation  is  accelerated 

*  Biologische  Gesetze,  p.  10. 

t  Arch.  ges.  Physiol.,  46,  189. 

%  Skand.  Arch.  f.  Physiol.,  7,  123. 


INTERNAL    WOP,K. 


353 


simply  for  the  sake  of  the  heat  produced  is  still  an  unsettled  ques- 
tion and  one  which,  for  our  present  purpose,  we  need  not  pause 
to  consider.     As  to  the  fact  of  the  increase  there  is  no  question. 

Critical  Temperature. — In  early  writings  upon  this  subject 
the  influence  of  external  temperature  in  increasing  or  diminishing 
the  heat  production  of  the  body  was  frequently  spoken  of  as  if  it 
were  of  unlimited  application,  and  the  same  idea  has  passed  more 
or  less  fully  into  the  popular  literature  of  the  subject.  But  little 
reflection  is  necessary,  however,  to  show  that  this  cannot  be  the 
case.  Common  observation  teaches  us  that  neither  our  own  metab- 
olism nor  that  of  our  domestic  animals,  as  roughly  measured  by 
the  consumption  of  food,  is  affected,  for  example,  by  the  difference 
between  winter  and  summer  to  any  such  extent  as  would  correspond 
to  the  difference  in  average  temperature.  Moreover,  if  every  rise 
in  external  temperature  diminished  the  heat  production,  there 
would  be  a  temperature  at  which  no  heat  production  at  all  would 
occur  and  at  which,  therefore,  life  could  exist  without  metabolism, 
which  is  a  contradiction  in  terms.  This  extreme  case  renders  clear 
the  fundamental  error  of  this  view,  viz.,  that  of  regarding  the  heat 
production  as  an  end  in  itself  and  not  as,  substantially,  an  incident 
of  the  general  metabolism. 

Carl  Voit*  was  the  first  to  demonstrate  by  exact  scientific 
experiments  the  limits  within  which  the  influence  of  temperature 
upon  metabolism  (the  so-called  chemical  regulation)  is  confined. 
His  experiments  were  a  continuation  of  those  of  Theodor  (p.  351), 
and  were  made  upon  a  man  weighing  about  70  kgs.  and  wearing 
ordinary  clothing.  After  exposure  for  some  time  to  the  tempera- 
ture to  be  tested  he  passed  six  hours  in  the  chamber  of  the  respira- 
tion apparatus,  fasting  and  in  complete  rest.  During  the  six  hours 
the  excretion  of  carbon  dioxide  and  nitrogen  was  as  follows: 


Temperature. 
Deg.  C. 

Carbon 

Dioxide. 

Grms. 

Urinary 

Nitrogen 

Grms. 

Temperature. 
Deg.  C. 

Carbon 

Dioxide. 

Grms. 

Urinary 

Nitrogen. 

Grms. 

4.4 

6.5 

9.0 

14.3 

16.2 

210.7 
206.0 
192.0 
155.1 
158.3 

4.23 
4.05 
4.20 
3.81 
4.00 

23.7 
24.2 
26.7 
30.0 

164.8 
166.5 
160.0 
170.6 

3.40 
3.34 
3.97 

*  Zeit.  f.  Biol.,  14,  57. 


354 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Later  and  more  comprehensive  experiments  with  animals  by 
Rubner  have  given  corresponding  results.  Thus  with  two  guinea- 
pigs  the  following  figures  were  obtained  in  24-hour  experi- 
ments :  * 


Mature  Animal. 

Young  Animal. 

Temperature 

CO,  per  Kg. 

Temperature 

Temperature 

CO,  per  Kg. 

of  Animal, 

and  Hour, 

of  Air, 

of  Animal, 

and  Hour, 

Deg.  C. 

Deg.  C. 

Grms. 

Deg.  C. 

Deg.  C. 

Grms. 

0 

37.0 

2.905 

0 

38.7 

4.500 

11.1 

37.2 

2.151 

10 

38.6 

3.433 

20.8 

37.4 

1.766 

20 

38.6 

2.283 

25.7 

37.0 

1.540 

30 

38.7 

1.778 

30.3 

37.7 

1.317 

35 

39.2 

2.266 

34.9 

38.2 

1.273 

40.0 

39.5 

1.454 

A  later  experiment  by  Rubner  f  upon  a  dog,  in  which  the  heat 
production  was  measured  by  a  calorimeter,  gave  the  following 
results : 

Temperature  of  Air.  Heat  Production  per  Kg. 

7.6°  C 83.5  Cals. 

15.0°  " 63.0     " 

20.0°  "  53.5     " 

25.0°  " 54.2     " 

30.0°  "  56.2     " 

The  uniform  testimony  of  these  various  experiments  is  that  for 
each  species  there  is  a  certain  external  temperature  at  which  the 
metabolism  and  consequent  heat  production  reach  a  minimum. 
With  man  in  ordinary  clothing  it  would  appear  to  lie  at  about 
15°  C.,t  with  the  dog  at  about  20°  C,  and  with  the  guinea-pig  at 
about  30°-35°  C.  Below  this  point  the  heat  production  rises  or 
falls  with  changes  of  external  temperature;  or,  in  other  words,  the 
constancy  of  the  body  temperature  is  secured,  in  part  at  least,  by 


*  Biologische  Gesetze,  p.  13. 

t  Archiv  f.  Hygiene,  11,  285. 

%  Rubner  (Biol.  Gesetze,  p.  30)  says  that  for  naked  man  it  is  about  37°  C. 


INTERNAL   IVORK.  355 

means  of  the  so-called  "  chemical "  regulation,  that  is,  by  variations 
in  the  production  of  heat. 

Above  this  point  the  heat  production,  instead  of  a  further  de- 
crease, shows  an  increase,  which,  however,  is  slight  as  compared 
with  the  differences  observed  as  a  result  of  the  "chemical"  regu- 
lation. Here  we  are  obviously  in  the  domain  of  the  "physical" 
regulation — the  regulation  by  changes  in  the  emission  constant  of 
the  skin.  This  temperature  at  which  the  chemical  regulation 
ceases,  and  which  presumably  varies  for  different  species  of  animals, 
Ranke  calls  the  critical  temperature.  Below  it  the  regulation  is 
chiefly  "chemical,"  above  it  chiefly  "physical."  The  slight  in- 
crease in  the  metabolism  above  the  critical  point  is  plausibly  ex- 
plained as  due  to  the  greater  activity  of  the  organs  of  circulation, 
respiration,  and  perspiration  required  for  the  "physical"  regula- 
tion. 

Rubner's  experiments  also  show  that  the  portion  of  the  thermic 
range  lying  above  the  critical  temperature  falls  into  two  distinct 
subdivisions.  For  a  certain  distance  above  that  point,  the  factors 
chiefly  concerned  in  the  regulation  of  the  body  temperature  are 
conduction  and  radiation,  which  keep  pace  with  the  rising  tem- 
perature in  the  manner  already  explained.  At  the  same  time, 
there  is  a  small  increase  in  the  rate  of  evaporation  of  water,  approxi- 
mately equivalent  to  the  slight  increase  in  the  metabolism  above 
the  critical  temperature  to  which  attention  has  just  been  called. 
Matters  go  on  in  this  way  through  a  certain  range  of  temperature 
until  the  regulative  capacity  of  the  vaso-motor  mechanism  is 
utilized  to  its  maximum.  If  the  external  temperature  still  rises, 
the  emission  of  heat  by  conduction  and  radiation  begins  to  decrease 
as  it  would  in  a  lifeless  object,  and  the  deficit  thus  occasioned  is 
made  up  by  a  sudden  increase  in  the  exhalation  of  water  vapor, 
coinciding,  in  man,  with  the  production  of  visible  perspiration. 
This  sudden  increase  in  the  activity  of  the  sweat-glands  is  accom- 
panied, as  we  should  expect,  by  an  increase  in  the  total  metabolism 
and  consequent  heat  production. 

These  phenomena  are  well  illustrated  by  Rubner's  experiments 
with  a  fasting  dog,  already  partially  cited  on  the  opposite  page. 
The  following  table  shows  the  amount  of  heat  carried  off  by  con- 
duction and  radiation  and  as  latent  heat  of  water-vapor  at  the 


356 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


several  temperatures,  and  the  same  facts  are  also  shown  graphically 
in  the  accompanying  diagram. 


Temperature 
of  Air. 
Deg.  C. 

Total  Heat 

Production, 

Cals. 

Disposed  of  by 

Conduction 

and 
Radiation. 

Cals. 

As  Latent 

Heat  of  Water 

Vapor, 

Cals. 

7.6 
15.0 
20.0 
25.0 
30.0 

83.5 

63.0 
53.5 
54.2 
56.2 

71.7 

49.0 
37.3 
37.3 
30.0 

11.8 
14.0 
16.2 
16.9 
26.2 

It  appears,  then,  that  a  certain  minimum  heat  production, 
corresponding  to  the  metabolism  at  the  critical  temperature,  is 
inseparably  connected  with  the  life  of  the  animal.  The  very  fact 
that  the  heat  production  at  this  temperature  is  a  minimum  shows 
that  its  amount  is  not  determined  by  the  needs  of  the  organism  for 
heat.  If  the  latter  were  the  controlling  condition,  a  rise  of  exter- 
nal temperature  should  still  further  reduce  the  generation  of  heat, 
while  as  a  matter  of  fact  it  is  accompanied  by  a  slight  increase  up 
to  the  point  where  the  amount  of  heat  produced  overpasses  the 
ability  of  the  organism  to  dispose  of  it  and  death  results.  The 
natural  conclusion  is  that  the  metabolism  at  the  critical  tem- 
perature is  that  which  is  necessary  for  the  performance  of  the 
various  functions  of  the  organism,  and  that  the  heat  production 
at  this  temperature,  therefore,  represents  the  amount  of  energy 
necessarily  consumed  in  the  internal  work  of  the  body.  This  is, 
of  course,  Rubner's  conclusion  (p.  346)  in  a  slightly  altered  form. 

The  case  is  not  unlike  that  of  a  room  in  which  a  fire  must  be 
kept  burning  for  some  purpose — a  kitchen,  for  example.  In  winter, 
changes  in  external  temperature  may  be  met  by  burning  more  or 


INTERNAL   WORK.  357 

less  fuel.  As  spring  advances,  the  fire  is  reduced  until  it  is  just 
sufficient  for  the  necessary  work.  If  the  weather  still  continues 
to  grow  warmer,  since  the  fire  cannot  be  further  reduced  the  excess 
of  heat  is  gotten  rid  of  by  opening  the  windows  more  or  less,  while, 
to  carry  out  the  analogy,  in  very  hot  weather  we  may  sprinkle  the 
floor  or  wet  the  walls  to  secure  relief  from  heat  through  the  evapora- 
tion of  water. 

Modification  of  Conception  of  Critical  Temperature. — 
In  our  discussion  thus  far  we  have  considered  chiefly  the  influence 
of  external  temperature  on  metabolism  and  heat  production.  This 
is,  however,  by  no  means  the  only  condition  affecting  the  heat 
balance  of  the  body.  Of  the  other  meteorological  factors,  three 
call  for  special  mention,  viz.,  wind,  insolation,  and  in  particular 
relative  humidity. 

Wind. — In  a  perfectly  still  atmosphere,  the  layer  of  air  next  to 
the  skin  becomes  warmed  and  loaded  with  water  vapor  and  con- 
stitutes to  a  certain  degree  a  protective  envelope  which  is  removed 
with  comparative  slowness  by  gaseous  diffusion.  A  current  of  air, 
by  removing  this  protecting  layer  and  bringing  fresh  portions  of  air 
in  contact  with  the  body,  increases  the  emission  of  heat  both  by 
conduction  and  by  evaporation  of  water.  This  is  in  accord  with  the 
common  experience  that  a  degree  of  cold  which  can  readily  be 
borne  when  the  air  is  still  becomes  intolerable  in  a  brisk  wind,  while, 
on  the  other  hand,  the  oppressiveness  of  a  very  hot  day  is  sensibly 
relieved  by  even  a  slight  breeze.  The  effect  of  wind,  then,  is  to 
transpose  the  thermic  range  of  the  animal  to  a  higher  place  in  the 
thermometric  scale,  and  to  correspondingly  raise  the  critical  tem- 
perature. 

Insolation. — The  direct  rays  of  the  sun  impart  a  considerable 
amount  of  heat  to  the  body.  The  effect  of  insolation,  therefore,  is 
the  reverse  of  that  of  wind,  viz.,  to  transpose  the  thermal  range 
and  the  critical  temperature  downward.  A  similar  effect  is  pro- 
duced, of  course,  by  the  sun's  heat  when  reflected  from  surrounding 
objects,  or  by  the  radiant  heat  from  hot  objects,  the  earth,  for  ex- 
ample. On  the  other  hand,  the  radiation  from  the  body  into  space 
during  the  night,  especially  at  high  altitudes  and  through  a  dry, 
clear  atmosphere,  may  have  a  very  considerable  effect  in  the  con- 
trary direction. 


35 8  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Relative  Humidity. — The  relative  humidity  of  the  air  affects  the 
emission  of  heat  in  two  principal  ways.  At  low  temperatures, 
where  the  evaporation  of  water  plays  a  subordinate  role,  it  increases 
the  rate  of  emission  by  increasing  the  conductivity  and  specific 
heat  of  the  air,  and  also  the  conductivity  of  the  skin  and  the  body 
covering  (hair,  fleece,  clothing),  these  effects  outweighing  its  in- 
fluence in  diminishing  the  relatively  small  amount  of  evaporation 
Moist  cold  is,  therefore,  more  trying  than  dry  cold. 

At  high  temperatures,  on  the  other  hand,  where  a  large  pro- 
portion of  the  heat  is  removed  by  evaporation,  a  high  relative 
humidity,  by  checking  this  evaporation,  hinders  the  emission  of 
heat,  this  effect  overbalancing  any  slight  increase  in  conductivity. 
Moist  heat  is  accordingly  more  oppressive  than  dry  heat. 

An  increase  in  the  relative  humidity,  then,  abbreviates  the 
thermal  range  at  both  ends,  while  at  moderate  temperatures  it 
appears  to  have  but  little  effect,  a  diminution  of  the  loss  by  evap- 
oration being  compensated  for  by  an  increase  in  radiation  and 
conduction. 

Critical  Thermal  Environment. — From  the  above  it  is 
obvious  that  the  so-called  critical  temperature  is  not  a  constant, 
even  for  the  same  species  or  the  same  individual,  but  that  other 
factors  than  the  temperature  of  the  air  materially  affect  it. 

What  is  constant  (relatively  at  least)  is  the  rate  at  which  heat 
is  produced  in  the  body  by  the  metabolism  necessary  to  sustain  its 
various  physiological  activities,  that  is,  by  its  internal  work  in 
order  to  maintain  the  normal  body  temperature,  the  total  outflow 
of  heat  through  its  various  channels  must,  at  its  minimum,  be  equal 
to  the  amount  thus  liberated  in  the  organism.  The  outflow  of 
heat,  as  we  have  seen,  is  affected  directly  or  indirectly  by  the 
external  conditions,  and  largely  by  the  three  just  mentioned.  In- 
numerable combinations  of  these  conditions  are  possible,  and  any 
one  of  them  whose  combined  effect  upon  the  animal  is  to  make 
the  outflow  of  heat  equal  to  the  rate  of  evolution  due  to  the 
internal  work  will  constitute  a  critical  point  in  the  above  sense. 
Any  change  in  such  a  set  of  conditions  which  tends  to  increase  the 
outflow  of  heat  will,  like  a  fall  in  temperature,  be  met  chiefly  by  an 
increased  heat  production.  Any  change  tending  in  the  opposite 
direction  will  be  compensated  for  by  the  effects  upon  the  organ- 


INTERNAL   WORK.  359 

ism  whch  have  already  been  described  and  which  result  in  maintain- 
ing the  rate  of  emission  of  heat  at  a  point  enough  higher  than  before 
to  provide  for  carrying  off  the  extra  heat  arising  from  the  physio- 
logical work  of  the  regulative  mechanism  itself.  In  other  words, 
instead  of  a  critical  temperature,  we  get  the  conception  of  a  critical 
thermal  environment,  which  may  be  reached  under  a  variety  of 
conditions,  and  below  which  we  have  the  domain  of  "  chemical " 
regulation,  while  above  it  is  the  region  of  "  physical "  regulation. 

Influence  of  Size  of  Animal  on  Heat  Production. — The  total 
metabolism  of  a  large  animal  is  necessarily  greater  than  that  of  a 
small  one  of  the  same  species,  but  it  is  not  proportional  to  the 
weight,  being  relatively  greater  in  the  smaller  animal  under  com- 
parable conditions. 

Relation  of  Heat  Production  to  Surface. — Bergmann  * 
appears  to  have  been  the  first  to  connect  the  fact  just  stated  with 
the  relatively  greater  surface  of  the  smaller  animal,  but  we  are  in- 
debted to  Rubner  f  for  the  first  quantitative  investigation  of  this 
phase  of  the  subject.  His  experiments  were  made  on  six  dogs 
whose  weights  varied  from  3  to  24  kilograms  each.  The  total 
metabolism  (proteids  and  fat)  of  each  of  these  animals  in  the  fasting 
state  was  determined  in  from  two  to  thirteen  experiments,  and 
from  their  results  the  average  heat  production  of  each  animal  was 
computed.  The  table  on  page  360  %  shows  the  air  temperature  and 
the  computed  heat  production  per  kilogram  live  weight  in  each 
experiment,  and  also  the  same  corrected  to  the  uniform  tempera- 
ture of  15°  C.  This  correction  is  made  on  the  basis  of  Theodor's 
experiments  (see  p.  351),  according  to  which  a  difference  of  1° 
Centigrade  caused  the  amount  of  oxygen  taken  up  by  the  cat  to 
vary  1.11  per  cent.  The  first  series  consists  of  a  selection  from 
Pettenkofer  &  Voit's  experiments. 

Whether  we  consider  the  observed  or  the  corrected  heat  pro- 
duction we  find  that  with  the  single  exception  of  the  corrected 
result  for  No.  VI  the  amount  per  unit  of  live  weight  increases  as  the 
weight  itself  decreases. 

*  Cited  by  Rubner. 

t  Zeit.  f.  Biol  ,  19,  535. 

J  The  figures  of  the  table  are  computed  from  those  given  by  Rubner  in 
loc.  cit.,  p  540,  and  differ  in  some  cases  from  the  summary  given  in  loc.  cit., 
p  542. 


360 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


No.  oi 
Ani- 
mal. 

Date. 

Live 

Weight, 

Kgs. 

Air  Tem- 
perature, 
Deg.  C. 

Heat  Production 
per  Kg. 

Observed, 
Cals. 

Corrected 
to  15°, 
Cals. 

I  . 

Pettenkofer  &  Voit's    J 
experiments           "j 

30.96 
29.87 
31.44 
30.38 

17.1 
17.7 
16.2 
13.9 

38.99 
31.82 
37.39 
36.54 

39.90 
32.77 
37.89 
36.09 

30.66 
24.11 
23.75 
23.27 

16.2 
15.0 
15.0 
15.0 

36.18 
41.40 
40.22 
41.10 

36  66 

June  19,  1883 

41  40 

"      21,      "    

40  22 

TT  \ 

"      23,      "    

41   10 

1 

1 

23.71 
19.80 
19.01 
18.79 

15.0 
16.9 
14.5 
16.0 

40.91 
47.95 
45.71 
42.79 

40  91 

Feb.  24,  1882 

48  91 

1 

"     28,     "    

45  48 

TTT  J 

Mch.    1,     "    

43.22 

1 

1 

19.20 
18.20 
17.20 

15.8 
13.9 
16.6 

45.48 
50.72 
41.54 

45.87 

f 

Jan.  12,  1880 

50.11 

IV 

"      14,      "    

42.29 

I 

17.70 
9.05 
8.83 
8.68 
8.53 
11.11 
10.87 

15.3 
19.2 
20.9 
20.2 
21.0 
18.4 
20.0 

46.13 
66.32 
60.28 
64.88 
60.66 
61.16 
57.86 

46.20 

r 

Dec.  21,  1881 

69.10 

"     22,     "    

64.19 

"     23,     "    

68.58 

"     24,     "... 

64.66 

V  \ 

May     2,  1882 

63.42 

"       3,     "    

61.04 

Average 

9.51 
6.84 
6.36 
6.14 
6.83 
6.69 
6.56 
6.40 
6.66 
6.50 
6.36 
6.21 
6.15 
5.98 

19.95 

15.8 
23.6 
20.7 
18.2 
18.0 
15.0 
16.5 
14.6 
16.4 
16.3 
15.9 
18.4 
19.2 

61.86 

65.01 
63.65 
58.13 
71.07 
76.85 
71.60 
75.03 
61.55 
54.91 
53.64 
52.57 
61.06 
54.24 

65.16 

r 

Dec.    5,1881 

65.77 

"       6,     "    

69.70 

9,     "    

61.79 

Feb.     1,1882 

73.56 

2,     "    

79.39 

"       3.     "    

71.60 

"       4,     "    

76.23 

VI  J 

Jan.  27,  1883 

61.11 

"     28.     "    

55.73 

"     29,     "    

54.39 

Feb.     2,     "    

53.09 

"      10,     "    

63  22 

"      11,     "    

56.73 

6.44 
3.34 
3.05 
2.91 

17.6 
15.0 
12.7 
20.6 

63.02 

84.45 
97.86 
80.00 

64.79 

f 

Jan    30,  1880 

84.45 

Feb.     1,     "    

95  41 

VII  \ 

3,     "    

84.88 

1 

I 

3.10 

16.1 

87.44 

88.25 

INTERNAL    IVORK. 
SUMMARY. 


36l 


Average  Live 

Weight, 

Kgs. 

Heat  Production  per  Kg. 

Relative  Heat 

No.  of 
Animal. 

Observed, 
Cals. 

Corrected 
to  15°, 
Cals. 

Production 

(Corrected), 

Cals. 

I 

30.66 
23.71 
19.20 
17.70 
9.51 
6.44 
3.10 

36.18 
40.91 
45.48 
46.13 
61.86 
63.02 
87.44 

36.66 
40.91 
45.87 
46.20 
65.16 
64.79 
88.25 

100 

II 

112 

Ill 

125 

IV 

126 

V 

178 

VI 

177 

VII 

241 

Rubner  also  determined  approximately  the  surface  exposed  by 
his  animals,  in  part  by  direct  measurement  and  in  part  by  calcu- 
lation, and  computed  the  heat  production  per  square  meter  of  sur- 
face, with  the  following  results : 


No.  of  Animal. 

Surface, 
Sq.  Cm. 

Heat 
Production  per 
Square  Meter, 

Cals. 

I 

10750 
8805 
7500 
7662 
5286 
3724 
2423 

1046 
1112 
1207 
1097 
1183 
1120 
1214 

II 

Ill 

IV 

V 

VI 

VII 

He  also  cites  *  the  results  of  experiments  by  Senator  on  the  heat 
production  of  fasting  dogs,  and  a  respiration  experiment  by  Reg- 
nault  &  Reiset,  as  follows: 


Live  Weight, 
Kgs. 

Calculated 
Surface, 
Sq.  Cm. 

1 

Heat  Production. 

No. 

Per  Kg. 

Live  Weight, 
Cals. 

Per  Square 
Meter  of  Sur- 
face, Cals. 

VIII 

10.80 
7.52 
6.09 
5.68 
5.40 
4.24 
5.59 

5423 
4285 
3722 
3534 
3462 
2924 
3508 

52.31 
53.76 
63.04 
68.40 
74.16 
69.12 
72.82 

1035 

IX 

944 

X 

1031 

XI 

1101 

XII 

1157 

XIII 

1003 

XIV 

1154 

*  Loc.  cit.,  p.  551. 


362 


PRINCIPLES   OP  ANIMAL    NUTRITION. 


With  one  exception,  the  results  per  square  meter  agree  very  well 
with  those  of  Rubner,  both  absolutely  and  relatively. 

Rubner  has  also  shown  in  later  experiments  *  that  the  same 
thing  is  substantially  true  of  guinea-pigs,  both  at  zero  and  at  the 
temperature  of  about  30  degrees,  at  which  the  heat  production  is  at 
its  mimimum  (critical  temperature).  He  likewise  points  outf 
that  the  well-known  rapid  metabolism  of  children  as  compared 
with  adults  is,  so  far  as  the  available  data  show,  quite  closely  pro- 
portional to  their  relative  surface,  and  observations  on  the  diet  of 
a  dwarf  J  gave  a  like  result. 

Richet,§  working  with  an  air-calorimeter  of  constant  pressure, 
in  which  the  heat  production  was  measured  by  the  amount  of 
water  displaced  by  the  expansion  of  the  air,  obtained  the  following 
results  on  rabbits,  and  similar  results  upon  guinea-pigs  are  also 
reported : 


Number  of 
Experiments. 

Live  Weight. 
Kgs. 

Heat 

per  Kg., 

Cals. 

Total  Heat 
Expressed  in 
c.c.  of  Water 

Displaced. 

The  Same 

per  Unit  of 

Surface. 

5 

2.0-2.2 
2.2-2.4 
2.4-2.6 
2.6-2.8 
2.8-3.0 
3.0-3.2 

4.730 
3.985 
3.820 
3.650 
3.570 
3.320 

119 
110 
115 
119 
125 
130 

10 

12 

4 

130 
129 
127 

6 

128 

7 

127 

It  would  appear  from  the  description  of  the  experiments  that 
only  the  heat  given  off  by  radiation  and  conduction  was  measured, 
no  specific  statements  being  made  as  to  ventilation  or  as  to  the  loss 
of  heat  as  latent  heat  of  water- vapor.  The  experiments  were  also  of 
short  duration,  ranging  from  sixty  to  ninety  minutes. 

The  same  author  in  later  experiments  |  determined  the  respi- 
ratory exchange^  of  rabbits  of  different  weights.     Computing  the 

*  Biologische  Gesetze,  pp.  17-18. 

t  Zeit.  f.  Biol.,  21,  390. 

%  Biologische  Gesetze,  p.  9. 

§  Archives  de  Physiol ,  1885,  II,  237. 

\\Ibid.,  1890,  pp.  17  and  483;  1891,  p.  74;  Comptes  rend  ,  109,  190. 

^f  By  means  of  an  apparatus  described  briefly  in  Comptes  rend.,  104,  435 


INTERNAL    WORK. 


363 


results  per  square  centimeter  of  surface  by  the  use  of  Meeh's  for- 
mula (p.  364)  he  obtained  the  following  figures,  while  similar 
results  are  also  reported  on  guinea-pigs,  rats,  and  birds. 


Number  of 
Experiments. 

Average  Live 
Weight,  Kgs. 

Carbon  Dioxide 

per  Square  Cm. 

of  Surface, 

Mgrs. 

4 

24.0 
13.5 
11.5 
9.0 
6.5 
5.0 
3.1 
2.35 

2.65 
2.60 
2.81 
2.81 
2.69 
2.57 
2.71 
2.70 

5 

7 

4 

3 

3 

6 

4 

E.  Voit  *  has  recently  published  an  extended  compilation  of 
results  bearing  upon  this  point,  including  experiments  on  man,  dogs, 
rabbits,  swine,  geese,  and  hens,  the  heat  production  being  in  most 
cases  computed  from  the  metabolism  of  carbon  and  nitrogen.  The 
results  when  computed  per  square  meter  of  surface,  while  they 
show  not  inconsiderable  variations  in  some  individual  cases,  never- 
theless as  a  whole  substantially  confirm  the  conclusion  that  the 
fasting  metabolism  is  in  general  proportional  to  the  surface.  Still 
more  recently  Oppenheimer  f  has  shown  that  the  law  also  holds 
good  for  infants. 

Causes  of  Variations. — In  comparing  experiments  made  upon 
different  animals  by  different  observers  at  different  times  some 
variation  in  the  results  would  naturally  be  expected.  The  experi- 
ments compiled  by  Voit  were  not  all  made  at  the  same  temperature, 
but  the  range  in  most  cases  is  relatively  small  and  can  hardly  have 
exerted  any  considerable  influence.  Differences  between  the  differ- 
ent animals  as  to  their  normal  rate  of  emission  of  heat  (thickness  of 
coat,  quality  of  skin)  may  perhaps  have  also  had  an  effect,  although 
probably  a  small  one. 

A  more  important  source  of  error  seems  to  lie,  as  Voit  points 
out,  in  the  computation  of  the  results  to  unit  surface,  what  is 
actually  measured,  of  course,  being  the  total  heat  production  of 
the  animal.     In  solids   which  are  of  the  same  shape,  that  is,  which 


*Zeit.  f.  Biol.,  41,  113. 


t  Ibid.,  42,  147. 


364  PRINCIPLES   OF  ANIMAL  NUTRITION. 

are  geometrically  similar  figures,  the  surface  is  proportional  to 
the  two-thirds  power  of  the  volume.  If  we  let  S  =  surface  and 
V=  volume,  then  S  =  kV$,  in  which  k  is  a  constant  for  any  given 
form.  Putting  W  =  weight,  if  the  bodies  have  the  same  specific 
gravity  we  may  substitute  W  for  V  in  the  above  equation,  and  we 
then  have 

S-kWt,     *-,*. 

On  the  assumption  that  the  bodies  of  animals  of  the  same  species 
constitute  similar  figures  and  have  the  same  specific  gravity,  the 
value  of  k  has  been  determined  for  several  species,  as  follows  (the 
weight  being  expressed  in  kilograms  and  the  surface  in  square 
centimeters) : 

Man 12.9  Meeh  (Zeit.  f.  Biol.,  15,  425). 

Dog 11.2  Rubner  (Ibid.,  19,  548). 

Rabbit 12 .9  Rubner  (Ibid.,  19,  553). 

Horse 9.02  Hecker  (Zeit.  f.  Veterinark.,  1894). 

Hen 10 .45  Rubner  (Zeit.  f.  Biol.,  19,  553). 

Guinea-pig....     8.89  Rubner  (Biol.  Gesetze,  p.  17). 

Rat 9.13) 

Fro  4  62  \  Rubner  (Zeit.  f.  Biol.,  19,  553). 

The  heat  production  per  unit  of  surface  in  most  of  the  foregoing 
experiments  is  computed  by  the  use  of  these  factors.  The  results 
of  such  computations,  however,  are  necessarily  approximations 
only.  While  animals  of  the  same  species  are  of  the  same  general 
shape,  we  can  by  no  means  regard  them  as  being  exactly  similar 
figures  in  the  geometrical  sense,  nor  can  we  safely  assume  them  to 
be  of  exactly  the  same  specific  gravity,  since  changes  in  the 
amount  of  contents  of  stomach  and  intestines,  and  particularly  in 
the  quantity  of  fat  in  the  body,  would  cause  greater  or  less  variations. 
Moreover,  the  state  of  fatness  has,  as  Voit  points  out,  still  another 
effect.  As  an  animal  grows  fat,  the  increase  in  size  is  mainly 
transverse  and  not  longitudinal,  the  effect  being  like  that  of  in- 
creasing the  diameter  of  a  cylinder  of  fixed  length.*  In  such  a 
case,  however,  the  increase  in  the  surface  is  not  proportional  to  the 
two-thirds  power  of  the  volume,  nor  to  the  square  root  of  the  vol- 

*  In  the  case  of  an  animal,  of  course,  we  have  the  additional  fact  that  the 
deposit  of  fat  is  not  of  uniform  thickness  over  the  whole  surface  of  the  body 


INTERNAL   WORK.  365 

ume,  as  Voit  states.  The  curved  surface  of  the  cylinder  will  be 
proportional  to  the  square  root  of  its  volume,  while  the  surface  of 
the  two  ends  will  be  proportional  to  the  volume,  and  the  ratio  of 
total  surface  to  volume  will  depend  upon  the  ratio  of  length  to 
diameter,  being  greater  as  the  latter  becomes  less. 

Obviously,  the  calculation  of  the  surface  of  an  animal  from  its 
weight  is  a  more  or  less  uncertain  one,  and  it  is  not  surprising  that 
the  results  should  be  somewhat  fluctuating.  It  seems  very  doubt- 
ful, however,  whether  the  larger  differences  found  in  Voit's  com- 
pilation can  be  explained  in  this  way,  and  Voit  shows  that  there 
is  another  factor  to  be  considered,  viz.,  the  mass  of  active  cells  in 
the  body,  which  has  a  material  bearing  on  the  results.  Before 
proceeding  to  a  discussion  of  this  point,  however,  it  is  desirable  to 
consider  briefly  the  significance  of  the  general  fact  of  the  close 
relation  between  heat  production  and  surface. 

Significance  of  Results. — Let  us  imagine  an  animal  exposed  to 
its  "critical  thermal  environment"  (p.  358)  to  gradually  shrink  in 
size  while  the  external  conditions  remain  the  same.  Under  such 
circumstances  the  loss  of  heat  to  its  surroundings  will  tend  to  in- 
crease relatively  to  its  mass — that  is,  the  body,  like  an  inanimate 
object,  will  tend  to  cool  more  rapidly:  This  tendency  can  be  met 
and  the  body  temperature  maintained  in  only  two  ways,  viz.,  either 
by  some  modification  of  its  surface — e.g.,  thicker  hair — which  will 
lower  what  we  may  call  its  emission  constant,  or  by  a  relative  in- 
crease in  its  rate  of  heat  production. 

The  results  which  we  have  been  considering  show  that  in 
general  the  emission  constant,  i.e.  the  rate  of  heat  emission  per 
unit  of  surface,  is  substantially  the  same  in  small  and  large  animals, 
and  that  the  greater  loss  of  heat  in  the  former  case  is  met  by  an 
increased  production.  In  this  aspect  the  effect  is  simply  an  ex- 
tension of  the  influence  of  falling  temperature,  the  increased  de- 
mand for  heat  being  met  by  an  increased  supply,  so  that  the  extent 
of  surface  appears  as  the  determining  factor  of  the  amount  of  met- 
abolism. 

In  the  case  of  an  animal  exposed  to  a  temperature  below  the 
critical  point,  however,  the  increased  demand  for  heat  appears  to  be 
met  largely  by  a  stimulation  of  those  processes  of  metabolism  which 
do  not  result  in  any  visible  form  of  work,  while  the  internal  work, 


366 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


in  the  more  restricted  sense  of  the  ordinary  functions  of  the  internal 
organs,  does  not  seem  to  be  materially  affected.  Are  we  justified 
in  assuming  the  same  thing  to  be  true  in  our  imagined  shrinkage 
of  an  animal?  In  other  words,  is  the  work  of  the  internal  organs 
proportional  to  the  mass  of  the  body  and  is  the  increased  heat 
production  in  the  smaller  animal  due  to  the  same  cause  as  that 
observed  when  an  animal  is  exposed  to  a  falling  temperature? 

It  appears  quite  clear  that  this  question  must  be  answered  in 
the  negative.  It  is  a  well-known  fact  that  the  circulation,  respira- 
tion, and  other  functions  are  as  a  rule  more  active  in  small 
than  in  large  animals,  and  this  greater  activity  must  necessarily 
result  in  the  evolution  of  relatively  more  heat.  If  we  raise  the 
temperature  of  the  surroundings  to  a  point  corresponding  to  the 
critical  thermal  environment,  we  may,  as  we  have  seen,  regard  the 
heat  production  as  representing  the  internal  work  in  the  narrower 
sense.  Rubner  *  reports  experiments  of  this  sort  upon  four  guinea- 
pigs  at  0°  C.  and  at  30°  C,  which  gave  the  following  results  for  the 
production  of  carbon  dioxide : 


CO.,  per  Hour  at  0°  C. 

COj  per  Hour  at  30°  C. 

Animal, 
Grms. 

Per  Kg. 
Weight. 
Grms. 

Per  Square 

Meter  Surface, 

Grms. 

Per  Kg. 
Weight, 
Grms. 

Per  Square 

Meter  Surface, 

Grms. 

617 
568 
223 
206 

2.905 
3.249 
4.462 
4.738 

27.85 
30.30 
30.47 
31.56 

1.289 
1.129 
1.778 
1.961 

12.35 
10.53 
12.14 
13.16 

With  the  first  and  third  of  these  animals  direct  experiment 
showed  that  the  minimum  production  of  carbon  dioxide  (critical 
point)  was  reached  at  about  30°-35°,  and  we  may  fairly  assume 
this  to  be  true  of  the  other  two.  At  30°  C,  then,  we  may  assume 
that  the  "chemical"  regulation  was  practically  eliminated  and  that 
the  observed  metabolism  was  that  due  to  the  work  of  the  internal 
organs.  Under  these  conditions,  as  the  figures  show,  the  metab- 
olism was  still  approximately  proportional  to  the  surface  of  the 
animal,  and  consequently  greater  per  unit  of  weight  in  the  smaller 
than  in  the  larger  animals. 

*  Biologische  Gesetze,  pp.  12-18. 


INTERNAL    WORK.  367 

Strong  confirmation  of  this  conclusion  is  afforded  by  the  exper- 
iments previously  cited.  In  many  of  them,  notably  in  Rubner's, 
the  range  of  size  is  so  great  that  to  regard  the  differences  in  heat 
production  as  arising  from  a  direct  stimulation  of  the  metab- 
olism, as  in  the  case  of  a  fall  in  the  external  temperature,  leads 
to  improbable  consequences.  Thus  a  comparison  of  the  largest 
with  the  smallest  dog  in  Rubner's  experiments  (p.  361)  shows 
that  if  we  regard  the  heat  production  of  the  .former  as  represent- 
ing simply  the  work  of  the  internal  organs,  over  56  per  cent,  of  the 
heat  production  of  the  smaller  animal  must,  on  the  supposition 
that  the  internal  work  is  proportional  to  the  mass  of  the  body,  have 
arisen  from  some  other  source.  Such  an  enormous  increase  in  the 
metabolism  of  the  body  simply  for  the  sake  of  heat  production 
can  hardly  be  regarded  as  probable.  Still  further,  if  we  assume 
(compare  p.  354)  a  temperature  of  about  20°  C.  to  represent  the 
critical  point  for  the  dog,  then,  on  the  hypothesis  that  the  necessary 
internal  work  per  unit  of  weight  is  the  same,  we  find  that  a  fall  of  one 
degree  in  temperature  must  have  produced  about  six  times  the 
effect  upon  the  metabolism  of  the  smallest  dog  that  it  did  on  that 
of  the  largest  one,  while  if  we  take  the  other  alternative  and  seek  to 
explain  the  results  on  the  assumption  of  a  higher  critical  tempera- 
ture for  the  smallest  dog,  we  find  for  the  latter  about  36£°  C. 

Taking  these  considerations  along  with  the  results  of  Rubner's 
trials  with  the  four  guinea-pigs,  it  seems  most  reasonable  to  assume, 
in  default  of  more  extensive  investigations  directed  to  this  specific 
point,  that  the  critical  temperature  is  substantially  the  same  for 
large  and  small  animals  of  the  same  species  and  that  the  work  of 
the  internal  organs  is  approximately  proportional  to  the  surface 
of  the  animal. 

Substantially  the  same  conclusion  has  been  reached  by  v.  Hoss- 
lin  *  from  a  quite  different  point  of  view.  He  points  out  that  the 
increased  production  of  heat  below  the  critical  temperature  is  not 
proportional  to  the  difference  in  temperature  between  the  body  and 
its  surroundings,  as  it  should  be,  according  to  Newton's  law,  if  the 
emission  constant  of  the  surface  remained  the  same.  Taking  as  an 
example  Theodor's  experiments  (p.  351)  he  makes  the  following 
comparisons : 

*  Arch.  f.  (Anat.  u.)  Physiol.,  1888,  p.  323. 


368 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


External 
Temperature, 

Difference  Between  Body  and 
External  Temperature. 

Carbon  Dioxide  in  12  Hours. 

Degrees. 

Total, 
Degrees. 

Relative. 

Total, 
Grms. 

Relative. 

30.8 
20.1 
12.3 
0.2 
-5.5 

7.2 
17.9 
25.7 
37.8 
43.5 

1.0 

2.5 
3.6 
5.25 
6.0 

12.03 
14.34 
17.76 
18.24 
19.83 

1.00 
1.19 
1.48 
1.52 
1.65 

It  would  appear  from  these  figures  that  even  below  the  critical 
temperature  the  "physical"  regulation  plays  a  large  part  in  the 
regulation  of  the  body  temperature,  being  simply  supplemented 
by  the  "chemical"  regulation,  and  that  therefore  the  demand  for 
heat  has  not  the  determining  influence  upon  the  heat  production 
which  Rubner  supposes.  According  to  v.  Hosslin  the  apparent 
dependence  of  the  total  metabolism  upon  the  surface  is  only  a  par- 
ticular case  of  a  general  morphological  law  and  he  points  out : 

First,  that  since,  according  to  him,  the  velocity  of  the  circula- 
tion does  not  vary  greatly  in  large  and  small  animals,  the  average 
amount  of  blood  passing  through  the  organs,  and  consequently 
their  supply  of  oxygen,  will  be  proportional  to  the  total  cross- 
section  of  the  blood-vessels,  which  again,  similar  form  being 
assumed,  will  be  proportional  to  the  two-thirds  power  of  the 
volume  (or  weight)  of  the  body. 

Second,  that  the  capacity  of  the  alimentary  canal  to  digest  and 
resorb  food  and  thus  to  supply  material  for  metabolism  is  limited 
in  the  same  proportion. 

Third,  that  the  work  of  locomotion — substantially  the  only 
form  of  external  work  in  the  wild  state — at  a  given  speed  is  pro- 
portional to  the  two-thirds  power  of  the  weight. 

In  short,  v.  Hosslin  claims  that  all  the  important  physiological 
activities  of  the  body,  including,  of  course,  its  internal  work  and  the 
consequent  heat  production,  are  substantially  proportional  to  the 
two-thirds  power  of  its  volume,  and  that  since  the  external  surface 
bears  the  same  ratio  to  the  volume,  a  proportionality  necessarily 
exists  between  heat  production  and  surface.  According  to  this 
view,  then,  the  heat  production  of  the  fasting  animal  at  the  criti- 
cal temperature  represents  the  internal  work,  which  is  proportional 


INTERNAL    WORK. 


369 


to  the  two-thirds  power  of  the  volume  of  the  body,  while  below 
this  point  there  is  superadded  a  stimulating  effect  upon  the  heat 
production,  which,  since  it  acts  through  the  surface,  we  may 
assume  to  be  proportional  to  the  latter. 

Comparison  of  Species. — In  the  foregoing  discussion  compari- 
sons have  been  made  between  large  and  small  animals  of  the  same 
species,  with  the  result  that  both  their  internal  work  and  their 
total  fasting  metabolism  appear  to  be  closely  proportional  to  their 
surface.  Going  a  step  further  and  comparing  the  average  results 
of  the  several  species  with  each  other,  E.  Voit  *  reaches  the  inter- 
esting and  striking  result  that  the  same  relation  of  total  fasting 
metabolism  to  surface  is  substantially  true  as  between  different 
species.  The  following  table  contains  the  averages,  with  the  addi- 
tion of  the  fasting  metabolism  of  the  horse  as  computed  by  Zuntz 
&  Hagemann,  which  Voit  believes  with  good  reason  to  be  too 
low: 


Average  Tem- 
perature, 
Deg.  C. 

Average 
Weight.  Kgs. 

Fasting  Metabolism. 

Per  Kg., 

Cals. 

Per  Square 
Meter,  Cals. 

Horse 

9.1  (?) 
20.1 
14.3 
18.0 
18.2 
15.0 
18.5 

441 
128 
64.3 
15.2 
2.3 
3.5 
2.0 

11.3 
19.1 
32.1 
51.5 
75.1 
66.7 
71.0 

>948 

1078 
1042 

Man 

Dog 

1039 
776 

Rabbit 

Goose 

967 
943 

Hen  .... 

With  the  exception  of  the  rabbit,  the  average  heat  production 
of  these  various  animals  per  unit  of  surface  does  not  show  any 
greater  variations  than  have  been  observed  between  different 
animals  of  the  same  species,  more  or  less  of  which,  as  we  have  seen, 
can  probably  be  accounted  for  by  errors  in  the  estimate  of  the 
surface  of  the  body. 

Accepting  the  fact  of  the  general  proportionality  of  heat  pro- 
duction to  surface,  and  passing  over  for  the  moment  the  excep- 
tional case  of  the  rabbit,  it  is  plain  that  the  considerations  which 
have   been    adduced   in    discussing    the   results   upon   the   same 
*  Loc.  cit.,  p.  120. 


37°  PRINCIPLES   OF  ANIMAL   NUTRITION. 

species  will  in  the  main  apply  to  a  comparison  of  different  species. 
It  is  true  that  what  data  we  have  indicate  that  there  may  be  more 
or  less  difference  between  the  critical  temperatures  for  different 
species,  but  in  view  of  the  enormous  range  in  the  size  of  the  animals 
experimented  on  this  cannot  largely  modify  the  results.  Any 
reasonable  assumptions  as  to  critical  temperatures  and  as  to  rates 
of  variation  per  degree  in  heat  production  would  still  leave  the 
corrected  results  substantially  proportional  to  the  surface.  Appar- 
ently we  must  conclude  that  in  all  these  different  species,  as  well 
as  in  larger  and  smaller  animals  of  the  same  species,  the  internal 
work,  as  measured  by  the  total  metabolism  at  the  critical  tem- 
perature, is  substantially  proportional  to  the  surface. 

How  generally  this  may  be  true  we  have  at  present  no  means 
of  judging.  It  is  clear,  however,  that  in  the  process  of  organic 
evolution  one  of  the  very  important  factors  has  been  the  demand 
for  heat  exerted  by  the  environment  upon  the  animal.  This  has 
been  met  to  some  extent  by  modifications  in  the  coat  of  the  animal, 
but  to  a  very  large  degree  by  changes  in  the  rate  of  heat  produc- 
tion, with  the  result  that,  other  things  being  equal,  those  forms  have 
survived  whose  normal  heat  production,  resulting  from  internal 
work  alone,  was  sufficient  to  maintain  their  temperature  under  the 
average  conditions  surrounding  them  without,  on  the  one  hand, 
calling  largely  into  play  the  processes  of  "chemical"  regulation, 
nor,  on  the  other  hand,  producing  so  much  heat  as  to  render  it 
difficult  for  the  body  to  get  rid  of  it. 

Relation  of  Heat  Production  to  Mass  of  Tissue. — As 
already  indicated,  E.  Voit.  in  his  article  cited  above,  has  shown 
that  while  the  heat  production  is  in  general  proportional  to  the  sur- 
face, there  is  also  another  determining  factor,  viz.,  the  mass  of  the 
active  cells  in  the  organism,  a  rough  measure  of  which  is  the  total 
nitrogen  of  the  body  exclusive  of  that  of  the  bones  and  the  skin. 
This  conclusion  is  based  chiefly  on  experiments  with  fasting  animals. 
As  the  weight  of  such  an  animal  decreases,  its  relative  surface  must 
increase,  and,  as  was  shown  on  p.  364,  probably  more  rapidly  than 
in  proportion  to  the  two-thirds  power  of  the  weight.  Under  these 
circumstances  we  should  naturally  expect  that  the  relative  heat 
production  would  increase,  but  as  a  matter  of  fact  it  rather  shows 
a  tendency  to  decrease.     E.  Voit,  in  discussing  the  results  of  Rubner 


INTERNAL    WORK. 


371 


and  others,  computes  the  heat  production  per  unit  of  surface,  and 
also  compares  it  with  the  amount  of  nitrogen  computed  to  be 
present  in  the  organs  of  the  animal  on  the  several  days  of  the  ex- 
periment. The  following  results  of  one  of  Rubner's  experiments 
with  rabbits  are  typical  of  those  obtained  in  this  way : 


Average 

Live 
Weight, 
Grms. 

Heat  Production  per  Day. 

Day  of  Fasting. 

Total, 
Cals. 

Per  Kg.. 

Cals. 

Per 

Square 

Meter  of 

Surface, 

Cals. 

Per  100 

Nitrogen, 
Cals. 

Third 

2185 
2093 
2007 
1923 
1841 
1735 
1646 
1507 

155 
117 
102 
97 
95 
88 
81 
72 

71.0 
55.9 
50.8 
50.5 
51.6 
50.7 
49.2 
47.8 

730 
556 
499 
488 
494 
463 
452 
428 

310 

Fifth 

243 

Seventh  

220 

Ninth 

221 

Tenth  and  twelfth 

227 

Thirteenth  and  fourteenth  . . 
Fifteenth  and  sixteenth  .... 
Seventeenth  and  eighteenth 

222 
218 
219 

The  heat  production  per  unit  of  surface  is  seen  to  decrease  at 
first  rapidly  and  later  more  slowly,  while  the  heat  production  per 
unit  of  weight  shows  but  a  slight  decrease  and  that  per  unit  of 
nitrogen  scarcely  any.  From  these  and  other  similar  results,  Yoit 
concludes  that  the  law  of  the  proportionality  of  heat  production  to 
surface  as  enunciated  by  Rubner  and  as  extended  by  himself  must 
be  limited  in  its  application  to  animals  in  like  bodily  condition, 
and  that  an  animal  with  a  low  stock  of  nitrogenous  tissue  will, 
under  the  same  conditions,  show  a  lower  heat  production  per  unit 
of  surface  than  a  well-nourished  animal.  The  exceptionally  low 
average  for  the  rabbit  noted  on  p.  369  he  explains  on  this  hypoth- 
esis as  resulting  from  the  frequent  use  for  such  experiments  of 
animals  in  a  poorly  nourished  and  "degenerate"  condition  re- 
sulting from  long  confinement. 

The  result  has  an  interesting  bearing  in  another  direction. 
Most  of  the  experiments  cited  by  Voit  were  probably  made  at  tem- 
peratures below  the  critical  points  for  the  several  animals.  In 
our  previous  discussion  we  have  assumed  that  under  these  circum- 
stances the  heat  regulation  is  accomplished  largely  by  "  chemical " 
means — by  variations  in  the  rate  of  production.     In  these  experi- 


SI- 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


ments,  on  the  contrary,  since  the  heat  production  decreased  along 
with  the  decrease  of  nitrogenous  tissue,  we  see  the  regulation  of 
body  temperature  effected  by  a  diminution  in  the  rate  of  emission 
of  heat,  which,  however,  was  in  most  cases  less  marked  than  in  the 
instance  just  cited.  Either  we  must  conclude  that  the  abnormal 
condition  arising  from  fasting  enables  the  animal  to  diminish  the 
rate  of  emission  of  heat  to  an  extent  not  possible  to  the  well- 
nourished  one,  or  we  may  suppose  that  in  the  latter  case  the  stimu- 
lation of  the  metabolism  by  the  abstraction  of  heat  begins  before 
the  possibilities  of  "physical"  regulation  have  been  exhausted; 
that,  in  other  words,  the  domains  of  "chemical"  and  "physical" 
regulation  overlap.  Obviously  the  latter  conclusion  is  entirely  in 
harmony  with  v.  Hosslin's  views  as  stated  on  pp.  367-8. 


§  3.  The  Expenditure  of  Energy  in  Digestion  and  Assimilation. 

General  Conception. 

Food  Increases  Metabolism. — That  the  consumption  of  food 
increases  the  metabolism  and  consequent  heat  production  in  the 
body  has  been  known  since  the  time  of  Lavoisier,  who  observed 
the  oxygen  consumption  of  man  to  increase  materially  (about  37 
per  cent.)  after  a  meal.  Regnault  &  Reiset  *  also,  among  their 
respiration  experiments  on  animals,  report  the  following  results 
for  the  oxygen  consumption  of  two  rabbits  while  fasting  and  after 
eating : 


Animal. 

Fasting, 
Grms. 

After  Eating, 
Grms. 

A 

2.518 
2.731 

3.124 
3.590 

B 

Subsequent  investigations  by  Vierodt,  Smith,  Speck,  Fredericq, 
v.  Mehring  &  Zuntz,  Wolfers,  Potthast,  Hanriot  &  Richet,f  Magnus- 
Levy,  Zuntz  &  Hagemann,  Laulanie,  and  others,  some  of  which  will 
be  considered  more  specifically  in  subsequent  paragraphs,  have  fully 
confirmed  these  earlier  results,  so  that  the  fact  of  an  increased  met- 
abolism consequent  upon  the  ingestion  of  food  is  undisputed. 

*  Ann.  de  Chim.  et  de  Phys.  (3),  26,  414. 

t  Ibid.  (6),  22,  520. 


INTERNAL    WORK.  373 

Cause  of  the  Increase. — Two  possible  explanations  of  the 
above  fact  naturally  suggest  themselves,  viz.,  that,  on  the  one  hand, 
the  more  abundant  supply  of  food  material  to  the  cells  of  the 
body  may  act  as  a  direct  stimulus  to  the  metabolic  processes,  or, 
on  the  other  hand,  that  the  increased  metabolism  may  arise  from 
the  greater  activity  of  the  organs  of  digestion,  or  finally,  that  both 
causes  may  act  simultaneously. 

The  results  obtained  by  Speck,*  who  found  that  the  increase 
began  very  promptly  (within  thirty  minutes)  after  a  meal,  would 
indicate  that  it  can  hardly  be  due  to  a  stimulating  action  of  the 
resorbed  food  upon  the  general  metabolism,  but  must  arise,  in 
large  part  at  least,  from  the  activity  of  the  digestive  organs. 
Specific  investigations  upon  this  point  were  undertaken  by  Zuntz  & 
v.  Mehring.f  They  found  that  glycerin,  sugar,  egg-albumin,  puri- 
fied peptones,  and  the  sodium  salts  of  lactic  and  butyric  acids  J 
when  injected  into  the  circulation  caused  no  material  increase  in 
the  amount  of  oxygen  consumed  as  determined  in  successive  short 
periods  by  the  Zuntz  form  of  respiration  apparatus.  It  is  well  estab- 
lished that  some  of  these  substances  do  increase  the  metabolism 
when  given  by  the  mouth,  and  the  authors  verified  this  fact  for  sugar 
and  for  sodium  lactate  and  likewise  showed  that  substances  like 
sodium  sulphate,  which  are  not  metabolized  in  the  body,  caused  a 
similar  rise  in  the  metabolism  when  introduced  into  the  digestive 
tract.  They  therefore  conclude  that  the  effect  of  the  ingestion  of 
food  upon  the  metabolism  is  due  chiefly  to  the  expenditure  of  energy 
required  in  its  digestion.  Wolfers  §  and  Potthast,||  in  experiments  sup- 
plementary to  those  just  mentioned,  obtained  confirmatory  results. 

On  the  other  hand,  Laulanie,!"  in  the  experiments  mentioned 
on  p.  180  in  their  bearings  upon  the  formation  of  fat  from  carbo- 
hydrates, obtained  almost  as  marked  an  increase  in  the  oxygen 
consumption  subsequent  to  the  injection  of  sugar  into  the  circula- 
tion as  after  its  administration  by  the  mouth. 

*  Arch,  exper.  Pathol,  and  Pharm.,  II,  1874,  p.  405. 
t  Arch.  ges.  Physiol.,  15,  634;  32,  173. 

%  The  results  of  their  experiments  upon  organic  acids  have  already  been 
cited  in  Chapter  V,  p.  157,  in  another  connection. 
§  Arch.  ges.  Physiol.,  32,  222. 
||  Ibid.,  32,  280. 
^f  Archives  de  Physiol.,  1896,  p.  791. 


374  PRINCIPLES   OF  ANIMAL   NUTRITION. 

On  the  whole,  however,  and  in  view  of  the  patent  fact  that  the 
activity  of  the  digestive  apparatus  consequent  upon  the  consump- 
tion of  food  must  lead  to  an  expenditure  of  energy,  the  results  of 
Zuntz  &  v.  Mehring  appear  to  have  been  generally  accepted  as  proof 
that  it  is  this  influence  rather  than  any  direct  effect  of  the  resorbed 
food  upon  the  metabolism  to  which  the  increase  of  the  latter  after 
a  meal  is  to  be  ascribed.  This  increased  expenditure  is  often, 
although  rather  loosely,  spoken  of  as  the  "work  of  digestion." 

Factors  of  Work  of  Digestion. — In  the  process  of  digestion 
we  are  probably  safe  in  assuming  that  the  muscular  work  of  pre- 
hension, mastication,  deglutition,  rumination,  peristalsis,  etc.,  con- 
stitutes an  important  source  of  heat  production.  A  secondary 
source  of  heat  production,  which  we  may  designate  as  glandular 
metabolism,  is  the  activity  of  the  various  secretory  glands  which 
provide  the  digestive  juices,  to  which  may  be  added  also  the  work 
of  the  resorptive  mechanisms.  Furthermore,  the  various  processes 
of  solution,  hydration,  cleavage,  etc.,  which  the  nutrients  undergo 
during  digestion  contribute  their  share  to  the  general  thermic  effect. 

Fermentations. — To  the  above  general  sources  of  heat  produc- 
tion during  the  digestive  process,  there  is  to  be  added  as  a  very 
important  one  in  the  case  of  ruminating  animals  the  extensive  fer- 
mentation which  the  carbohydrates  of  the  food  undergo.  We  have 
already  seen  that  a  considerable  fraction  of  the  gross  energy  of  these 
bodies  is  carried  off  in  the  potential  form  in  the  combustible  gases 
produced.  A  further  portion  is  liberated  as  heat  of  fermentation. 
This  latter  portion  forrqs  a  part  of  the  metabolizable  energy  of  the 
food  as  defined  in  the  preceding  chapter,  since  it  assumes  the  kinetic 
form  in  the  body.  Since,  however,  it  appears  immediately  as  heat, 
it  can  be  of  use  to  the  body  only  indirectly,  as  an  aid  in  maintaining 
its  temperature.  While,  therefore,  it  does  not  constitute  work  in 
the  strict  sense  of  the  term,  the  heat  produced  by  fermentation 
constitutes  a  part  of  the  expenditure  of  metabolizable  energy  in 
digestion,  and  therefore  is  included  under  the  term  "  work  of  diges- 
tion "  in  the  general  sense  in  which  the  term  is  frequently  used. 

Warming  Ingesta. — The  food,  and  particularly  the  water,  con- 
sumed by  an  animal  have  to  be  warmed  to  the  temperature  of  the 
body.  To  the  extent  that  this  warming  of  the  ingesta  is  accom- 
plished at  the  expense  of  the  heat  generated  by  the  muscular,  gland- 


INTERNAL    WORK.  375 

ular,  and  fermentative  actions  indicated  above,  it  does  not  call  for 
any  additional  expenditure  of  energy  and  so  does  not,  from  the 
statistical  point  of  view,  constitute  part  of  the  "work  of  digestion." 
If,  however,  at  any  time  the  warming  of  the  ingesta  requires  more 
heat  than  is  produced  by  these  processes — if,  for  example,  a  large 
amount  of  very  cold  water  is  consumed — it  is  evident  that  the 
surplus  energy  required  will  be  withdrawn  from  the  stock  otherwise 
available  for  other  purposes,  and  to  this  extent  will  increase  the 
expenditure  of  energy  consequent  upon  digestion. 

The  Expenditure  of  Energy  in  Assimilation. — While  our 
knowledge  of  the  changes  which  the  nutrients  undergo  after  re- 
sorption is  very  meager,  we  may  regard  it  as  highly  probable  that 
they  undergo  important  transformations  before  they  are  fitted  to 
serve  directly  as  sources  of  energy  for  those  general  vital  activities 
of  the  body  represented  in  gross  by  the  fasting  metabolism. 

Thus  the  proteoses  and  peptones  produced  in  the  course  of 
digestive  proteolysis  are  synthesized  again  to  proteids,  while  the 
proteids,  when  the  supply  is  large,  undergo,  as  was  shown  in  Chap- 
ter V,  rapid  nitrogen  cleavage,  leaving  a  non-nitrogenous  residue 
as  a  source  of  energy.  According  to  some  authorities,  as  we  have 
seen,  the  resorbed  fat  undergoes  conversion  into  dextrose  in  the 
liver  before  entering  into  the  general  metabolism  of  the  body. 
Even  the  carbohydrates,  at  least  so  far  as  they  are  not  directly 
resorbed  as  dextrose,  seem  to  undergo  more  or  less  transformation 
before  entering  into  the  general  circulation. 

In  brief,  there  seems  good  reason  to  believe  that  the  crude  mate- 
rials resulting  from  the  digestion  of  the  food  undergo  more  or  less 
extensive  chemical  transformations  before  they  are  ready  to  serve 
as  what  Chauveau  calls  the  "potential"  of  the  body — that  is,  as 
the  immediate  source  of  energy  for  the  vital  functions.  Of  the 
nature  and  extent  of  these  transformations  we  are  largely  ignorant. 
So  far  as  they  are  katabolic  in  their  nature,  a  liberation  of  energy  is 
necessarily  involved.  Any  anabolic  processes  of  course  would 
absorb  energy,  but  the  energy  so  absorbed  must  come  ultimately 
from  the  katabolism  of  other  matter,  and  in  all  probability  there 
would  be  more  or  less  escape  of  kinetic  energy  in  the  process. 

Moreover,  as  was  pointed  out  in  the  opening  paragraphs  of 
Chapter  II  in  discussing  the  general  nature  of  metabolism,  as  well 


376  PRINCIPLES   OF  ANIMAL   NUTRITION. 

as  in  the  Introduction,  the  vital  activities  are  intimately  connected 
with  the  katabolic  processes  going  on  in  the  protoplasm  of  the 
cells.  As  was  there  stated,  it  is  highly  probable  that  the  molecules 
of  the  protoplasm  are  much  more  complex  than  those  of  the  pro- 
teids,  fat  and  carbohydrates  of  the  food  (compare  pp.  17  and  224)# 
To  what  extent  it  is  necessary  that  the  resorbed  nutrients  shall  be 
synthesized  to  these  more  complex  compounds  before  they  can 
serve  the  purposes  of  the  organism  we  are  hardly  in  position  to 
say,  but  so  far  as  it  is  required  it  can  be  accomplished  only  by  an 
expenditure  of  energy  derived  ultimately  from  the  food  and  con- 
stituting a  part,  and  not  impossibly  a  large  part,  of  the  work  of 
assimilation. 

Summary.  —  The  considerations  of  the  foregoing  paragraphs 
make  it  plain  that  the  exercise  of  the  function  of  nutrition,  as  is  the 
case  with  the  other  functions  of  the  body,  involves  the  expenditure 
of  energy.  In  general,  we  may  say  that  this  energy  is  expended  for 
the  two  purposes  indicated  in  the  title  of  this  section,  viz.,  for  diges- 
tion, or  the  transformation  of  the  crude  materials  of  the  food  and 
their  transference  to  the  fluids  of  the  body,  and  for  assimilation,  or 
the  conversion  of  these  resorbed  materials  into  the  "  potential "  of 
the  organism.  Each  of  these  two  general  purposes  is  served  by  a  va- 
riety of  processes,  and  the  attempt  to  assign  to  each  its  exact  share 
in  the  increased  metabolism  brought  about  by  the  ingestion  of  food 
is  a  physiological  problem  at  once  interesting  and  complicated. 

For  our  present  purpose,  however,  viz.,  a  consideration  from  the 
statistical  point  of  view  of  the  income  and  expenditure  of  energy 
by  the  organism,  we  are  concerned  primarily  with  the  total  ex- 
penditure caused  by  the  ingestion  of  food  rather  than  with  the 
single  factors  composing  it.  As  a  matter  of  convenience  it  may  be 
permissible  to  retain  the  designation  above  given,  viz.,  the  work  of 
digestion  and  assimilation,  but  it  should  not  be  forgotten  that  other 
processes  may  conceivably  be  concerned  in  the  matter.  In  par- 
ticular, any  increased  heat  production  resulting  from  a  direct  stimu- 
lation of  the  metabolic  processes  or  of  the  incidental  muscular 
activity  of  the  animal  by  the  resorbed  food,  such  for  example,  as 
Zuntz  &  Hagemann  *  have  observed  with  the  horse  as  a  result  of 
abundant  feeding,  particularly  with  Indian  corn,  would  be  included 
under  the  term  as  here  used. 

*  Landw.  Jahrb.,  27,  Supp.  Ill,  234  and  259. 


INTERNAL    WORK.  377 

Experimental  Results. 

General  Methods. — It  follows  from  what  has  been  said  above 
that  two  general  methods,  or  more  properly  two  modifications  of 
one  general  method,  may  be  employed  to  determine  the  total  ex- 
penditure of  energy  due  to  the  ingestion  of  food. 

First,  since  the  energy  expended  in  the  various  processes  out- 
lined above  is  ultimately  converted  into  heat,  we  may  determine  the 
heat  production  of  the  animal  while  fasting  and  compare  with  it  the 
heat  production  during  the  digestion  and  assimilation  of  a  known 
amount  of  food.  The  excess  of  heat  produced  in  the  latter  case  as 
compared  with  the  former  will  represent  the  increased  expendi- 
ture of  energy  in  the  work  of  digestion  and  assimilation. 

Second,  we  may  determine  the  total  income  and  outgo  of  energy 
in  the  fasting  and  in  the  fed  animal  by  one  of  the  methods  indicated 
in  Chapter  VIII.  In  this  case  the  extent  to  which  the  net  loss  of 
energy  by  the  body  has  been  diminished  by  means  of  the  food  will 
show  how  much  of  the  metabolizable  energy  of  the  latter  has  been 
utilized  by  the  organism  in  place  of  that  previously  drawn  from  the 
metabolism  of  tissue.  The  part  of  the  metabolizable  energy  not 
thus  utilized  has  obviously  been  expended  in  some  of  the  various 
operations  of  digestion,  assimilation,  etc.  The  two  methods  are  com- 
plementary, in  the  one  case  the  expenditure  for  digestion,  assimila- 
tion, etc.,  being  determined  directly  and  in  the  other  by  difference. 

A  point  of  some  importance,  at  least  logically,  is  that  the  deter- 
minations should  be  made  below  the  point  of  maintenance.  The 
term  assimilation  as  above  defined  includes  all  those  processes  by 
which  the  resorbed  nutrients  are  prepared  for  their  final  metabo- 
lism in  the  performance  of  the  vital  functions.  When  we  give  food 
in  excess  of  the  maintenance  requirement,  however,  there  is  added 
to  this  the  set  of  processes  by  which  the  excess  food  is  converted 
into  suitable  forms  for  more  or  less  temporary  storage  in  the 
body.  These  may  be  presumed  to  consume  energy,  and  as  it  would 
seem,  to  a  more  or  less  variable  extent.  At  any  rate,  we  have 
no  right  to  assume  in  advance  that  the  relative  expenditure  of  en- 
ergy above  the  maintenance  point  in  the  storage  of  excess  material 
is  the  same  as  that  below  the  maintenance  point  for  the  processes 
of  assimilation  as  above  defined.  In  other  words,  it  is  not  necessa- 
rily nor  even,  it  would  seem,  probably  the  case  that  the  resorbed 


378  PRINCIPLES   OF  ANIMAL   NUTRITION. 

portion  of  a  maintenance  ration  is  first  converted  into  the  same 
materials  (particularly  fat)  that  are  deposited  in  the  body  when 
excess  food  is  given,  and  that  these  materials  are  then  metabolized 
in  the  performance  of  the  bodily  functions.  It  is  at  least  conceiv- 
able, if  not  likely,  that  a  much  less  profound  transformation,  and 
one  involving  a  smaller  loss  of  energy,  suffices  to  prepare  the  re- 
sorbed  nutrients  for  their  functions  as  "  potential "  than  is  required 
for  their  storage  as  gain  of  tissue. 

Finally,  the  comparison  need  not  necessarily  be  made,  and  in- 
deed in  case  of  most  agricultural  animals  cannot  well  be  made,  with 
the  fasting  state.  While  this  method  is  the  simpler  when  practi- 
cable, a  comparison  of  the  total  heat  production  or  of  the  balance 
of  energy  on  two  different  rations  (both  being  less  than  the  mainte- 
nance requirement)  will  afford  the  data  for  a  computation  by  differ- 
ence (exactly  similar  to  that  employed  in  the  determination  of 
metabolizable  energy  in  Chapter  X)  of  the  expenditure  of  energy  in 
the  digestion  and  assimilation  of  the  food  added  to  the  basal  ration. 

The  most  important  quantitative  investigations  upon  the  work 
of  digestion  are  those  of  Magnus-Levy  *  on  the  dog  and  on  man, 
and  those  of  Zuntz  &  Hagemann  f  upon  the  horse. 

Experiments  on  the  Dog.  —  In  Magnus-Levy's  experiments 
the  respiratory  exchange  of  the  animal  was  determined  by  means 
of  the  Zuntz  apparatus  at  intervals  of  one  or  two  hours  during 
fasting  and  after  feeding.  The  single  periods  were  twenty-five  to 
thirty  minutes  long,  and  the  external  conditions  were  maintained 
as  uniform  as  possible. 

Fat. — Fat  (in  the  form  of  bacon  free  from  visible  lean  meat), 
when  given  in  quantities  not  materially  exceeding  in  heat  value 
the  fasting  metabolism,  resulted  in  a  slight  increase  of  the  latter, 
beginning  about  one  to  three  hours  after  eating,  reaching  its  maxi- 
mum between  the  fifth  and  ninth  hours,  and  disappearing  about  the 
twelfth  hour.  The  maximum  increase  observed  was  12  per  cent., 
seven  hours  after  eating.  In  amounts  largely  exceeding  the  equiv- 
alent of  the  fasting  metabolism  the  effect  of  fat  was  somewhat 
more  marked  and  longer  continued,  a  maximum  increase  of  19.5 
per  cent,  being  observed  in  one  case  seven  hours  after  eating,  while 

*  Arch.  ges.  Physiol.,  55,  1. 
f  Landw.  Jahrb.,  27,  Supp.  III. 


INTERNAL    WORK. 


379 


the  metabolism  was  still  slightly  above  its  fasting  value  after  eight- 
een hours.  The  respiratory  quotient  in  every  case  sank  to  a  value 
closely  corresponding  to  that  for  the  oxidation  of  pure  fat. 

The  experiments  do  not  permit  an  exact  estimate  of  the  total 
increase  of  the  metabolism  during  the  twenty-four  hours,  since 
the  observations  were  not  always  made  at  hourly  intervals  and 
but  few  of  the  trials  extended  over  a  full  day.  By  selecting, 
however,  the  two  in  which  the  data  are  most  complete  and  com- 
puting as  accurately  as  may  be  the  average  rate  of  consumption 
of  oxygen  per  minute,  it  is  possible  to  obtain  an  approximate 
expression  for  the  total  heat  production.  For  this  purpose  the 
average  oxygen  per  minute  is  multiplied  by  1440  and  this  product 
by  the  calorific  equivalent  of  the  oxygen,  viz.,  3.27  Cals.  per  gram 
in  this  case,  and  the  following  results  obtained,  the  heat  production 
during  fasting  being  in  each  instance  that  found  in  the  particular 
experiment  under  consideration: 


Fat 
Eaten, 
Grms. 

Energy 

of  Food, 

Cals. 

Heat  Production  in  24  Hours. 

No.  of 
Experiment. 

Fasting, 
Cals. 

With 
Food, 
Cals. 

Increase. 

Cals. 

Per  Cent, 
of  Food. 

100 

64  and  68 

131.6 
305.5 

1250 
2902 

972 
1055 

991 
1142 

19 

87 

1.53 
2.99 

Carbohydrates. — Carbohydrates  produced  a  more  marked 
effect  upon  the  metabolism  than  did  fat,  and  one  which  showed 
itself  more  promptly.  In  the  experiments  on  the  dog  the  food 
consisted  of  rice,  either  alone  or  with  the  addition  of  small  amounts 
of  fat,  sugar,  or  meat;  in  other  words,  the  animal  was  on  a  mixed 
diet  in  which  carbohydrates  predominated. 

On  the  average  of  a  series  of  six  experiments  in  which  the  food 
consisted  of  500  grams  of  rice,  200  grams  of  meat,  and  25  grams  of 
fat,  the  metabolism  increased  by  fully  30  per  cent,  within  the  first 
hour  and  continued  to  increase  more  slowly  until  the  maximum  of 
39  per  cent,  was  reached  at  the  sixth  to  eighth  hour.  From  that 
time  it  decreased  to  25  per  cent,  in  the  twelfth  hour  and  then  rather 
suddenly  dropped  nearly  to  the  fasting  value.  The  respiratory 
quotient  rose  from  0.78  during  fasting  to  0.90  in  the  first  hour,  and 


38o 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


reached  very  nearly  1.00  by  the  third  hour,  remaining  at  substan- 
tially this  value  for  sixteen  to  eighteen  hours  and  not  falling  to  the 
fasting  value  in  twenty-four  hours.  Two  parallel  experiments  in 
which  400  grams  of  meat  were  fed  showed  that  a  part,  but  by  no 
means  all,  of  the  above  increase  was  to  be  ascribed  to  the  200  grams 
of  meat.  The  small  amount  of  fat  given  can  hardly  have  affected 
the  result.  The  author  estimates  that  of  the  total  calculated  in- 
crease of  22  per  cent,  over  the  fasting  metabolism  about  6  per 
cent,  may  have  been  due  to  the  proteids  of  the  food  and  the 
remainder  to  the  carbohydrates.  This  conclusion  is  confirmed  by 
the  results  of  two  experiments  in  which  rice,  sugar,  and  fat  were 
given.  The  increase  in  the  metabolism  was  of  precisely  the  same 
character  as  in  the  other  experiments,  but  less  in  amount. 

In  all  these  experiments  the  food  was  in  excess  of  the  fasting 
metabolism.  In  another  series  in  which  the  food,  consisting  of  rice, 
either  alone  or  with  a  small  amount  of  sugar,  was  about  equivalent 
to  the  fasting  metabolism,  the  increase  in  the  metabolism  was 
slightly  less,  although  otherwise  the  results  were  similar  to  those 
of  the  other  trials. 

Computing  the  results  per  twenty-four  hours,  as  in  the  case  of  the 
fat,  we  have  the  following  approximate  figures  for  the  three  series : 


Heat  Production  in  24  Hours. 

Food* 
Grms. 

Metab- 
oliza- 

ble 
Energy 

of 
Food.t 
Cals. 

No.  of 

Fast- 
ing, 
Cals. 

With 
Food, 
Cals. 

Increase. 

Cals. 

Per 

Cent,  of 
Food. 

68,  70,      ( 

71,  73,     \ 

74,  and  75  ( 

Proteids 71.3    ) 

Carbohydrates. .  375 . 0    >■ 
Fat 31.0    ) 

2121 

1040 

1271 

231 

10.89 

84  and  87  \ 

Proteids 28.1    ) 

Carbohydrates. .  457 . 5    \ 
Fat 25.0    ) 

2226 

1132 

1292 

160 

7.19 

107 i 

Proteids 18.75) 

Carbohydrates..  225.00  \ 
Fat ) 

999 

991 

1080 

89 

8.91 

Rice  estimated  to  contain  75  per  cent,  carbohydrates  and  1  per  cent, 
nitrogen. 

t  Computed  by  the  writer,  using  Rubner's  factors. 


INTERNAL   WORK. 


38i 


Proteids. — Proteids  in  the  form  of  meat  or  a  mixture  of  meat 
and  flesh-meal,  with  in  some  cases  small  amounts  of  fat,  caused  a 
very  marked  and  prompt  increase  in  the  metabolism  of  the  dog. 
The  maximum  effect  was  usually  reached  about  the  third  or  fourth 
hour  and  continued  with  but  slight  diminution  up  to  the  seventh 
or  eighth  hour  with  small  rations  and  as  long  as  to  the  twelfth  or 
fifteenth  hour  with  large  rations.  As  in  the  case  of  fat  and  carbo- 
hydrates, the  increase  was  greater  with  large  rations,  but  its  amount 
largely  exceeded  that  caused  by  either  of  the  two  former  groups  of 
nutrients,  reaching  in  some  cases  90  or  more  per  cent,  of  the  fasting 
value. 

The  results  were  more  irregular  than  in  the  preceding  experi- 
ments, and  were  apparently  influenced  by  a  peculiar  effect  of  the 
food  upon  the  type  of  respiration.  The  author,  however,*  com- 
putes from  three  selected  series  of  experiments  the  following 
approximate  averages  for  the  twenty-four  hours : 


Proteids 
Eaten, 
Grms. 

Metabo- 

lizable 

Energy 

of  Food, 

Cals. 

[Heat  Production  in  24  Hours. 

No.  of 
Experiment. 

Fasting, 
Cals. 

With 
Food, 
Cals. 

Increase. 

Cals. 

Per  Cent, 
of  Food. 

83  and    89 

102    "     106 

95     "       96 

82.5 
230.0 
370.6 

338 
943 
1520 

1030 

963 

1059 

1086 
1079 
1303 

56 
116 
244 

16.57 
12.30 
16.05 

The  amount  of  the  proteid  metabolism  was  not  determined  in 
these  experiments,  but  the  author  points  out  that  they  were  made 
on  the  first  day  of  the  feeding,  and  that  it  is  probable  that  the 
proteid  metabolism,  and  consequently  the  heat  production,  would 
have  increased  more  or  less  had  the  feeding,  particularly  with 
excess  of  food,  been  continued  longer. 

Bone,  when  fed  in  large  quantities  to  the  dog,  was  found  to 
cause  a  greater  increase  in  the  metabolism  than  corresponded  to  the 
nitrogenous  matter  estimated  to  have  been  resorbed  from  it,  and 
the  difference  is  ascribed  to  the  mechanical  effect  upon  the  digestive 
tract. 

*  hoc.  cit.,  p.  78. 


382 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


Experiments  on  Man. — Magnus-Levy's  experiments  upon  man 
were  made  substantially  like  those  upon  the  dog,  the  subject  lying 
upon  a  sofa,  as  completely  at  rest  as  possible,  and  breathing  through 
a  mouth-piece. 

Fat. — Two  experiments  with  fat,  computed  in  the  same  way  as 
those  upon  the  dog,  gave  the  following  results : 


Fat 
Eaten, 
Grms. 

Energy 

of  Food, 

Cals. 

Heat  Production  in  24  Hours. 

No.  of  Experiment. 

Fasting, 
Cals. 

With 
Food, 
Cals. 

Increase. 

Cals. 

Per  Cent, 
of  Food. 

81 

94.0 
195.6 

893 
1855 

1537 
1524 

1547 
1582 

10 
58 

1.12 

21 

3.13 

Carbohydrates. — Numerous  experiments  on  a  man  were  made 
in  which  the  diet  consisted  chiefly  of  bread,  and  a  smaller  number 
in  which  the  effect  of  sugar  was  studied.  With  bread  the  increase 
in  the  metabolism  was  more  prompt  than  in  the  experiments  on  the 
dog,  but  smaller  in  amount,  varying  from  about  12  to  as  high  as  33 
per  cent.,  according  to  the  amount  eaten.  By  the  end  of  the  third 
hour  the  effect  had  nearly  disappeared,  but  it  was  then  followed 
by  a  second  increase,  less  in  amount  but  continuing  longer,  which 
the  author  suggests  may  have  been  due  to  the  commencement  of 
intestinal  digestion.  With  sugar  (both  cane  and  grape)  the  increase 
was  equally  prompt,  although  rather  less  in  amount,  but  dis- 
appeared entirely  after  two  or  three  hours.  None  of  the  experi- 
ments extended  over  more  than  ten  hours  and  usually  over  less,  and 
the  data  given  are  insufficient  for  a  satisfactory  computation  of  the 
total  increase  for  the  twenty-four  hours.  The  respiratory  quotient 
was  considerably  raised,  but  did  not  reach  1.00  in  any  case. 

Proteids. — Experiments  upon  the  effect  of  proteids  on  the 
respiratory  exchange  yielded  results  similar  to  those  obtained  with 
the  dog,  but  do  not  permit  of  a  satisfactory  computation  of  averages 
for  the  twenty-four  hours. 

Mixed  Diet. — Results  with  a  mixed  diet  the  ingredients  of 
which  are  not  specified  have  been  reported  by  Johansson,  Lander- 


INTERNAL   WORK. 


383 


gren,  Sonden  &  Tigerstedt.*  The  experiments  were  made  in  a 
large  Pettenkofer  respiration  apparatus  and  extended  over  twenty- 
two  hours,  the  results  being  computed  to  twenty-four  hours.  The 
total  heat  production,  as  computed  from  the  carbon  and  nitrogen 
balance,  and  the  computed  metabolizable  energy  of  the  food  werej 


First        day . 

Second 

Third 

Fourth 

Fifth 

Sixth 

Seventh 

Eighth 

Ninth 


Energy  of  Food, 
Cals. 


4141.4 
4277.9 

0 

0 

0 

0 

0 
4355.9 
3946.4 


Heat  Production, 
Cals. 


(?) 
2705.3 
2220.4 
2102.4 
2024 . 1 
1992.3 
1970.8 
2436.9 
2410.1 


The  above  figures  furnish  a  striking  example  of  the  constancy  of 
the  fasting  metabolism,  and  of  the  marked  increase  brought  about 
by  the  consumption  of  food.  Omitting  the  results  for  the  first  day 
of  fasting  and  for  the  first  day  of  the  experiment  we  obtain  the 
following  averages: 

Average  energy  of  food 4193.4  Cals. 

Metabolism  : 

With  food 2517.4  " 

Fasting 2022.4  " 

Increase. 

Total 495.0  " 

Per  cent,  of  food 11 .  76  Per  cent. 

It  is  to  be  noted,  however,  that  the  food  in  this  experiment  was 
considerably  in  excess  of  the  fasting  requirements,  so  that  there 
was  a  notable  storage  of  material  and  energy  in  the  body. 

Summary. — The  results  of  the  foregoing  approximate  computa- 
tions of  the  increased  expenditure  of  energy  for  twenty-four  hours 
are  summarized  in  the  following  table,  which  also  includes  a  com- 
parison of  the  metabolizable  energy  of  the  food  with  the  fasting 
metabolism : 

*  Skand.  Arch.  Physiol.,  7,  29. 


384 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


Food. 

Metabolizable 
Energy  of 
Food,  Cals. 

Excess  Above 

Fasting 

Metabolism, 

Cals. 

Digestive  Work 
in  Per  Cent, 
of  Metaboliz- 
able Energy. 

Fat: 

Experiments  on  man | 

"  dog j 

893 

1855 
1250 
2902 

-644 
+  331 

+  278 
+  1847 

1.12 

3.13 
1.53 
2.99 

Average 

2.19 

Chiefly  Carbohydrates  : 

Experiments  on  dog J 

2121 

2226 

999 

+  1081 
+  1094 

+  8 

10.89 
7.19 
8.91 

8.99 

Proteids  : 

Experiments  on  dog •] 

338 
943 
1520 

-692 

-20 

+  461 

16.57 
12.30 
16.05 

Average 

14.97 

Mixed  Diet  : 

Experiments  on  man  . . . 

4193 

+  2171 

11.76 

It  is  clear  that  proteids  caused  the  greatest  increase  in  the 
metabolism  and  fat  the  least,  while  the  carbohydrates  occupied  an 
intermediate  position.  In  the  case  of  fat  the  increase  in  the  heat 
production  seems  to  show  a  slight  tendency  to  become  greater 
with  amounts  of  food  largely  in  excess  of  the  fasting  metabolism, 
but  with  the  carbohydrates  and  proteids  no  distinct  effect  of  this 
sort  is  apparent. 

These  results,  particularly  those  on  proteids,  afford  a  good  illus- 
tration of  the  fact  that  the  increase  in  the  heat  production  caused 
by  the  ingestion  of  food  is  not  due  solely  to  the  increased  muscular 
work  involved,  since  if  we  were  to  suppose  the  latter  to  be  the  case 
it  is  not  apparent  why  the  proteids,  which  are  digested  pretty 
promptly  and  with  comparative  ease,  should  cause  seven  times  as 
much  work  as  the  fats.  The  results  certainly  suggest  strongly  that 
a  large  part  of  the  heat  production  in  the  former  case  arises  from 
the  considerable  chemical  cleavage  which  the  proteids  undergo  in 
digestion  and  still  more  from  the  stimulative  effect  of  food  proteids 


INTERNAL  WORK.  385 

on  the  nitrogen  cleavage;  in  other  words,  that  what  was  called 
on  p.  375  the  work  of  assimilation  is  an  important  factor. 

Results  on  Fat. — The  relatively  small  increase  in  the  metabo- 
lism resulting  from  the  ingestion  of  fat  is  worthy  of  notice  as  bear- 
ing upon  the  hypothesis,  already  several  times  referred  to,  that  it 
undergoes  a  cleavage  into  dextrose,  carbon  dioxide,  and  water  in 
the  liver,  and  that  the  resulting  dextrose  is  the  material  which 
serves  as  the  source  of  potential  energy  for  the  general  metabolism. 
As  was  pointed  out  in  Chapter  V  (p.  153),  however,  the  dextrose 
derived  from  one  gram  of  fat  according  to  the  commonly  accepted 
equation  would  contain  about  6.1  Cals.  of  potential  energy  out  of 
the  9.5  Cals.  contained  in  the  original  fat.  In  other  words,  over 
one  third  of  the  energy  of  the  fat  would  be  liberated  as  heat  in  the 
intermediary  metabolism  supposed  to  take  place  in  the  liver. 
While  the  heat  production  was  not  directly  measured  in  Magnus- 
Levy's  experiments,  and  while  the  method  of  computation  em- 
ployed may  be  open  to  criticism  in  details,  his  results  certainly  fail 
to  indicate  any  such  large  increase  in  the  metabolism  as  this  hypoth- 
esis would  require. 

It  should  be  noted,  in  conclusion,  that  the  above  experiments 
did  not  include  a  determination  of  the  work  of  mastication  and  in- 
gestion of  the  food,  and  also  that,  according  to  the  author,  there 
was  little  if  any  production  of  fat  in  the  experiments  in  which  carbo- 
hydrates were  fed. 

Experiments  on  the  Horse. — Zuntz,  Lehmann  &  Hagemann  * 
have  investigated  the  effect  of  digestive  work  and  also  of  the  masti- 
cation of  the  food  on  the  metabolism  of  the  horse,  the  respiratory 
exchange  being  determined  by  the  Zuntz  method  and  a  correction 
made  for  the  cutaneous  and  intestinal  respiration.  In  addition  to 
this,  however,  other  data  were  secured  which  serve  the  authors  as 
the  basis  for  computations  of  the  energy  metabolism  of  the  animal 
and  of  the  available  energy  of  the  digested  food.  Since  their  most 
important  conclusions  as  to  digestive  work  are  based  in  large  part 
on  the  results  of  these  computations  it  is  necessary  to  consider 
their  method  in  some  detail. 

Method  of  Computation. — At  six  different  times  between  the 

*  Landw.  Jahrb.,  27,  Supp.,  Ill,  pp.  271-285. 


386  PRINCIPLES   OF  ANIMAL   NUTRITION. 

years  1888  and  1891  digestion  experiments  were  made  *  in  which 
the  total  nitrogen  metabolism  f  and  the  carbon  of  the  food  and  of 
the  visible  excreta  were  determined.  The  ration  in  every  case 
consisted  of  hay  and  a  mixture  of  six  parts  of  oats  with  one  of  cut 
straw:  the  chemical  composition  of  these  feeds  was  quite  similar 
in  the  several  experiments,  the  greatest  variation  being  in  the  last 
experiment  (October  16-22,  1891). 

From  the  results  of  these  experiments  the  metabolism  of 
energy  in  the  respiration  experiments  is  computed  in  the  following 
manner : 

First,  the  results  of  the  several  digestion  experiments  are  com- 
bined in  such  a  way  as  to  give  an  average  corresponding  to  the  ration 
during  the  respiration  experiment.  E.g.,  in  Period  1  {loc.  cit.,  p.  256) 
the  ration  consisted  of  6  kgs.  of  oats,  1  kg.  of  straw,  and  6  kgs.  of 
hay.  As  no  single  digestion  experiment  was  made  on  just  this 
ration,  the  results  of  the  first  one  are  taken  four  times,  those  of  the 
second  three  times,  and  those  of  the  third  once,  and  the  sums  divided 
by  eight.  These  averages  are  taken  as  representing  the  digestibility 
and  the  urinary  carbon  and  nitrogen  during  the  respiration  experi- 
ment. 

Second,  from  the  average  carbon  and  nitrogen  of  the  urine  as 
thus  obtained  its  content  of  urea  and  hippuric  acid  is  computed, 
and  from  these  data,  on  the  assumption  of  average  composition  for 
the  metabolized  proteids,  the  portion  of  the  elements  of  the  latter 
completely  oxidized  in  the  body,  from  which  again  the  amount  of 
oxygen  required  and  of  carbon  dioxide  produced  is  computed. 

Third,  from  the  computed  amount  of  crude  fiber  digested, 
assuming  it  to  have  the  composition  C8H10O5  and  that  100  grams 
yield  4.7  grams  of  methane,  is  computed  the  oxygen  required  for 
its  oxidation  and  the  carbon  dioxide  resulting. 

Fourth,  after  subtracting  the  amounts  of  oxygen  and  carbon 
dioxide,  as  above  computed,  corresponding  to  the  proteids  and 
crude  fiber  oxidized,  from  the  totals  found  in  the  respiration  experi- 
ment, the  remainders  are  divided  between  fat  and  carbohydrates 

*  Loc.  cit.,  pp.  211-236. 

t  The  nitrogen  of  the  feces  was  determined  in  the  air-dried  material. 
Subsequent  experience  has  shown  that  there  is  some  loss  of  nitrogen  in  air- 
drying. 


INTERNAL    WORK. 


387 


in  the  manner  described  on  page  76  on  the  assumption  that  the  fat 
has  the  composition  C  76.54  per  cent.,  H  12.01  per  cent.,  0  11.45 
per  cent.,  and  the  carbohydrates  that  of  starch. 

Fifth,  on  the  basis  of  the  chemical  processes  thus  computed  the 
amount  of  energy  set  free  is  estimated  from  the  known  (average) 
heats  of  combustion  of  the  materials  oxidized. 

While  the  calculation  involves  numerous  assumptions,  and 
while,  therefore,  the  result  is  of  the  nature  of  an  approximation 
most  of  the  assumptions  are  so  nearly  correct  as  not  to  contain  the 
possibility  of  serious  error.  The  two  which  seem  most  questionable 
are  the  peculiar  method  of  computing  the  digestibility  of  the  food 
and  the  proteid  metabolism,  and  the  computation  of  the  proximate 
composition  of  the  urine. 

Influence  of  Food  Consumption  on  Metabolism. — The 
influence  of  the  ingestion  of  food  in  increasing  the  oxygen  con- 
sumption and  the  energy  metabolism  of  the  animal  is  illustrated  by 
the  following  tabulation  of  the  results  obtained  in  Period  b  (loc.  cit., 
p.  282).  (The  animal  was  standing  quietly,  but  otherwise  was  in 
a  state  of  rest.) 


In  the  Morning, 
Fasting. 

Immediately  After  Feeding. 

Later  Stage  After 
First  Feeding. 

No.  of 

Per  Kg.  Live 

Weight  per 

Minute. 

si 

i-l 

O  C 

'£% 

m    4) 

!* 

0 

H 

Feed  Eaten. 

Per  Kg.  Live 

Weight  per 

Minute. 

1 

ffijg 
1 

Per  Kg.  Live 

Weight  per 

Minute. 

3 

lid 

I!1 

Oats 
and 

Straw, 
Grms. 

Hay, 
Grms. 

Ill 

lis 

og 
0 

a  0  g 

og 
0 

4>    2"c3 

c  i)  5 

w-g 

0 

S3 

0 

49 

T2300 
|_2300 

2100 

2280 
3180 
3150 
[23(H) 
2330 
0 
[0 

1500"1 
1500J 

1000 
1420 
0 
0 
1000] 
1430 
1650 
2500] 

3.602 
3.613 

18.365 
18.798 

19.159 
19.220 
19 . 304 
16.134 
18.318 

20.450 
19.333 

50 

2  0 

51    

3.226 
3.304 
3.516 
3.246 
3.130 
3.499 
3.310 

16.380 
16.784 
17.613 
16.359 
16.928 
17.748 
16.219* 

10.5 
10.5 
10.5 
11.0 
10.5 
11.0 

17.5 

3.418 
4.039 
3.745 
3.584 

17.431 

20.889 
18.913 
17.647 

0.6 
0.8 
0.5 
0.6 

3.823 
3.737 
3.739 
3.169 
3.564 

4.174 
3.914 

52. . . 

53 

54 

55 

56 

57 

58 

59 

3.475 
3.446 
3.242 

17.516 

17.474 
16.272 

11.0 
11.0 
11.0 

3.716 
3.385 

18.931 
17.247 

0.5 
0.5 

60 

61 

Averages  . . 

3.339 

16.929 

11.5 

2173 

917 

3.648 

18.510 

0.6 

3.704 

18.787 

3.5 

Animal  was  uneasy. 


388 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


The  average  energy  metabolism  thirty-six  minutes  after  eating, 
computed  as  previously  described,  is  somewhat  more  than  9  per  cent, 
greater  than  that  shortly  before  eating,  and  a  still  further  increase 
was  observed  at  the  end  of  three  hours.  The  effect  is  precisely 
similar  to  that  observed  in  Magnus-Levy's  experiments.  It  was 
not,  however,  followed  through  the  twenty-four  hours,  as  in  some 
of  those  experiments. 

Comparison  of  Hay  and  Grain. — It  was  found  further  that 
coarse  fodder  (hay)  produced  a  much  more  marked  effect  than  did 
grain.  The  following  comparison  of  the  average  of  the  experi- 
ments of  Period  c  on  an  exclusive  hay  diet  with  that  of  Period  /  on 
a  mixed  ration  illustrates  this  fact : 


Period  c. 

Period  /. 

2.6  hrs. 
About  10.5  kgs.* 

2.8    hrs. 

Ration : 

Hay  

4.75  kgs. 
6.00    " 

Oats 

1.00    " 

Total  digested  nutrients  (fat  X 
2.5)   

4125      grms.  t 

3.9837  c.c. 

3.6586    " 

19.552    cals. 

5697.    grms.t 
3  6986  c.c. 

Per  kilogram  and  minute : 

Carbon  dioxide  given  off 

Energy  set  free  (computed) 

3.6695    " 
18.339    cals 

Notwithstanding  the  greater  total  weight  of  food  consumed  in 
Period  /,  and  the  much  larger  amount  of  digestible  matter  contained 
in  it,  the  oxygen  consumption  and  the  computed  amount  of  energy 
liberated  are  notably  greater  in  Period  c,  on  the  hay  ration.  The 
average  time  which  had  elapsed  since  the  last  feeding,  as  well  as  the 
external  conditions,  having  been  substantially  the  same  in  both 
periods,  J  and  the  animal  having  been  in  a  state  of  rest,  the  effect 
is  ascribed  to  an  increase  in  the  expenditure  of  energy  in  diges- 
tion due  to  the  difference  in  the  physical  properties  of  the  two 
rations.     This  difference  is  chemically  characterized  by  the  greater 

*  The  exact  amount  of  hay  eaten  is  not  stated.  The  digestible  matter 
is  computed  from  the  composition  of  the  hay  by  the  use  of  Wolff's  coeffi- 
cients. 

t  Computed  in  the  manner  described  above,  p.  386. 

X  It  varied  considerably  in  the  individual  experiments  composing  Period  /. 


INTERNAL   WORK.  389 

proportion  of  crude  fiber  in  the  hay  ration.  Ascribing  the  differ- 
ence in  digestive  work  entirely  to  the  crude  fiber,  the  authors  en- 
deavor to  estimate  the  expenditure  of  energy  on  this  ingredient 
as  follows: 

Digestive  Work  for  Crude  Fiber. — The  hay  ration  con- 
tained 1572  grams  less  of  (estimated)  digestible  matter  and  648 
grams  more  of  total  crude  fiber  than  the  mixed  ration.  The  com- 
puted evolution  of  energy  per  head  for  the  twenty-four  hours  was 
greater  by  772  Cals.  in  the  hay  period.  On  the  basis  of  Magnus- 
Levy's  results  the  authors  assume  that  the  expenditure  of  energy 
in  the  digestion  of  the  nutrients  exclusive  of  crude  fiber  equals  9 
per  cent,  of  the  total  energy  of  the  digested  matter.  For  1572 
grams  (fat  being  reduced  to  its  starch  equivalent)  this  amounts  to 
4.1X1572X0.09  =  580  Cals.  Accordingly,  the  energy  metabo- 
lism should  have  been  580  Cals.  less  in  Period  c  than  in  Period  /. 
It  was  actually  772  Cals.  greater,  a  difference  of  1352  Cals.  This 
difference  is  ascribed  to  the  presence  of  the  648  grams  more  of  total 
crude  fiber,  and  corresponds  to  2.086  Cals.  per  gram.  With  an 
average  digestibility  of  55  per  cent,  this  would  equal  3.793  Cals. 
per  gram  of  digested  crude  fiber,  an  amount  slightly  exceeding  its 
metabolizable  energy  as  computed  on  p.  331.  In  other  words,  it 
would  appear  that  all  the  metabolizable  energy  of  the  crude  fiber 
(or  even  more,  should  the  digestibility  fall  below  the  percentage 
assumed)  is  consumed  in  the  work  of  digestion  and  converted  into 
heat,  leaving  none  available  for  external  work,  and  this  result  seems 
to  coincide  strikingly  with  the  results  obtained  by  Wolff  *  by  an 
entirely  different  method.     (Compare  Chapter  XIII,  §2.) 

It  is  to  be  observed,  however,  that  the  basis  of  Zuntz  &  Hage- 
mann's  computation  is  the  difference  between  the  energy  required 
for  the  digestion  of  the  648  grams  of  crude  fiber  and  that  required 
for  the  digestion  of  an  equal  amount  of  fiber-free  nutrients.  To 
get  at  the  total  expenditure  upon  the  digestion  of  the  crude  fiber 
we  should  make  the  following  computation : 

The  nutrients  other  than  crude  fiber  digested  were  in  Period  / 
5124  grams  and  in  Period  c  2608  grams,  a  difference  of  2536  grams. 
The  corresponding  difference  in  the  work  of  digestion  would,  on  the 

*  Grundlagen,  etc.,  Neue  BeitrSge,  1887,  p.  94 


39°  PRINCIPLES   OF  ANIMAL   NUTRITION. 

above  assumptions,  be  4.1X2516X0.09  =  928  Cals.  Adding  this, 
as  before,  to  the  observed  difference  of  772  Cals.  gives  a  total  of 
1700  Cals.  as  the  effect  of  the  648  grams  of  crude  fiber,  which  equals 
2.623  Cals.  per  gram.  With  a  digestibility  of  55  per  cent.,  this 
corresponds  to  4.768  Cals.  per  gram  of  digested  crude  fiber,  or 
materially  more  than  its  metabolizable  energy. 

Uncertainties  of  the  Computation. — The  whole  method  of 
computation,  however,  is  open  to  serious  criticism  on  at  least  two 
points,  aside  from  the  rather  indefinite  statements  as  to  the  amount 
of  hay  consumed  in  Period  c  and  as  to  the  distribution  of  the  ration 
between  the  three  feedings  in  Period  /. 

First,  the  estimate  for  the  work  of  digestion  of  the  nutrients 
other  than  crude  fiber  which  forms  the  basis  of  the  computation  is 
derived  chiefly  from  the  experiments  of  Magnus-Levy  on  dogs  and 
man.  Those  experiments  were  not  only  made  with  highly  digesti- 
ble food,  but  the  digestive  work  is  computed  as  a  percentage  of  the 
total  (gross)  energy  of  the  food.  The  food  of  the  horse  contained 
in  the  dry  matter  40.94  per  cent,  of  indigestible  substances  in 
Period  /  and  54.37  per  cent,  in  Period  c,  or  if  we  leave  out  of  account 
the  crude  fiber  the  corresponding  figures  are  31.99  per  cent,  and 
58.45  per  cent.  A  considerable  part  of  the  work  of  digestion  un- 
doubtedly consists  of  muscular  work,  which  must  be  performed 
on  the  indigestible  as  well  as  the  digestible  matter  of  the  food. 
Moreover  these  indigestible  matters,  by  their  mechanical  stimulus 
and  by  acting  in  a  certain  sense  as  diluents,  may  perhaps  cause  a 
more  abundant  secretion  of  the  digestive  juices.  These  facts  are 
entirely  ignored  when  the  figures  for  digestive  work  derived  from 
experiments  on  dogs  and  man  are  applied  simply  to  the  digested 
food  of  the  horse. 

Second,  the  method  of  computation  assumes  that  the  difference 
between  the  metabolism  on  the  two  rations  which  was  observed  2.7 
hours  after  eating  would  have  retained  the  same  absolute  (not  rela- 
tive) value  during  the  twenty-four  hours.  The  justification  for  this 
assumption  is  found  in  a  comparison  *  of  the  results  of  a  single  res- 
piration experiment,  made  one  half  hour  after  feeding,  with  the 
average  of  two  experiments  in  which    the   excretion   of  carbon 

*  Loc  cit.,  p.  218. 


INTERNAL    WORK.  39* 

dioxide  was  determined  for  twenty-four  hours  in  a  Pettenkofer 
respiration  apparatus.  After  allowing  for  the  work  of  mastica- 
tion in  the  latter  experiment  the  results  were  found  to  agree 
within  8.8  per  cent.  The  authors,  therefore,  conclude  that  with 
regular  feeding  the  respiratory  exchange  during  the  forenoon 
hours,  when  their  experiments  were  made,  corresponds  substan- 
tially to  the  average  metabolism  for  the  twenty-four  hours,  exclu- 
sive of  the  work  of  mastication.  It  is  to  be  remarked,  however, 
that  this  conclusion  is  not  fully  in  harmony  with  the  results 
quoted  on  p.  387,  which  plainly  show  a  marked  decrease  in  the 
metabolism  during  the  night.  Moreover,  numerous  other  deter- 
minations of  the  respiratory  exchange  at  the  same  hours  and  on 
similar  food  show  quite  wide  variations.  In  view  of  this  discrep- 
ancy, as  well  as  of  the  somewhat  narrow  basis  of  comparison,  it 
certainly  appears  questionable  whether  a  computation  of  Periods 
c  and  /  for  twenty-four  hours  can  be  safely  made. 

Zuntz  &  Hagemann's  results  unquestionably  show  that  the 
work  of  digestion  is  greater  with  coarse  fodder  than  with  grain. 
That  this  difference  is  due,  at  least  in  large  part,  to  the  greater 
amount  of  crude  fiber  in  the  former  is  extremely  probable.  In 
view,  however,  of  the  two  sources  of  uncertainty  just  pointed  out, 
as  well  as  of  the  numerous  minor  assumptions  involved  in  the  calcu- 
lations, we  must  conclude  that  the  data  available  are  insufficient 
for  an  accurate  quantitative  estimate  of  the  digestive  work  re- 
quired by  crude  fiber. 

Work  of  Mastication. — The  foregoing  computations  relate 
to  the  expenditure  of  energy  in  the  digestion  of  the  food  after  it  has 
entered  the  stomach.  The  same  authors  have  also  determined  the 
increase  in  the  gaseous  exchange  caused  by  mastication,  degluti- 
tion, etc.  For  this  purpose  they  compare  *  the  excretion  of  carbon 
dioxide  and  the  consumption  of  oxygen  during  the  time  actually 
occupied  in  eating  with  the  corresponding  amounts  during  rest  as 
found  from  the  average  of  a  number  of  experiments  made  under 
identical  conditions.  On  the  assumption  that  the  proteid  metabo- 
lism is  unaltered,  the  proportion  of  carbohydrates  and  fat  metabo- 
lized and  the  corresponding  amounts  of  energy  are  computed  by 

*  Loc.  cit.,  p.  271. 


392 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


the  method  described  on  pp.  76  and  252.    The  following  is  a  sum- 
mary of  the  results  computed  per  kilogram  of  feed : 


Fodder. 

No.  of 
Experi- 
ments. 

Oxygen 

Consumed, 

Liters. 

CO„ 
Excreted, 

Liters. 

Equivalent 

Energy, 

Cals. 

Oats  and  cut  straw  (6:1).... 
Hay 

8 
8 
8 
2 

7 

12.964 

33.840 

20.072 

7.133 

6.171 

10.679 

27.813 

17.677 

6.205 

4.980 

64.17 
167.44 

Hay,  oats,  and  cut          

Maize  and  cut  straw  (6:1)... 

100.79 
35.72 
30.42 

47  00 

13.80 

As  was  to  have  been  expected,  the  work  of  mastication  proves 
to  be  much  greater  in  the  case  of  hay  than  in  that  of  grain.  Maize 
gave  a  remarkably  low  result,  while  the  lowest  was  obtained  with 
green  fodder.  Even  when  the  results  on  the  latter  are  computed 
per  kilogram  of  dry  matter,  they  are  still  about  40  per  cent,  lower 
than  those  on  hay.  A  few  experiments  on  old  horses  with  defect- 
ive teeth  gave  somewhat  higher  results  for  the  mixture  of  oats 
and  cut  straw. 

The  absolute  amount  of  energy  expended  in  mastication,  etc.,  is 
very  considerable.  On  the  average  of  three  periods,  on  a  ration 
consisting  of  5.6  kgs.  of  oats,  0.93  kgs.  of  cut  straw,  and  5.18  kgs. 
of  hay,  it  is  computed  at  1287.1  Cals.,  an  amount  equal  to  11.2  per 
cent,  of  the  total  metabolism  during  rest. 

Conclusions. — The  researches  of  Zuntz  &  Hagemann  are  of 
great  value  in  that  they  demonstrate  the  large  proportion  of  the 
energy  of  the  food  which  is  consumed  in  its  prehension,  mastication, 
digestion,  and  assimilation  in  the  case  of  herbivorous  animals,  and 
that  this  proportion  is  largely  influenced  by  the  physical  character 
of  the  food.  Thus  the  hard  but  brittle  maize  required  much  less 
energy  for  its  mastication  than  the  softer  but  tougher  and  more 
woody  oats,  and  the  dry  matter  of  the  green  alfalfa  decidedly  less 
than  that  of  the  hay.  These  results  indicate  quite  clearly  that  no 
accurate  estimates  of  the  work  of  mastication  can  (at  least  in  the 
present  state  of  our  knowledge)  be  based  on  the  chemical  compo- 
sition of  feeding-stuffs.  As  noted  above,  Zuntz  &  Hagemann 
attempt  to  compute  the  work  of  digestion  upon  that  basis.     It 


INTERNAL   WORK.  393 

certainly  seems  open  to  question,  however,  whether  in  this  case  also 
other  properties  than  those  expressed  by  the  percentage  of  crude 
fiber  may  not  materially  affect  the  result,*  and  it  will  be  wise,  until 
the  subject  receives  further  investigation,  to  accept  their  compu- 
tations as  tentative  and  approximate.! 

*  Compare  Kellner's  results  on  cattle,  Chapter  XIII,  §  1. 

t  A  somewhat  extended  critique  by  Pfeiffer  of  these  researches,  together 
with  replies  by  Zuntz  &  Hagemann,  will  be  found  in  Landw.  Vers.  Stat.,  54, 
101;   55,  117;   56,  283  and  289. 


CHAPTER  XII. 
NET  AVAILABLE  ENERGY— MAINTENANCE. 

The  organic  matter  contained  in  the  body  of  an  animal  we  have 
learned  to  regard  in  the  light  of  a  certain  capital  of  stored-up  energy, 
at  the  expense  of  which  the  vital  activities  of  the  organism  are 
carried  on.  The  function  of  the  food  is  to  make  good  the  losses 
thus  occasioned.  The  food  is  frequently  spoken  of  as  "the  fuel  of 
the  body."  In  a  certain  limited  sense  the  comparison  is  admissible, 
but  it  may  easily  be  pushed  too  far,  and  a  closer  analogy  is  that  with 
a  stream  of  water  supplying  a  reservoir  and  serving  to  replenish  the 
drafts  made  upon  it  for  water. 

The  food  in  the  form  in  which  it  is  consumed,  however,  is  by  no 
means  ready  to  enter  directly  into  the  composition  of  the  tissues  of 
the  body  and  add  to  its  store  of  potential  energy,  but  on  the  con- 
trary, as  we  have  seen,  a  very  considerable  amount  of  energy  must 
be  expended  in  the  separation  of  the  indigestible  matters  from  the 
digestible  and  in  the  conversion  of  the  latter  into  such  forms  as  are 
suitable  for  the  uses  of  the  living  cells  of  the  body. 

When,  therefore,  we  give  food  to  a  quiescent  fasting  animal  we 
do  two  things:  we  supply  it  with  metabolizable  energy,  depending 
in  amount  upon  the  quantity  and  nature  of  the  food,  to  take  the 
place  of  the  energy  expended  in  its  internal  work,  but  we  at  the 
same  time  increase  its  expenditure  of  energy  by  the  amount  neces- 
sary to  separate  the  metabolizable  from  the  non-metabolizable 
energy  of  the  food. 

The  case  is  analogous  to  that  of  a  steam-boiler  which  is  fired 
by  means  of  a  mechanical  stoker  driven  by  steam  from  the  same 
boiler.  Each  pound  of  coal  fed  into  the  fire-box  is  capable  of 
evolving  a  certain  amount  of  heat,  representing  its  metabolizable 
energy  in  the  above  sense,  and  that  heat  is  capable  of  producing  a 

394 


NET  AVAILABLE  ENERGY— MAINTENANCE.  395 

certain  quantity  of  steam.  A  definite  fraction  of  the  latter,  how- 
ever, is  required  to  introduce  the  next  pound  of  coal  into  the  furnace 
and  therefore  is  not  available  for  driving  the  main  engine.  To 
recur  to  the  illustration  of  the  reservoir,  it  is  as  if  the  water,  instead 
of  simply  flowing  into  the  reservoir,  actuated  a  pump  or  a  hydraulic 
ram  which  lifted  part  of  it  to  the  required  level. 

Gross  and  Net  Availability. — As  stated  in  Chapter  X,  the 
difference  between  the  potential  energy  of  the  food  and  that  of  the 
excreta  represents  the  maximum  amount  of  energy  which  is  avail- 
able to  the  organism  for  all  purposes.  This  quantity  has  some- 
times been  designated  as  gross  available  energy,  but  has  here  been 
called  metabolizable  energy. 

A  portion  of  this  metabolizable  energy,  however,  as  just  pointed 
out,  has  to  be  expended  in  the  various  processes  which  have  been 
grouped  together  under  the  term  work  of  digestion  and  assimilation. 
This  portion  ultimately  takes  the  form  of  heat,  thus  tending  to 
increase  the  heat  production  of  the  animal  by  a  corresponding 
amount,  and  becomes  unavailable  for  other  purposes  in  the  body, 
since,  so  far  as  we  know,  the  organism  has  no  power  to  convert  heat 
into  other  forms  of  energy.  The  remainder  of  the  metabolizable 
energy  of  the  food  represents  the  amount  which  that  food  con- 
tributes directly  towards  the  maintenance  of  the  capital  of  potential 
energy  in  the  body.  It  is  the  measure  of  the  net  advantage  derived 
by  the  body  from  the  introduction  into  it  of  the  food.*  From  this 
point  of  view  the  energy  remaining  after  deducting  the  expenditure 
caused  by  the  ingestion  of  the  food  from  its  metabolizable  or  gross 
available  energy  has  been  called  the  net  available  energy.  There 
are  obvious  objections  to  the  use  of  the  words  available  and  avail- 
ability in  two  senses,  but  no  better  term  for  net  available  has 
yet  been  suggested,  while  the  use  of  available  energy  in  the  sense 
of  metabolizable  energy  has  become  quite  general.  It  appears 
necessary,  therefore,  to  retain  for  the  present  the  modifying  words 
gross  and  net  to  avoid  ambiguity. 

Distinction  between  Availability  and  Utilization. — The 
net  available  energy  of  the  food  in  the  above  sense  represents  the 

*  As  will  appear  later,  this  somewhat  broad  statement  appears  to  be  sub- 
ject to  modification  in  certain  cases  in  which  there  is  an  indirect  utilization 
of  the  heat  resulting  from  the  work  of  digestion  and  assimilation. 


396  PRINCIPLES  OF  ANIMAL   NUTRITION. 

net  contribution  which  it  makes  to  the  demands  of  the  vital  func- 
tions for  energy  or,  in  other  words,  its  value  as  part  of  a  mainte- 
nance ration.  This  must  be  clearly  distinguished  from  its  value 
for  the  storage  of  additional  energy  in  the  body — that  is,  its  value 
for  productive  purposes.  In  the  latter  case  it  is  quite  possible  that 
the  conversion  of  the  digested  nutrients  into  suitable  forms  for 
storage  (fat  of  adipose  tissue,  ingredients  of  milk  solids,  proteids  of 
new  growth,  etc.)  involves  a  greater  expenditure  of  energy  than  is 
required  to  convert  them  into  forms  fitted  to  serve  as  sources  of 
energy  to  the  body  cells  (work  of  assimilation).  The  consideration 
of  this  question  belongs  in  the  succeeding  chapter,  but  meanwhile 
it  is  important  to  bear  in  mind  that  the  net  available  energy,  in 
the  sense  in  which  the  term  is  here  employed,  is  a  distinct  con- 
ception from  that  of  the  utilization  of  energy  in  fattening,  milk 
production,  etc.,  and  has  reference  to  the  availability  of  the  energy 
of  the  food  for  maintenance. 

It  is  evident  from  the  above  paragraphs  that  the  value  of  a 
feeding-stuff  to  the  animal  is  not  measured  solely  by  its  metaboliz- 
able  energy,  since  materials  containing  the  same  proportion  of  the 
latter  may  require  the  expenditure  of  very  unequal  amounts  of 
energy  for  their  digestion  and  assimilation  and,  therefore,  may 
contain  very  unequal  amounts  of  net  available  energy.  Plainly, 
then,  it  is  a  matter  of  much  importance  to  know  the  net  avail- 
ability of  the  metabolizable  energy  of  the  various  nutrients  and 
feeding-stuffs,  and  thus  to  learn  the  proportions  in  which  they  may 
replace  each  other. 

§  i.  Replacement  Values. 

We  have  already  seen  (Chapter  V,  p.  148)  that,  aside  from  a 
certain  minimum  of  proteids,  the  several  nutrients  can  mutually 
replace  each  other  to  a  very  large  if  not  to  an  unlimited  extent, 
either  one  or  all  serving,  according  to  circumstances,  to  supply  the 
demand  for  energy. 

In  1882  v.  Hosslin  *  published  an  extended  discussion  of  Petten- 
kofer  &  Voit's  respiration  experiments  from  this  point  of  view, 
using  such  data  regarding  the  potential  energy  of  the  nutrients  as 
were  then  available.     He  calls  attention  to  the  wide  range  of  re- 

*  Virchow's  Archiv,  89,  333. 


NET  AVAILABLE  ENERGY-MAINTENANCE. 


397 


placement  possible,  quoting  also  Lawes  &  Gilbert's  conclusions  * 
on  the  same  point  drawn  from  their  experiments  on  fattening  swine, 
and  asserts  that  the  nutrients  replace  each  other  according  to  their 
content  of  available  energy.  Danilewsky  f  also  advanced  similar 
views,  but  Rubner  %  appears  to  have  been  the  first  to  investigate 
the  subject  experimentally. 

Isodynamic  Values. — We  have  already  seen  that  the  total 
metabolism  of  a  fasting  animal  is  approximately  constant,  repre- 
senting the  rate  at  which  the  store  of  matter  and  energy  in  the  body 
is  drawn  upon  to  support  the  necessary  internal  work.  If  we  deter- 
mine the  total  metabolism  of  such  an  animal  and  then  give  it  a 
known  quantity  of  some  nutrient,  as  fat,  e.g.,  the  loss  of  tissue  will 
be  diminished  by  a  certain  amount,  which  will  represent  the  net 
available  energy  of  the  nutrient  and  which  may  be  compared  with 
the  amount  fed.  Similarly,  a  second  and  third  nutrient  may  be  fed 
and  thus  their  relative  values  for  the  prevention  of  loss  of  tissue  be 
determined.  For  example,  a  dog  after  fasting  for  six  days  was 
given  on  the  seventh  and  eighth  days  720  and  760  grams  respect- 
ively of  fresh  lean  meat.  The  average  nitrogen  and  fat  metab- 
olism for  the  fifth  and  sixth  days  (fasting)  and  the  seventh  and 
eighth  days  was  as  follows :  § 


Food. 

Total  Nitrogen 

Excretion 

Grms. 

Fat 

Metabolized 

Grms. 

Temperature 
Deg  C. 

Nothing  (fifth  and  sixth  days) .... 
Meat  (seventh  and  eighth  days) .  . 

3.16 
20.63 

75.92 
30.72 

18.0 
19.2 

Difference 

+  17.47 

-45.20 

+  1  2 

The  result  of  the  feeding  with  meat  was,  of  course,  a  great  in- 
crease in  the  proteid  metabolism.  The  increase  of  17.47  grams  in 
the  nitrogen  excreted  was  equivalent  to  113.38  grams  of  dry  matter 
of  the  meat.  The  metabolism  of  this  amount  of  proteid  matter, 
therefore,  enabled  the  organism  to  diminish  the  metabolism  of  fat 

*  Phil  Trans  ,  160,  541 

t  Die  Kraftvorrate  der  Nahrungsstoffe ;  Arch.  ges.  Physiol.,  1885,  p.  230. 
JZeit.  f.  Biol,  19,313 

§The  original  account  of  the  experiments  is  contained  in  Zeit.  f.  Biol., 
19,  313;  these  figures  are  the  corrected  values  given  in  ibid.,  22,  45. 


39» 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


by  45.20  grams.  For  the  prevention  of  loss  of  tissue  in  this  experi- 
ment, then,  250  parts  of  the  dry  matter  of  the  meat  were  apparently 
equivalent  to  100  parts  of  fat.  The  food,  however,  was  given  at  the 
temperature  of  the  room.  To  warm  it  and  the  100  c.c.  of  water 
consumed  to  the  temperature  of  the  body  would  require  an  amount 
of  heat  equal  to  that  produced  by  the  oxidation  of  1.4  grams  of  fat- 
Adding  this  to  the  45.2  grams  above  gives  46.6  grams  of  fat  as  the 
equivalent  of  113.38  grams  of  dry  matter  of  the  meat,  or  a  ratio  of 
100:243. 

Another  similar  experiment  gave  as  a  final  result  a  ratio  of 
100:  253,  or  after  correction  for  the  warming  of  the  food  100:  243, 
and  a  third  longer  experiment  with  extracted  lean  meat  (syntonin) 
yielded  the  ratio  100:  227,  or  corrected  as  before  100:  225. 

If  now,  from  the  results  of  Rubner's  determinations  of  the  met- 
abolizable energy  of  the  proteids  (p.  276),  we  compute  the  amount  of 
each  which  contains  the  same  quantity  of  metabolizable  energy  as 
100  grams  of  fat  and  compare  it  with  the  above  ratios  we  have  the 
following  as  the  amounts  equivalent  to  100  grams  of  fat : 


Dry  Matter  of — 

Computed 
from  Met- 
abolizable 
Energy, 
Grms. 

Found  in 

Experiments 

on  Animals, 

Grms. 

Lean  meat: 

First  experiment 

Second        "         

235 
235 
213 

243 
243 
225 

The  computed  and  observed  equivalents  differ  by  only  4.3  per 
cent,  and  5.6  per  cent,  respectively,  and  hence  Rubner  concludes 
that  protein  replaces  fat  in  metabolism  substantially  in  inverse  pro- 
portion to  its  "  physiological  heat  value,"  or,  in  other  words,  to  its 
metabolizable  energy. 

Rubner  has  also  made  similar  experiments  with  cane-sugar  and 
starch,  comparing  them  in  each  case  with  the  body  fat,  as  in  the 
above  experiments,  and  has  also  made  trials  in  which  grape-sugar 
was  substituted  for  the  fat  of  the  food.  In  computing  the  results 
of  these  experiments  any  change  in  the  proteid  metabolism  was 
reduced  to  its  equivalent  in  fat  as  computed  from  its  metabolizable 


NET  AVAILABLE  ENERGY-MAINTENANCE. 


399 


energy.     The  following  table  contains  the  final  results,  including 
those  on  proteids  just  given : 


EQUIVALENT  TO   100   GRMS.   OF  FAT. 


Dry  Matter  of — 

Computed 

from 

Metabolizable 

Energy, 

Grms. 

Found  in 

Experiments 

on  Animals, 

Grms. 

235 
213 

235 

229 

255 

243 

243 

225 

(      234 

\      235 

(      234 

232 

(      258 

\      254 

(     255 

The  equivalents  found  by  experiment  correspond  quite  closely 
with  those  computed  from  the  metabolizable  energy,  and  on  these 
facts  Rubner  bases  the  law  of  isodynamic  replacement,  which  may 
be  briefly  stated  as  follows:  In  amounts  less  than  a  maintenance 
ration  the  nutrients  replace  each  other  in  inverse  proportion  to  their 
metabolizable  energy.  The  quantities  which  thus  replace  each  other 
are  accordingly  said  to  be  isodynamic.  It  need  scarcely  be  pointed 
out  that  the  minimum  of  proteids  required  for  the  maintenance  of 
the  nitrogenous  tissues  is  not  included  under  this  law. 

Rubner  is  careful  to  limit  this  law  to  small  amounts  of  food. 
In  his  earlier  publications  he  states  that  it  holds  only  below  the  main- 
tenance ration ;  later  *  he  asserts  that  it  obtains  up  to  an  excess  of 
about  50  per  cent,  over  the  maintenance  ration. 

Isoglycosic  Values. — Mention  has  already  been  made  of  the 
theory  of  isoglycosic  values  maintained  by  Chauveau  and  his  school, 
according  to  which  the  net  available  energy  of  the  digested  nutrients 
is  measured  by  the  amount  of  sugar  they  are  considered  to  be  capa- 
ble of  producing  in  the  organism  according  to  the  equations  given 
in  Chapter  II.  Chauveau  f  computes  that  the  metabolism  of  100 
parts  of  proteids  according  to  Gautier's  scheme  (p.  51),  together 
with  the  partial  oxidation  of  the  resulting  fat  (p.  38),  would  yield 
*  Biologische  Gesetze,  p.  20.  f  Comptes  rend.,  126,  1073. 


400 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


81.5  parts  of  dextrose.      Laulani6  *  computes  that  100  parts  of 

fat,  carbohydrates,    and   albumin    would   produce   the   following 

amounts  of  dextrose : 

100  parts  of  fat  produce 161  parts  of  dextrose 

100     "      "  Starch  produce 110     "      "       " 

100     "      "  sucrose  produce 105     "      "       " 

100     "      "  albumin  produce 80     "      "       " 

The    corresponding   isoglycosic   values  would   be   as   follows, 

Rubner's  isodynamic  values  being  added  for  comparison : 


Isodynamic 
Weights. 

Isoglycosic 
Weights. 

Fat     

100 
229 
235 
255 
235 
213 

100 
146 
153 
161 

201 

It  is  evident  that  the  chief  point  of  difference  is  the  relative 
value  of  fat  and  carbohydrates. 

Experiments  on  Maintenance. — As  regards  the  relative  values  of 
the  several  nutrients  in  a  maintenance  ration  the  above  conclusions 
are  in  part  based  on  theoretical  considerations  and  in  part  are  de- 
ductions from  the  experiments  upon  the  influence  of  work  on  the 
respiratory  quotient  and  upon  the  nature  of  the  non-nitrogenous 
material  metabolized  which  were  considered  in  Chapter  VI,  pp. 
211  to  225.  Contejean,f  however,  has  made  direct  experiments 
upon  the  replacement  values  of  fat  and  carbohydrates. 

His  experiments  were  made  with  dogs.  In  the  first  series  the 
animal,  weighing  about  20  kgs.,  received  a  basal  ration  of  500  grams 
of  meat  (1000  in  the  first  period),  estimated  to  be  ample  to  main- 
tain nitrogen  equilibrium.  To  this  were  added  in  the  several 
periods  varying  amounts  of  lard,  sugar,  and  gelatin.  The  live 
weight  of  the  animal  was  taken  daily  at  the  same  hour  and  under 
uniform  conditions,  and  the  urinary  nitrogen  was  determined. 
No  mention  is  made  of  the  fecal  nitrogen.     The  total  heat  produc- 

*  Energetique  musculaire,  p.  101. 
f  Archives  de  Physiol.,  1896,  p.  803. 


NET  AVAILABLE  ENERGY— MAINTENANCE. 


401 


tion  for  the  four  days  of  each  experiment  (excluding  preliminary 
feeding)  is  computed  from  the  proteid  metabolism  as  measured 
by  the  urinary  nitrogen,  on  the  assumption  that  all  the  fat  con- 
tained in  the  meat  and  all  the  non-nitrogenous  nutrients  added 
were  metabolized.  An  exception  is  made  in  the  fourth  period, 
however,  in  which  the  author  computes  from  a  comparison  of  the 
gains  of  nitrogen  and  of  live  weight  that  there  was  a  gain  of  about 
50  grams  ( ?)  of  fat  by  the  animal.  The  results  are  contained  in 
the  first  six  columns  of  the  following  table : 


Food. 

Gain  or 
Loss  of 
Weight, 
Grms. 

Gain  or  Loss  of 

Esti- 
mated 
Heat 
Pro- 
duction, 
Cals. 

Cor- 
rected 

■a 

P4 

Nitrogen, 
Grms. 

Equivalent 
Flesh, 
Grms. 

Heat 
Pro- 
duction, 
Cals. 

I 
II  | 

III] 

ivj 

vj 
vi  1 

1000  grms.  meat 

500      "        "   > 

40      "     lard f 

500      "     meat ) 

80      "      lard j 

500      "     meat ) 

100      "     lard [ 

500  "  meat. .  . .  ) 
100      "     sugar  . . .  j 

500      "     meat ) 

100      "      gelatin  . .  \ 

-395 
-170 

+  50 

+  335  (?) 

+  152 

-105 

+  19.39 

-  1.86 

+   1.81 
+  6.36 
+  6.04 

-  1.10 

+  570 

-  55 

+  53 

+  187  (?) 
+  170 

-  32 

4548 
3903 

5326 

5486 

3811 

4088 

6190 
4981 

5354 

4566 

3980 

4773 

Making  the  comparison  of  fat  and  carbohydrates,  as  the  essen- 
tial point,  it  would  appear  from  Contejean's  results  that  100  grams 
of  sugar  was  fully  as  efficient  as  80  grams  of  fat,  while  according 
to  Rubner's  figures  about  180  grams  of  sugar  would  be  required. 
Corresponding  to  this  is  the  lower  computed  heat  production  in  the 
sugar  period,  the  excess  in  the  fat  periods  being  ascribed  to  the 
cleavage  of  fat  believed  to  occur  in  the  liver. 

If,  however,  there  is  justification  for  computing  the  gain  of  fat 
by  the  body  in  the  fourth  period  by  subtracting  the  gain  of  flesh 
from  the  total  gain  in  weight,  the  same  method  is  equally  applicable 
to  the  other  periods.  By  its  use  the  figures  of  the  last  column  of 
the  table  have  been  computed  by  the  writer.  While  the  heat 
production  in  the  sugar  period  as  thus  estimated  is  still  below  that 
of  the  fat  periods,  the  rather  wide  range  in  the  results  of  the  latter 
serves  to  illustrate  the  uncertainties  of  such  computation. 


402 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


In  a  second  series  of  experiments  a  ration  of  150  grams  of  meat 
and  100  grams  of  lard  appeared  to  be  equivalent  to  one  of  300  grams 
of  meat  and  50  of  lard.  In  a  third  series,  fat,  sugar,  and  gelatin 
were  each  given  for  two  days  to  a  fasting  dog,  the  live  weight  *  and 
urinary  nitrogen  being  determined  daily.  The  results  were  as 
follows : 


Date. 

Live  Weight,  Kgs. 

Food. 

Urinary 

Nitrogen, 

Grms. 

Dec.  24 

25.780 

25.125 

24.765 

24. 780 +.095  feces 

24.616+. 064     " 

24.215 

23.920 

23. 870 +.038     " 

23.500 

23.200 

Nothing 

it 

200  grms.  sugar 

200      " 

Nothing 

200  grms.  fat 

200     " 

Nothing 

200  grms.  gelatin 

"     25 

5.56 

"     26 

6.05 

"     27 

5.59 

"     28 

4.13 

"     29 

4.59 

"     30 

6.56 

"     31... 

6.85 

4.97 

"       2 

28.77 

Neglecting  the  variations  in  the  urinary  nitrogen,  Contejean 
makes  the  following  comparison  of  the  daily  loss  of  live  weight, 
from  which  he  draws  the  conclusion  that  200  grams  of  sugar, 
equivalent  to  792  Cals.,  was  more  efficient  in  maintaining  the  ani- 
mal than  200  grams  of  fat,  equivalent  to  1876  Cals. 


Gain  or  Loss  of  Live  Weight  per  Day. 

Average  for  fasting —377  grains 

Sugar  ; 

First  day +110  grams 

Second  day -100      " 

Average +5      " 

Fat: 

First  day -295  grams 

Second  day -   12      " 

Average — 154      " 

*  In  taking  the  live  weight  any  feces  voided  during  the  previous  twenty- 
four  hours  were  added  to  the  weight  of  the  animal,  so  that  the  computed  gain 
or  loss  of  weight  does  not  include  the  feces. 


NET  AVAILABLE  ENERGY— MAINTENANCE.  403 

Experiments  in  which  Work  was  Done. — Somewhat  earlier  in 
point  of  time  than  the  above  experiments  by  Contejean  were  similar 
ones  by  Chauveau  *  in  which  the  animal  performed  a  uniform 
(unmeasured)  amount  of  work  per  day.  No  attempt  was  made  to 
determine  the  equivalence  between  food  or  body  metabolism  and 
the  work  performed,  but  the  latter  was  simply  used  as  a  means  of 
increasing  the  metabolism,  while  the  relative  value  of  the  several 
nutrients  in  maintaining  the  store  of  energy  in  the  body  was  esti- 
mated from  the  effect  upon  the  live  weight.  The  experiments, 
therefore,  are  not,  properly  speaking,  work  experiments,  but  belong 
in  the  same  category  as  those  of  Contejean — that  is,  they  aim  to 
show  in  what  proportions  the  nutrients  may  replace  each  other  in 
a  maintenance  ration. 

In  the  first  series  the  basal  ration  consisted  of  400  grams  of 
lean  meat,  to  which  was  added  in  alternate  six-  or  five-day  periods 
either  51  grams  of  lard  or  an  isodynamic  quantity  (121  grams)  of 
cane-sugar.  In  one  period  128.5  grams  of  dextrose  was  used  in- 
stead of  the  cane-sugar.  The  animal  (bitch)  averaged  about  16.8 
kgs.  in  weight.  The  gain  or  loss  of  weight  in  each  period  (differ- 
ence between  first  and  last  weighings)  was  as  follows : 

Period  1 Lard 0  grams 

"       2 Cane-sugar + 170  " 

"       3 Lard -   10  " 

"       4 Cane-sugar +290  " 

"       5 Lard -265  " 

"'    6 Dextrose 0  " 

"      7 Lard -295  " 

In  the  first  four  periods  the  cane-sugar  seems  to  have  caused  a 
gain  in  weight  as  compared  with  practical  maintenance  on  the  lard. 
During  the  last  three  periods  the  animal  was  in  heat  and  a  loss  of 
weight  upon  the  lard  ration  resulted,  which  was  arrested  on  the 
dextrose  ration.  Water  was  given  ad  libitum  for  several  hours  after 
the  work,  but  withdrawn  at  least  twelve  hours  before  weighing. 
No  record  is  given  of  the  amount  of  it  consumed  or  of  the  water 
content  of  the  materials  fed. 

In  another  experiment,  in  which  twice  as  much  work  was  done, 
*  Comptes  rend.,  125,  1070;   126,  795,  930,  1072. 


4°4  PRINCIPLES   OF  ANIMAL   NUTRITION. 

fat  and  cane-sugar  replaced  each  other  in  isoglycosic  proportions, 
viz.,  110  grams  of  fat  and  168  of  cane-sugar.  In  this  case  the 
amount  of  water  consumed  was  uniform,  viz.,  400  grams.  The  gain 
or  loss  of  live  weight  in  five-day  periods  was : 

Period  1 Sugar +  35  grams 

"      2 Fat -160      " 

"      2 "  —Omitting  first  day .  .    -  20      " 

A  third  experiment,  in  which  amounts  of  sugar  intermediate 
between  the  isoglycosic  and  isodynamic  equivalents  of  the  fat 
were  fed,  showed  a  gain  on  the  former  as  compared  with  practi- 
cally no  change  on  the  fat. 

In  a  second  series  of  experiments  isoglycosic  amounts  of  lard 
(110  grams)  and  cane-sugar  (168  grams)  were  alternated  every  five 
or  three  days  for  eighty-five  days,  the  basal  ration  consisting  of  500 
grams  of  lean  meat,  and  400  grams  of  water  being  consumed  per 
day.  The  estimated  heat  values  of  these  rations  were  respectively 
1513  Cals.  and  1145  Cals.,  but  notwithstanding  this  difference  they 
appeared  to  be  equally  efficient  in  maintaining  the  live  weight. 

Whatever  weight  may  attach  to  the  deductions  from  the  exper- 
iments upon  work  production,  it  is  hardly  necessary  to  urge  that 
such  a  method  of  investigation  as  that  employed  in  the  above 
trials,  while  it  may  afford  useful  indications,  is  altogether  too 
crude  to  disprove  the  theory  of  isodynamic  values  based  upon 
Rubner's  more  elaborate  experiments. 

Respiration  Experiments. — Kaufmann  *  has  also  reported  respi- 
ration experiments  in  support  of  the  views  regarding  the  interme- 
diary metabolism  promulgated  by  Chauveau.  In  his  experiments 
the  nitrogen  excretion,  respiratory  exchange,  and  heat  production 
of  clogs  variously  fed  were  determined,  in  five-hour  periods,  by 
means  of  a  radiation  calorimeter  in  which  the  products  of  respira- 
tion were  allowed  to  accumulate.  (See  pp.  69  and  248.)  From  the 
theoretical  equations  given  in  Chapter  II  he  computes  the  figures 
given  on  the  opposite  page  for  the  consumption  of  oxygen,  produc- 
tion of  carbon  dioxide,  and  heat  evolution  in  the  various  reactions. 

Besides  determinations  of  the  fasting   metabolism  the  experi- 
ments included  feeding  exclusively  with  meat  and  also  with  rations 
rich  in  carbohydrates  and  in  fat.    For  each  diet,  on  the  basis  of  the 
*  Archives  de  Physiol.,  1896,  pp.  329,  342,  and  757. 


NET  AVAILABLE  ENERGY— MAINTENANCE. 


405 


Per  Grm.  of  Substance. 

Heat 

Oxygen 

Con- 
sumed, 
Liters. 

Carbon 

Dioxide 

Produced, 

Liters. 

of  Oxygen 
Con- 
sumed, 
Cals. 

0.481 
0.713 
1.045 
0.840 
2.043 
0.744 

0.4777 
0.5480 
0.8720 
0.2257 
1.4290 
0.7440 

2.234 
3.180 
4.857 
3.417 
9.500 
3.762 

4.646 
4.460 
4.647 
4.067 
4.650 
5.056 

"           "  dextrose  and  urea 

"  C02,  H20  "       "     

Stearin      "    "       "       "     dextrose  . .  . 
"           "    "    and  H20 

determination  of  the  respiratory  products,  the  author  assumes  a 
scheme  of  metabolism  in  accordance  with  the  theory,  and  finds  that 
the  heat  production  as  computed  on  this  assumption  agrees  quite 
closely  with  that  actually  determined. 

Aside  from  questions  of  method,  particularly  whether  a  five- 
hour  period  is  sufficiently  long,  it  is  to  be  remarked  that  the  results 
of  Kaufmann's  experiments  are  ambiguous.  They  show  that  it  is 
possible  to  interpret  the  facts  in  accordance  with  his  theory,  but 
they  do  not  exclude  the  possibility  of  other  explanations.  For  this 
reason  it  seems  unnecessary  to  cite  the  experiments  in  detail,  and 
for  the  same  reason  they  are  at  best  but  confirmatory  evidence  in 
favor  of  the  theory  of  isoglycosic  values. 


§  2.  Modified  Conception  of  Replacement  Values. 

The  theory  of  isodynamic  replacement  as  announced  by  Rubner 
constituted  the  first  systematic  application  of  the  general  laws  of 
energy  to  the  problems  of  animal  nutrition.  As  such  it  has  exerted 
a  profound  influence  upon  subsequent  study  of  the  subject  in  that 
it  has  been  chiefly  instrumental  in  leading  to  a  practical  application 
of  the  long-known  fact  that  the  food  is  primarily  a  supply  of  energy. 
It  was  based,  of  course,  upon  the  conception  that  the  law  of  the 
conservation  of  energy  obtains  in  the  animal  body,  and  in  subsequent 
experiments,  which  have  been  described  in  Chapter  IX,  Rubner 
gave  at  least  a  partial  demonstration  of  the  truth  of  this  concep- 
tion. 

Rubner' s  general  ideas  still  form  the  basis  of  our  views  regard- 


4°&  PRINCIPLES    OF  ANIMAL    NUTRITION. 

ing  the  metabolism  of  energy  in  the  body,  but,  as  was  natural,  his 
first  conclusions  have  undergone  more  or  less  modification,  in  part 
at  his  own  hands. 

Digestive  Work. — The  law  of  isodynamic  replacement  as 
stated  above  is  equivalent  to  saying  either  that  all  the  metabolizable 
energy  of  the  food  below  a  maintenance  ration  is  net  available 
energy  or  that  the  percentage  availability  of  all  the  nutrients 
experimented  with  is  the  same.  The  latter  supposition,  however, 
appears  to  be  negatived  by  the  results  of  Magnus-Levy  and  others 
on  digestive  work. 

If,  however,  a  fraction  of  the  metabolizable  energy  of  the  food 
is  applied  to  the  work  of  digestion  and  assimilation,  it  is  plain  that 
this  fraction  cannot  serve  directly  for  tissue  building.  In  his  first 
paper,  Rubner,  while  not  denying  the  fact  of  the  consumption  of 
energy  in  digestive  work,  appears  to  regard  its  amount  as  insignifi- 
cant, although  what  he  specifically  claims  is  that  the  total  metabo- 
lism below  the  maintenance  ration  is  not  increased  by  the  inges- 
tion of  food.  In  support  of  this  view  he  gives  the  results  of  three 
experiments  in  which  fat  was  fed;  that  is,  the  nutrient  which,  ac- 
cording to  Magnus-Levy's  later  results,  causes  the  least  digestive 
work.  Of  these,  one  on  a  dog,  in  which  approximately  a  mainte- 
nance ration  was  given,  showed  no  increase  of  the  metabolism  over 
the  fasting  state.  In  the  other  two  experiments,  one  on  a  dog  and 
one  on  a  rabbit,  more  fat  was  consumed  than  corresponded  to  the 
fasting  metabolism,  and  an  increase  of  the  latter  was  observed 
amounting  to  approximately  3  per  cent,  and  12  per  cent,  respec- 
tively. '  Feeding  with  bone  also  caused  an  increase  of  about  12 
per  cent. 

In  a  later  publication,*  however,  he  recognizes  the  apparent 
inconsistency  between  the  effects  of  small  and  large  amounts  of 
food,  and  propounds  a  hypothesis  to  explain  it  which,  in  its  general 
features  at  least,  seems  in  harmony  with  the  observed  facts.  This 
hypothesis  is  outlined  in  the  following  paragraphs,  although  in  a 
slightly  different  manner  than  by  Rubner. 

Indirect  Utilization  of  Heat  Resulting  from  Digestive 
Work. — In  Chapter  XI  we  acquired  the  conception  of  the  critical 
thermal  environment.  According  to  the  ideas  there  advanced, 
*  Biologische  Gesetze,  Marburg,  1887,  p.  20. 


NET  AVAILABLE  ENERGY— MAINTENANCE.  407 

the  heat  production  of  a  quiescent,  fasting  animal  below  the  critical 
point  is  made  up  of — 

1.  The  heat  produced  by  the  internal  work. 

2.  The  heat  produced  by  the  processes  of  " chemical"  regula- 
tion. 

The  first  of  these  we  may  regard  as  substantially  constant,  while 
the  latter  varies  to  meet  varying  conditions  and  thus  maintain  the 
constancy  of  body  temperature.  When  we  give  food  to  such  an 
animal  we  introduce  a  third  source  of  heat,  viz.,  the  work  of  diges- 
tion and  assimilation.  Other  conditions  remaining  the  same,  the 
tendency  would  be  to  raise  the  temperature  of  the  body,  and  this 
tendency  can  be  overcome  either  by  means  of  "  chemical "  or  "  physi- 
cal" regulation.  Recurring  to  the  illlustration  of  the  room  on 
p.  356,  it  is  as  if  a  second  fire  were  kindled  in  it.  To  maintain  con- 
stant temperature,  either  the  first  fire  must  be  lowered  or  the  win- 
dows must  be  opened. 

The  fact,  however,  that  below  the  critical  point  the  heat  regula- 
tion of  the  body  appears  to  be  largely  "  chemical "  renders  it  prob- 
able that  the  regulation  is  effected  by  the  former  method;  that  is, 
that  the  heat  produced  by  the  work  of  digestion  is  utilized  to  warm 
the  body  and  that  correspondingly  less  energy  is  withdrawn  from 
that  stored  in  the  tissues  of  the  body.*  Under  these  circum- 
stances the  total  heat  production  of  the  animal  would  not  be  in- 
creased by  the  ingestion  of  food,  and  all  the  metabolizable  energy 
of  the  food  would  be  apparently  available ;  that  is,  we  should  have 
the  phenomenon  of  isodynamic  replacement. 

Digestive  Work  Above  Critical  Point. — The  statements  of 
the  last  paragraph  refer  to  conditions  below  the  critical  point. 
Above  this  point  no  such  indirect  utilization  of  the  heat  resulting 
from  digestive  work  is  possible,  since  the  heat  production  has 
already  been  reduced  to  the  minimum  due,  as  was  concluded  on 
p.  356,  to  internal  work.  The  excess  of  heat  arising  from  the  work 
of  digestion  is  then  disposed  of  by  "  physical "  means. 

Thus  Rubner  f  obtained  the   following  results  for  the  carbon 

*  Loewy  (Arch.  ges.  Physiol.,  46,  189;  quoted  by  Magnus-Levy,  ibid.,  p. 
116)  claims  to  have  shown  that  such  a  substitution  or  compensation  does 
not  take  place  in  man. 

f  Biologische  Gesetze,  pp.  17-25. 


408 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


dioxide  produced  per  square  meter  by  guinea-pigs  at  0°  C.  and  at 
30°  C.  (critical  temperature),  when  fasting  and  after  the  consump- 
tion of  food  ad  libitum. 

PER  SQUARE   METER  OF  SURFACE. 


Fasting.* 

Fed. 

Live  Weight, 
Grms. 

At  0°  C. 
C02,  Grms. 

At  30°  C. 

CO.,,  Grms. 

Live  Weight, 
Grms. 

At  0°  C. 
CO,,  Grms. 

At  30°  C. 
CO„,  Grms. 

617 
568 
223 
206 

27.85 
30.30 
30.47 
31.56 

12.35 
10.53 
12.14 
13.16 

670 
520 
240 
220 

Average . .  . 

29.49 
29.08 
34.07 
30.59 

14.10 
16.19 
17.69 
18.94 

Average. .  . 

30.05 

12.05 

30.81 

16.73 

Already  cited  on  p. 


Comparing  the  averages  we  see  that  at  0°  C,  considerably  below 
the  critical  point,  the  consumption  of  food  did  not  materially  in- 
crease the  total  metabolism  per  unit  of  surface.  On  the  other  hand, 
at  a  temperature  close  to  the  critical  point  the  average  heat  pro- 
duction was  increased  nearly  39  per  cent,  by  the  consumption  of 
food. 

It  appears  also  that  at  this  higher  temperature  the  heat  produc- 
tion of  the  fed  animals  was  no  longer  proportional  to  their  surface, 
but  was  relatively  greater  in  the  smaller  animals.  Rubner  explains 
this  by  the  supposition  that  (the  animals  being  fed  ad  libitum)  the 
consumption  of  food  by  the  animals  was  in  proportion  to  their  fast- 
ing metabolism ;  that  is,  to  their  surface.  Under  these  circumstances 
the  factor  of  surface  enters  twice,  and  the  heat  production  is  approx- 
imately proportional  to  the  square  of  the  surface. 

Rubner  *  has  also  made  calorimetric  determinations  of  the  heat 
production  of  a  dog  at  different  temperatures  with  the  results 
shown  on  the  opposite  page.  Not  only  did  the  feeding  increase 
the  heat  production,  but  it  eliminated  the  effect  of  rising  tempera- 
ture in  diminishing  it;  that  is,  it  lowered  the  critical  temperature. 

Critical  Amount  of  Food. — The  very  probable  hypothesis  of 
a  substitution  of  the  heat  produced  by  the  work  of  digestion  for  that 

*  Sitzungsber.  der  k.  bayer.  Akad.  d.  Wiss.,  Math.-pbys.  Classe,  15,  452. 


NET  AVAILABLE  ENERGY-MAINTENANCE. 


409 


Fasting. 

Fed  Small  Amount  of  Meat. 

Temperature, 
Deg.  C. 

Heat 

Production, 

Cals. 

Temperature, 
Deg.  C. 

Heat 

Production, 

Cals. 

13.2 
19.5 
27.4 

39.65 
35.10 
30.82 

19.5 
18.2 
23.7 

24.8 

42.64 
41.13 
41.83 
41.10 

arising,  below  the  critical  point,  from  the  "  chemical "  regulation  of 
the  body  temperature  affords  a  very  reasonable  explanation  of  the 
apparent  discrepancy  between  the  law  of  isodynamic  replacement 
as  propounded  by  Rubner  and  the  no  less  certain  fact  that  the  work 
of  digestion  and  assimilation  makes  a  demand  on  the  body  for 
energy,  which  energy  finally  takes  the  form  of  heat  and  is  not 
available  for  other  purposes. 

A  consequence  of  this  hypothesis,  however,  which  is  sufficiently 
obvious  has  indeed  been  pointed  out,  but  hardly  seems  to  have  re- 
ceived the  attention  which  it  deserves  in  view  of  its  important 
bearing  on  the  theoretical  aspects  of  metabolism. 

If  we  give  increasing  amounts  of  food  to  a  fasting  animal  we 
progressively  increase  the  evolution  of  heat  due  to  digestive  work, 
and  this  heat,  according  to  the  hypothesis,  if  the  thermal  environ- 
ment is  below  the  critical  point,  is  substituted  for  the  heat  pre- 
viously produced  by  the  metabolism  of  tissue.  There  must  be  a 
limit  to  the  possibility  of  this  substitution,  however,  just  as  there 
must  be  to  the  "  chemical "  regulation  of  body  temperature  (p.  353), 
since  otherwise  there  would  be  a  ration  on  which  all  the  heat  of  the 
body  was  derived  from  the  work  of  digestion  and  the  internal  work 
was  performed  without  evolution  of  heat.  The  limit  is  indeed  the 
same  in  both  cases  and  is  reached  when  all  the  heat  previously 
evolved  by  the  processes  of  "chemical"  regulation  has  been  re- 
placed by  the  heat  arising  from  digestive  work.  Beyond  that 
point  the  conditions  are  the  same  as  in  the  fasting  animal  above 
the  critical  point,  and  the  excess  of  heat  is  gotten  rid  of  by 
"  physical "  regulation.  We  may  call  the  amount  of  food  whose  in- 
gestion produces  the  quantity  of  heat  necessary  to  just  reach  this 
limit  the  critical  amount  of  food.     Below  that  amount  the  apparent 


4io 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


availability  of  the  metabolizable  energy  of  the  food  will  be  100  per 
cent,  or  we  shall  have  isodynamic  replacement.  Above  that 
amount  we  shall  have  an  availability  depending  upon  the  relation 
of  the  work  of  digestion  and  assimilation  to  the  total  metabolizable 
energy. 

Graphic  Representation. — The  critical  amount  of  food  will 
depend  chiefly  upon  two  things,  viz.,  the  distance  below  the  critical 
thermal  environment  at  which  the  experiment  is  made  and  the 
amount  of  energy  that  has  to  be  expended  in  the  digestion  and 
assimilation  of  the  food.  The  greater  the  former  quantity,  the 
more  of  the  total  metabolism  of  the  animal  will  be  due  to  the  "  chemi- 
cal" regulation  and  therefore  capable  of  being  replaced,  while  the 
greater  the  work  of  digestion  the  less  food  must  be  consumed  to 
furnish  by  its  digestive  work  the  heat  necessary  to  a  complete 
substitution. 


On  the  two  coordinate  axes  OX  and  OY  let  distances  along  OX 
represent  the  metabolizable  energy  of  the  food  consumed  and  dis- 
tances along  OY  the  effect  of  this  food  upon  the  store  of  potential 
energy  in  the  body.     In  the  first  instance,  let  us  take  the  case  of  a 


NET  AVAILABLE  ENERGY— MAINTENANCE.  411 

fasting  animal  and  suppose  the  thermal  environment  to  be  at  the 
critical  point.  The  distance  OA  may  then  represent  the  loss  of 
potential  energy  (tissue)  from  the  body  caused  by  the  internal  work. 
If  now  we  supply  the  animal  with  food  80  per  cent,  of  whose  met- 
abolizable  energy  is  available,  with  any  given  amount  of  energy 
thus  supplied,  as  0B  =  AC,  SO  per  cent,  of  that  energy,  represented 
by  CD,  will  serve  to  maintain  the  store  of  potential  energy  in  the 
body,  while  20  per  cent.,  or  DB' ,  will  be  absorbed  by  the  work  of 
digestion,  etc.,  and  converted  into  heat.  Accordingly  if  we  assume 
that  the  work  of  digestion  is  proportional  to  the  amount  of  food 
eaten,  the  line  AD  will  indicate  the  availability  of  the  particular 
food  and  may  be  represented  algebraically  by  the  equation 


in  which  a  =  tan  D AC = the  percentage  availability. 

We  may  also  represent  the  heat  production  on  the  same  axes. 
With  no  food  it  will  be  OE  equal  to  OA.  With  an  amount  of  food 
equal  to  OB  it  will  be  equal  to  OE  +  DB'  =  BF,  and  the  line  EF, 
expressed  algebraically  by 

y=(l-a)x, 

will  represent  the  law  of  heat  production. 

Let  us  next  suppose  that,  the  animal  being  again  deprived  of  food, 
the  external  demand  for  heat  is  increased,  by  a  fall  of  temperature, 
e.g.,  and  that  to  meet  this  demand  the  metabolism  is  increased  by 
an  amount  AG,  and  the  heat  production  consequently  by  the  equal 
amount  EH.  If  we  now  give  the  same  food  as  before,  its  real  availa- 
bility will  be  unchanged  and  will  be  represented  by  the  line  GI, 
parallel  to  AD.  Up  to  the  critical  amount  of  food,  however,  the 
heat  resulting  from  the  digestive  work  will,  as  we  believe,  be  sub- 
stituted progressively  for  that  represented  by  EH  and  resulting 
from  the  metabolism  AG.  The  apparent  availability,  therefore, 
will  be  represented  by  the  line  GK,  making  an  angle  of  45°  with  the 
axes,  and  the  heat  production  by  the  line  HL,  parallel  to  OX. 
When  the  food  consumed  reaches  an  amount  OM  at  which  the  line 
GK  intersects  AD,  the  limit  of  this  substitution  is  reached,  since 
the  amount  of  digestive  work,  KN,  equals  the  amount  of  additional 
metabolism  AG  caused  by  the  fall  in  temperature.     In  other  words. 


412  PRINCIPLES   OF  ANIMAL   NUTRITION. 

OM  is  the  critical  amount  of  food.  Beyond  this  amount  the  energy 
expended  in  the  work  of  digestion  will  become  waste  energy,  serving 
simply  to  increase  the  outflow  of  heat,  and  the  apparent  and  real 
availability  of  the  food  will  coincide. 

Plainly,  the  critical  amount  of  food  will  vary  with  circumstances. 
If  the  experiment  is  made  at  or  above  the  critical  thermal  environ- 
ment for  the  fasting  animal  the  smallest  quantity  must  cause  an 
increase  in  the  heat  production  and  the  critical  amount  will  be  0 
(or,  mathematically,  a  negative  quantity).  As  the  external  con- 
ditions fall  below  the  critical  thermal  environment,  the  point  K  will 
be  further  and  further  removed  from  A  until  finally  the  point  of 
intersection  might  even  lie  above  OX,  that  is,  above  the  mainte- 
nance ration.  The  relative  availability  of  the  food,  too,  will  be  a 
factor  in  determining  the  critical  amount.  Thus  if  the  true  availa- 
bility of  the  food  were  expressed  by  the  line  AP  instead  of  AD,  the 
point  of  intersection  would  lie  at  R  and  OR'  would  be  the  critical 
amount  of  food. 

§  3.   Net  Availability. 

The  modified  conception  of  replacement  values  discussed  in 
the  preceding  section  and  in  the  introductory  paragraphs  of  this 
chapter  renders  it  evident  that  both  the  theory  of  isodynamic  re- 
placement, as  first  announced  and  later  modified  by  Rubner,  and 
the  rival  theory  of  isoglycosic  replacement  are  but  aspects  of  the 
more  general  question  of  the  availability  of  the  metabolizable 
energy  of  the  food.  That  the  several  nutrients  are  of  use  to  the 
body  and  can  replace  each  other  in  the  food  in  inverse  ratio  to 
their  available  energy  is  simply  a  necessary  consequence  of  the  law 
of  the  conservation  of  energy.  The  important  question  is  how 
much  of  their  energy  is  really  available.  Rubner 's  theory  regards 
all  the  metabolizable  energy  of  the  food  as  virtually  available, 
directly  or  indirectly,  for  maintenance,  and  this  view  has  been  quite 
generally  accepted.  Chauveau's  theory  of  isoglycosic  replacement 
has  the  merit  of  distinctly  recognizing  the  fact  of  a  possible  expen- 
diture of  energy  in  the  assimilation  of  the  digested  food,  but,  on  the 
other  hand,  it  takes  no  account  of  the  digestive  work,  and  moreover, 
so  far  as  maintenance  values  are  concerned,  rests,  as  we  have  seen, 
upon  a  rather  insecure  foundation.     Plainly,  the  real  question  at 


NET  AVAILABLE  ENERGY-MAINTENANCE.  4*3 

issue  can  only  be  settled  by  experiments  in  which  the  actual  availa- 
bility of  the  energy  of  the  food  or  of  its  various  ingredients  is  deter- 
mined. 

Determinations  of  Net  Availability. 

Since  the  net  available  energy  of  the  food  is  equal  to  its  metabo- 
lizable  energy  minus  the  energy  expended  in  digestion  and  assimila- 
tion, the  two  general  methods  for  the  determination  of  the  latter 
quantity  which  were  outlined  in  the  preceding  chapter  (p.  377)  are* 
also,  from  the  converse  point  of  view,  methods  for  the  determination 
of  net  availability.  In  our  study  of  digestive  work  we  considered 
chiefly  the  results  of  direct  determinations  of  the  increase  in  the 
heat  production  due  to  the  ingestion  of  food;  for  our  present  pur- 
pose the  results  of  any  accurate  determinations  of  the  metabolism 
upon  varying  known  amounts  of  the  same  food  may  be  used. 

The  experimental  evidence  available  is  far  from  being  as  full  as 
could  be  wished,  but  in  the  following  paragraphs  the  attempt  has 
been  made  to  summarize  such  data  as  are  accessible.  In  consider- 
ing these  results  it  should  be  remembered  that,  as  explained  on 
p.  396,  the  net  available  energy  means  the  energy  available  for 
maintenance.  In  a  considerable  number  of  the  experiments  to  be 
considered,  more  or  less  gain  was  made  by  the  animals,  but  it  seems 
better  to  give  the  results  of  each  series  of  experiments  in  full,  re- 
serving a  discussion  of  the  results  with  productive  rations  for  a 
subsequent  chapter. 

Experiments  on  Carnivora. — The  most  extensive  data  regarding 
the  metabolism  of  the  carnivora  in  its  relations  to  the  food  supply 
are  those  afforded  by  the  investigations  of  Pettenkofer  &  Voit  and 
of  Rubner.  These  have  already  been  considered  in  Chapter  V 
from  the  standpoint  of  matter  and  chiefly  in  a  qualitative  way; 
we  have  now  to  study  them  quantitatively  in  their  bearing  upon 
the  income  and  expenditure  of  energy  by  the  body. 

In  Pettenkofer  &  Voit's  experiments,  and  in  the  earlier  ones  by 
Rubner,  the  quantities  of  energy  involved  must  be  computed  from 
the  chemical  data.  In  Rubner's  experiments  upon  the  source  of 
animal  heat,  cited  in  Chapter  IX,  the  actual  heat  production  of  the 
animals  was  determined,  but  in  no  case  was  there  a  direct  determi- 
nation of  the  total  income  and  expenditure  of  energy,  and  in  par- 
ticular the  data  as  to  the  energy  of  the  food  are  incomplete.     For 


4i4 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


the  study  of  replacement  values  by  Rubner's  method  the  latter 
factor  was  not  necessary,  but  for  a  determination  of  the  percentage 
availability  of  the  energy  of  the  food  it  is  indispensable.  In  the 
following  paragraphs  the  necessary  computations  of  energy  have 
been  made  by  the  writer,  using  Rubner's  factors  so  far  as  possible.* 

In  the  case  of  Pettenkofer  &  Voit's  experiments  the  average 
results  given  in  Chapter  V  have  been  made  the  basis  of  the  compu- 
tation. 

Proteids. — From  the  average  results  obtained  by  Pettenkofer 
&  Voit  f  with  different  amounts  of  lean  meat  (see  p.  104),  the  met- 
abolizable  energy  of  the  food  and  of  the  resulting  gain  (or  loss)  by 
the  body  may  be  computed  as  follows : 


Metabolizable 

Energy  of 

Food, 

Cals. 

Computed  Heat  Production. 

Gain  by 

Food, 
Grms. 

From 

Proteids, 

Cals. 

From  Fat, 
Cals. 

Total, 
Cals. 

Body, 
Cals. 

0 
500 
1000 
1500 

0 
442 
883 
1325 

146 
530 
954 
1325 

895 
443 
179 
-38 

1041 
973 
1133 
1287 

-1041 
-531 

-250 

+  38 

*  The  following  factors  were  used  in  computing  these  experiments: 
Metabolizable  Energy  of  Food  : 

Bacon  (Speck),  92.2  per  cent,  fat  (Zeit.  f.  Biol.,  30,  138). 

1  grm.  pork  fat,  9.423  Cals.  (ibid.,  21,  333). 

1  grm.  butter  fat,  9.216  Cals.  (U.  S.  Dept.  Agr.,  Office  of  Expt.  Stations, 
Bull.  21,  p.  127). 

1  grm.  cane-sugar,  4.001  Cals.  (Zeit.  f.  Biol.,  21,  266). 

1  grm.  grape-sugar,  3 .  692  Cals.  (Stohmann,  Zeit.  f .  Biol.,  22,  40). 

1  grm.  starch,  4 .  123  Cals.  (Stohmann,  ibid.,  19,  376). 

Fresh  lean  meat,  3.4  per  cent,  nitrogen. 

1  grm.  nitrogen  in  meat,  25.98  Cals.  (Zeit.  f.  Biol.,  21,  321). 

1  grm.  nitrogen  in  syntonin,  26.66  Cals.  (ibid.,  21,  309). 
Energy  of  Metabolism  : 

1  grm.  excretory  nitrogen  (urine  and  feces). 

(a)  No  proteids  fed  : 

Birds,  24.35  Cals.  (Zeit.  f.  Biol.,  19,  367). 
Mammals,  24.94  Cals.  (ibid.,  22,  43). 

(b)  Meat  fed,  25.98  Cals.  (Ibid.). 

(c)  Syntonin  fed,  26.66  Cals.  (ibid.). 

1  grm.  carbon  in  fat,  12.31  Cals.  (ibid.). 
t  Zeit.  f.  Biol.,  7,  489. 


NET  AVAILABLE  ENERGY— MAINTENANCE. 


415 


As  compared  with  the  fasting  state,  the  883  Cals.  of  metaboliz- 
able  energy  supplied,  for  example,  in  1000  grams  of  meat  diminished 
the  loss  of  energy  by  the  body  by  1041  —  250  =  791  Cals.  The  latter 
quantity,  then,  represents  the  extent  to  which  the  883  Cals.  supplied 
in  the  food  aided  in  maintaining  the  stock  of  potential  energy  in  the 
body,  while  the  remaining  92  Cals.  was  consumed  in  the  work  of 
digestion  and  assimilation  as  defined  on  previous  pages;  that  is,  it 
increased  by  this  amount  the  heat  production  of  the  animal.  Ac- 
cordingly we  compute  that  in  this  case  89.6  per  cent,  of  the  metabo- 
lizable  energy  of  the  meat  was  available,  while  the  digestive  work 
consumed  10.4  per  cent.  Computing  the  other  experiments  in  the 
same  way  we  have — 


Metabolizable 
Energy  of 
Food,  Cals. 

Gain  Over 

Fasting 

Metabolism, 

Cals. 

Net  Avail- 
ability, 
Per  Cent. 

442 
883 
1325 

510 

791 

1079 

115.4 
89.6 
81.5 

From  Rubner's  experiments  *  with   proteids  (see  p.  106)  the 
following  figures  are  computed  in  the  same  manner  as  those  above : 


Food, 
Grms. 

Metab- 
olizable 
Energy 
of  Food, 
Cals. 

Heat 
Produc- 
tion, 
Cals. 

Gain. 

Net 
Avail- 
ability, 

Per 
Cent. 

Total, 
Cals. 

Over 
Fasting 
Metab- 
olism, 

Cals. 

Tem- 
perature, 
Deg.  C. 

Meat -j 

«    J, 

0 
415 

0 
740 

0 
740 

0 
390 
350 

0 
580 

0 
367 

0 
654 

0 
939 

0 
347 
309 

0 
512 

573* 
596* 
793* 
825* 
931* 
959* 
261f 
334f 
379f 
528f 
681f 

-573 
-229 
-793 
-171 
-931 

-  20 
-261 
4-   13 

-  70 
-528 
-169 

19.2 

344 

93.74 

19.6 
18.0 

622 

95.15 

19.2 
14.9 

Extracted  meat  j 
Meat \ 

« r 

911 

274 
191 

359 

97.03 

78.98 
61.80 

70.12 

15.6 

1 

Computed.  f  Calorimetric  determination. 

*  Zeit.  f.  Biol.,  22,  43-48;  30,  117-135. 


416 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


To  the  above  results  we  may  add  those  of  Magnus-Levy's  deter- 
minations (p.  381)  of  the  work  of  digestion  and  assimilation  in  the 
dog  on  a  meat  diet  as  follows : 


Metabolizable 

Expended  in 

Digestion  and 

Assimilation, 

Cals. 

Net  Available. 

Grms.                        Food, 
Cals. 

Total, 
Cals. 

Per  Cent. 

82.5 
230.0 
370.6* 

33S 
943 
1520 

56 
116 
244 

282 
827 
1276 

83.43 
87.70 
83.95 

*  In  excess  of  maintenance  requirements. 

The  wide  range  of  the  results  obtained  by  Rubner  would  seem  to 
indicate  either  that  the  net  availability  of  the  energy  of  the  pro- 
teids  may  vary  with  different  animals  and  under  different  conditions 
or  that  the  experimental  methods  were  not  sufficiently  sharp  for  the 
purpose  now  in  view.  The  value  of  an  average  drawn  from  such 
results  is  questionable,  but  for  the  sake  of  comparison  it  is  included 
below  along  with  those  derived  from  Voit's  and  Magnus-Levy's 
experiments,  Voit's  first  result  being  omitted  because  impossible. 
The  figures  express  the  average  net  availability  as  a  percentage 
of  the  metabolizable  energy. 

Voit's  experiments 85.60  per  cent. 

Rubner's  experiments 82.80   "       " 

Magnus-Levy's  experiments 85. 03   "       " 

Fat. — Computing  the  results  obtained  by  Pettenkofer  &  Voit  * 
and  by  Rubner  f  upon  the  effects  of  fat  on  the  total  metabolism 
(see  pp.  144-146)  in  the  same  manner  as  those  upon  the  proteids, 
and  adding  Magnus-Levy's  results  (p.  379),  we  have  the  table 
opposite. 

Rubner's  and  Magnus-Levy's  results  do  not  differ  widely,  and 
their  average,  96.4  per  cent.,  indicates  a  relatively  small  expendi- 
ture of  energy  in  the  digestion  and  assimilation  of  fat,  which  does 
not  appear  to  materially  increase  above  the  maintenance  require- 
ment. Most  of  Pettenkofer  &  Voit's  experiments  give  materially 
lower  results  above  that  point,  and  the  one  case  in  which  the  food 

*  Zeit.  f.  Biol.,  5,  370;  7,  440-443;  9,  3-13. 
t  Ibid.,  19,  328-334;  30,  123. 


NET  AVAILABLE  ENERGY-MAINTENANCE. 


417 


Metab- 
olizable 

Energy 

of  Food , 

Cals. 


Total, 

Cals. 


Over 
Fasting 
Metab- 
olism or 

Basal 
Ration, 

Cals. 


Net 
Avail- 
ability, 
Per  Cent. 


Pettenkofer  &  Voit's  Experiments  : 

Nothing 

100  grms.  fat 

350     "       fat  

500     "       meat 

500     "       meat;  100  grms.  fat  . 
500     "  "       200       "       "    . 

Rubner's  Experiments  : 

Nothing 

200  grms.  bacon 

Nothing 

39 .  75  grms.  butter  fat 

Nothing 

100  grms.  fat 

Nothing 

40  grms.  bacon 

Magnus-Levy's  Experiments  : 

Fasting 

131 . 6  grms.  fat 

Fasting 

305 . 5  grms.  fat 


0 

942 

3298 

442 

1384 

2326 


0 
1738 

0 
356 

0 
942 

0 
348 


0 
1250 

0 
2902 


-1086 
-275 
+  878 
-554 
+  329 
+837 


-658 

+  1016 

-373 

-17 
-466 
+428 
-261 

+  49 


-972 
+  259 
-1055 

+  1760 


811 
1964 


883 
1391 


1674 
356 
894 
310 

1231 
2815 


86.1 
59.6 


93.7 

73.8 


100.0 
94.9 
89.1 

98.5 
97.0 


supply  was  below  the  amount  required  for  maintenance  also  gives 
a  rather  low  availability  as  compared  w  th  that  obtained  by  the 
other  experimenters. 

Carbohydrates. — Tabulating  as  in  the  previous  cases  the  re- 
sults of  Pettenkofer  &  Voit  *  and  of  Rubner  f  (see  pp.  146-152), 
and  adding  those  of  Magnus-Levy  (p.  380),  we  have  the  figures 
shown  on  the  next  page. 

As  was  the  case  with  fat,  most  of  Pettenkofer  &  Voit's  experi- 
ments give  figures  notably  lower  than  those  obtained  by  the  other 
two  investigators.  The  averages  of  the  latter,  omitting  the  figures 
which  exceed  100  per  cent.,  are: 

Rubner's  experiments 88 . 9  per  cent. 

Magnus-Levy's  experiments 91. 0   "       " 


*  Zeit,  f.  Biol.,  9,  485. 


t  Ibid.,  19,  357-379;    22,  273. 


4i8 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Food. 


Pettenkofer  &  Voit's  Experiments  : 

Nothing* 

450  grms.  starch;  16.9  grms.  fat 
597     "  "       21.2     " 

700     "  "       20.2     "        " 

500     "       meat 

500     "       meat;  200    grms.    starch 
500  grms.  meat ;  200  grms.  dextrose 


Rubner's  Experiments  : 

Nothing 

76 .  12    grms.  cane-sugar 
104.97 


Nothing 

97.3  grms.  cane-sugar 

17.0     "  "        "      

143.0     "  "        "      

Nothing 

42 .  96  grms.  starch  (digested) . 

Nothing 

57 .  38  grms.  starch  (digested) . 

Nothing 

94.36  grms.  cane-sugar;  67.96  grms. 

starch;  4.7  grms.  fat 

300  grms.  meat;  63 . 7  grms.  dextrose 
300    "         "       79.7    " 
300    "         "     115.5    " 

Magnus-Levy's  Experiments  : 

Chiefly  rice 


Metab- 
olizable 
Energy 
of  Food, 
Cals. 

Gain. 

Total, 
Cab. 

Over 

Fasting 
Metab- 
olism or 

Basal 
Ration, 

Cals. 

0 
2015 
2661 
3076 

-1098 
+  353 
-198 
+  853 

1451 
900 
1951 

442 
1316 
1180 

-554 

+  137 
+  108 

691 
662 

0 
305 
420 

-436 

-116 

-22 

320 

414 

0 

389 

68 

572 

-451 

-87 
-374 
+  190f 

364 

77 
641 

0 

177 

-302 
-138 

164 

0 
244 

-354 
-140 

214 

0 

-302 

702 

+  365f 

667 

500 
559 
691 

-126 

-84 
+  34 

42 
160 

2121 

2226 

999 

+850 

+  934 

-81 

1890 

2066 

910 

Net 
Avail- 
ability, 
Per  Cent. 


72.0 
33.8 
63.4 

79.1 
89.7 


104.9 
98.6 

93.6 
113.2 
112.0 

92.6 

87.8 


95.0 


71.lt 
83.  n 


89.1 
92.8 
91.1 


*  Fasting  metabolism  estimated  from  previous  experiments, 
t  Gain  of  carbon  assumed  to  be  all  in  the  form  of  fat. 
%  Of  dextrose  added. 


Experiments  on  Herbivora. — Comparatively  few  experiments 
have  been  reported  from  which  the  net  availability  of  the  food  of 
herbivorous  animals  can  be  computed,  and  as  regards  the  common 
farm  animals  in  particular  there  is  an  almost  entire  lack  of  data, 
although  numerous  experiments  upon  the  relative  value  of  various 


NET  AVAILABLE  ENERGY-MAINTENANCE. 


419 


materials  for  productive  feeding  have  been  reported  and  will  be 
considered  in  the  following  chapter. 

Fat. — Rubner's  experiments  include  one  *   in  which   fat  was 
fed  to  a  rabbit  with  the  following  results : 


Metabolizable  energy  of  food  . 

Total  gain 

Gain  over  fasting  metabolism 
Net  availability  


Fasting. 


0      Cals. 
-101     " 


Fed  26 . 1  Grms. 
Bacon. 


227  Cals. 
+ 122     " 
223     " 

98. 2# 


In  connection  with  his  investigations  upon  cellulose,  v.  Knie- 
riem  f  also  experimented  upon  the  influence  of  fat  on  the  metabo- 
lism of  the  rabbit.  The  basal  ration  consisted  of  milk,  to  which 
was  added  in  the  second  period  3.94  grams  of  dry  butter  fat  per  day. 
Computing  the  amounts  of  energy  by  the  use  of  Rubner's  factors 
the  results  were : 


Metabolizable 
Energy,  Cals. 

Gain,  Cals. 

Net  Availability, 
Per  Cent. 

207.3 
169.8 

-19.5 
-55.2 

Milk 

Difference 

37.5 

35.7 

95  2 

Carbohydrates. — Rubner  %  reports  three  experiments  with 
cane-sugar  on  a  cock  from  which  the  following  results  are  com- 
puted : 


Metaboliz- 
able Energy 
of  Food, 
Cals. 

Gain. 

Net 

Food. 

Total, 
Cals. 

Over  Fast- 
ing Metab- 
olism, Cals. 

Availability, 
Per  Cent. 

0 
136 

0 
180 
200 

-239 
-121 
-258 
-101 
-  53 

118 

157 
205 

34  grms.  cane-sugar 

86.8 

45  grms.  cane-sugar 

50     "         "       "       

87.2 
102.5 

*  Zeit.  f.  Biol.,  19,  333 

t  Ibid 

,  21,  119. 

%  Ibid.,  19 

,366. 

420 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


From  the  comparisons  of  cellulose  and  cane-sugar  made  by  v. 
Knieriem  (loc.  cit.)  and  cited  on  p.  161,  the  following  figures  for  the 
net  availability  of  the  energy  of  the  latter  substance  may  be  com- 
puted : 


■g 

3 

n 

"5 
6 

Food  per  Day. 

Metab- 
olizable 
Energy 
of  Food, 
Cals. 

Gain. 

Net 

w 

Total, 
Cals. 

Over 

Basal 

Ration, 

Cals. 

Avail- 
ability, 
Per  Cent. 

TTT 

5 

4 
3 

Milk...                              

350.1 
393.7 
480.7 

-37.9 
-15.9 
+  69.9 

22.0 
107.8 

IV 
V 

"    + 1 1  grms.  cane-sugar .  . 
"    +33     "         "         "      .. 

50.5 

82.5 

A  series  of  experiments  by  May  *  upon  the  effect  of  fever  on 
metabolism  affords  incidentally  a  few  data  bearing  on  the  availa- 
bility of  the  energy  of  dextrose.  In  his  experiment  No.  5  {loc.  cit., 
p.  23)  the  ingestion  of  30  grams  of  grape-sugar,  an  amount  approxi- 
mately equivalent  to  the  fasting  metabolism,  caused  no  increase  in 
the  computed  heat  production  as  compared  with  that  during  fasting. 
In  this  experiment  there  was  no  fever.  In  Experiment  No.  6  (p.  25)-, 
with  fever,  the  ingestion  of  the  same  amount  of  grape-sugar  pro- 
duced a  computed  gain  of  2.88  grams  carbon  as  fat,  but  caused  no 
increase  in  the  computed  heat  production.  Experiment  No.  7 
(p.  26)  was  similar  to  No.  6,  but  showed  a  decrease  in  the  computed 
heat  production,  which,  however,  coincided  with  a  decrease  in  the 
fever.  On  the  whole,  May's  results  appear  in  accord  with  Rubner's 
hypothesis  of  a  substitution  of  the  heat  resulting  from  digestive 
work  for  that  arising  from  the  metabolism  of  tissue. 

Pentoses. — Cremer's  experiments  f  with  rhamnose  upon  rabbits, 
cited  in  Part  I,  p.  157,  afford  data  for  computing  the  net  availa- 
bility of  this  representative  of  the  pentoses.  For  this  purpose 
Cremer  computes  from  the  excretion  of  nitrogen  and  carbon  (neg- 
lecting the  feces),  in  the  manner  described  in  Chapter  VIII,  p.  253, 
the  amount  of  energy  liberated  by  the  metabolism  of  protein  and 
fat  in  the  body,  assuming  that  the  rhamnose,  after  deducting  the 
small  amounts  in  feces  and  urine,  was  completely  oxidized.  The 
following  are  the  results  for  each  day  of  the  four  experiments : 


*  Zeit.  f.  Biol.,  30,  1. 


t  Ibid.,  42,  451. 


NET  AVAILABLE  ENERGY— MAINTENANCE. 


421 


Food,  Grms. 

Metabolizable 

Energy  of  Food, 

Cals. 

Loss  by  Body, 
Cals. 

Experiment  I  : 

0 

45.3 

0 

0 

66.8 
0 

0 

74.1 

0 

0 

0 

0 

72.9 
22.0 

147.4 
114.0 
113.3 

180.7 
111.6 

184.8 

129.1 

54.3 

113.1 

113.4 

146.0 
141.4 
53.3 
98.1 

Rhamnose,  11.584  grms 

Experiment  II : 

Rhamnose,  17.09  grms 

Experiment  HI : 

Nothing 

Rhamnose,  18 .96  grms 

"        (av'ge  of  two  days) 

Experiment  IV  : 

Nothing 

Rhamnose,  18 .  66  grms 

5.64*  "     

*  The  total  amount  of  rhamnose  (24 . 3  grms.)  was  given  on  the  first  day, 
but  it  is  estimated  from  the  results  for  the  carbon  excretion  that  this  amount 
of  it  was  not  metabolized  until  the  second  day. 

The  results  as  to  net  availability  obtained  by  comparison  with 
the  several  fasting  days  vary  considerably,  as  the  following  state- 
ment shows,  several  of  them  exceeding  100  per  cent. : 

Experiment  I. 

Compared  with  first  day 74  per  cent. 

"  "    third  day Negative 

Experiment  II. 

Compared  with  first  day 103  per  cent. 

"    third  day 110   "       " 

Experiment  III. 

Compared  with  first  day 101  per  cent. 

"    third  day 79   "       " 

Second  and  third  with  fourth  and  fifth  days.  80   "       " 

Experiment  IV. 

Third  compared  with  second  day 121  per  cent. 

"    fourth  day 88   "       " 


422 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


The  great  variations  in  the  results,  as  well  as  the  large  propor- 
tion of  cases  in  which  the  availability  appears  to  exceed  100  per 
cent.,  show  that  little  value  attaches  to  them  as  quantitative  deter- 
minations, although  they  undoubtedly  show  that  rhamnose  pos- 
sesses a  comparatively  high  nutritive  value. 

Crude  Fiber. — The  experiments  of  v.  Knieriem  have  already 
been  cited  in  Chapter  V  in  their  general  bearings  upon  the  metabo- 
lism of  matter.  As  was  there  noted,  certain  corrections  were  neces- 
sary on  account  of  the  residue  of  undigested  cellulose  remaining 
in  the  digestive  canal  at  the  close  of  the  experiment.  The  results 
given  below  are  based  on  those  computed  by  the  author,  as  sum- 
marized on  p.  161,  on  the  assumption  that  the  resorption  of  the 
remaining  digestible  crude  fiber  was  complete  after  two  days. 


1 
0 

"o 

i 

Food  per  Day. 

Metab- 
olizable 
Energy 
of  Food, 
Cals. 

Gain. 

Net 

i 
I 

Total, 
Cals. 

Over 

Basal 

Ration, 

Cals. 

Avail- 
ability, 
Per  Cent. 

T 

9 
10 

5 

Milk 

341.7 
374.6 
350.1 

-46.8 
-  6.9  | 
-37.9 

39^9 
31.0 

II 
TTT 

"    +22  grms.  crude  fiber  j 

for  eight  davs \ 

Milk 

121.3* 
126. 5t 

*  Compared  with  Period  I. 


t  Compared  with  Period  III. 


It  is  evident  from  the  above  figures  that  while  the  experiments 
show  qualitatively  a  nutritive  value  for  the  cellulose,  they  are  in- 
sufficient for  a  quantitative  determination  of  its  amount. 

In  striking  contrast  with  these  results  are  the  conclusions  drawn 
by  Zuntz  &  Hagemann  from  their  experiments  upon  the  horse 
which  have  already  been  considered  in  the  previous  chapter  (pp. 
389-391).  As  was  there  explained  in  detail,  these  investigators 
have  estimated  the  expenditure  of  energy  in  the  digestion  of  crude 
fiber  from  a  comparison  of  the  computed  heat  production  in  two 
sets  of  experiments  in  which  the  proportion  of  coarse  fodder  eaten 
differed  considerably,  it  being  assumed  that  9  per  cent,  of  the  metab- 
olizable  energy  of  the  nutrients  other  than  crude  fiber  was  consumed 
in  their  digestion.  On  this  basis  the  digestive  work  caused  by  the 
crude  fiber  is  computed  at  2.086  Cals.  per  gram,  or  rather  more  than 


NET  AVAILABLE  ENERGY— MAINTENANCE.  423 

its  average  metabolizable  energy.  In  other  words,  it  is  computed 
that  under  the  conditions  of  these  experiments,  with  a  ration  more 
than  sufficient  for  maintenance,  the  net  availability  of  the  energy 
of  the  crude  fiber  was  practically  zero.  The  authors  report  no 
experiments  upon  rations  below  the  maintenance  requirement,  but 
appear  to  regard  the  metabolizable  energy  of  the  crude  fiber  as  being 
indirectly  available,  under  such  conditions,  substantially  in  the 
manner  assumed  by  Rubner  and  already  explained. 

As  has  been  noted,  Zuntz  &  Hagemann's  conclusions  as  to  the 
value  of  crude  fiber  for  work  production  are  in  apparent  harmony 
with  those  of  Wolff,  which  will  be  discussed  in  the  next  chapter,  but 
on  the  other  hand  they  contrast  sharply  with  the  results  of  Kellner 
(see  Chapter  XIII,  §  1),  who  observed  a  high  percentage  utilization 
of  the  energy  of  one  form  of  crude  fiber  in  the  ration  of  fatten- 
ing cattle.  On  previous  pages  some  reasons  were  presented  for 
questioning  the  quantitative  accuracy  of  Zuntz  &  Hagemann's 
computations,  but  even  aside  from  these  their  conclusions  as  re- 
gards the  value  of  crude  fiber  are  difficult  to  reconcile  with  obvious 
facts.  Thus  they  compute  (loc.  cit,  p.  280)  that  the  expenditure 
of  energy  in  the  mastication  and  digestion  of  average  straw  is 
greater  than  its  metabolizable  energy,  so  that  for  the  horse  this 
material  has  a  negative  value.  When  forming  part  of  a  mainte- 
nance ration  we  may  probably  assume  that  below  the  critical 
amount  of  food  (p.  408)  the  heat  generated  during  the  digestion  of 
the  straw  would  be  of  use  to  maintain  the  body  temperature,  but 
this  could  not  possibly  suspend  the  expenditure  of  energy  in  the 
various  forms  of  internal  work,  such  as  respiration  and  circulation. 
Since,  however,  by  hypothesis,  the  straw  can  contribute  no  energy 
directly  for  these  purposes,  it  follows  that  the  consumption  of  this 
material  alone  cannot  reduce  the  loss  of  tissue  below  the  amount 
requisite  to  supply  energy  for  the  internal  work,  while  on  an 
exclusive  straw  ration  above  the  critical  amount  of  food  the  more 
straw  the  animal  consumed  the  sooner  it  would  starve. 

Organic  Acids. — The  results  of  a  considerable  number  of  ex- 
periments in  which  salts  of  organic  acids  were  mjected  into  the 
blood  have  already  been  presented  in  Chapter  V  (p.  157).  The 
general  result  was  that  lactic  and  butyric  acids  caused  little  or 
no  increase  in  the  heat  production  of  the  animal — in  other  words 


424 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


that  practically  all  their  potential  energy  was  available  to  prevent 
loss  of  tissue.  In  such  experiments,  of  course,  there  is  no  digestive 
work  in  the  proper  sense.  What  they  indicate  is  that  what  we 
have  called  rather  loosely  the  work  of  assimilation  for  these  sub- 
stances is  practically  zero.  Acetic  acid,  on  the  other  hand,  was 
found  by  Mallevre  to  increase  the  consumption  of  oxygen  by  from 
10  to  17  per  cent.,  indicating  a  considerable  waste  of  energy  directly 
or  indirectly.  The  general  nature  of  these  experiments  is  not  such, 
however,  as  to  afford  data  of  much  direct  value  in  relation  to  the 
question  of  the  availability  of  the  energy  of  ordinary  foods. 

Timothy  Hay. — The  experiments  described  in  the  foregoing 
paragraphs  relate  to  pure  or  nearly  pure  nutrients.  Experiments 
upon  a  steer  have  been  made  by  the  writer  in  conjunction  with 
Fries  in  which  the  availability  of  the  apparent  metabolizable  energy 
of  timothy  hay  has  been  determined.  To  a  basal  ration  consider- 
ably below  the  maintenance  requirement,  consisting  of  3250  grams 
of  hay  and  400  grams  of  linseed  meal,  three  different  additions  of 
timothy  hay  were  made,  the  digestibility  of  the  ration  in  each 
period  being  determined,  and  likewise  the  total  balance  of  nitrogen, 
carbon,  and  energy  by  means  of  the  respiration-calorimeter.  The 
results  as  to  energy,  uncorrected  for  the  very  small  differences  in 
the  organic  matter  of  the  basal  ration  consumed  and  for  the 
changes  in  the  live  weight  of  the  animal,  were  as  follows :  * 


Period  A. 

Period  B. 

Period  C. 

Period  D. 

Outgo, 
Cals. 

Income, 
Cals. 

Outgo, 
Cals. 

Income, 
Cals. 

Outgo, 
Cals. 

Income, 
Cals. 

Outgo, 
Cals. 

Income, 
Cals. 

Feed       

14,923 

8,590 

974 

1,251 

9,482 

20,297 

11,477 
1,125 
1,374 

11,222 

25,198 

29,647 

Excreta : 

Feces 

6,446 
863 
996 

6,618 

14,276 
1,220 
1,896 

12,255 

Methane 

Metabolizable  . .  . 

14,923 

14,923 
6,618 
2,449 

20,297 
10,206 

20,297 
9,482 
"724 

25,198 

10,606 
616 

25,198 
11,222 

29,647 

Yl',i83 
1,072 

29,647 
12,255 

Heat  produced  .  . 

9,067 

9,067 

9,067 

10,206 

10,206 

11,222 

1  1 .222 

12,255 

12,255 

*  Proc.  Soc.  Prom.  Agr.  Sci.,  1902. 


NET  AVAILABLE  ENERGY— MAINTENANCE. 


425 


Subtracting  the  results  on  the  basal  ration  of  Period  A  from  those 
of  the  other  periods,  as  in  previous  cases,  we  have  the  following : 


Metabolizable 

Energy, 

Cals. 

Gain  of  Tissue, 
Cals. 

NetAvailability, 
Per  Cent. 

Period  B 

9,482 
6,618 

-724 
-2,449 

A 

2,864 

11,222 
6,618 

1,725 

616 
-2,449 

60   24 

Period  C 

A 

4,604 

12,255 
6,618 

3,065 

1,072 
-2,249 

66.57 

Period  D 

A 

Difference 

5,637 

3,521 

62  46 

63  09 

Strictly  speaking,  only  the  first  of  the  above  percentages  repre- 
sents the  net  availability  for  maintenance,  since  the  other  two 
include  some  gain.  From  the  difference  observed  between  the 
metabolism  of  the  animal  standing  and  lying,  however,  it  was 
computed  approximately  what  the  gains  would  have  been  had  the 
same  position  been  maintained  for  the  whole  twenty-four  hours, 
with  the  following  results: 


Metabolizable 
Energy,  Cals. 

Gain,  Standing, 
Cals. 

Net  Availability, 
Per  Cent. 

Period   B 

9,482 
6,618 

-1,606 
-3,507 

"      A 

2,864 

11,222 
6,618 

1,901 

-550 
-3,507 

66  37 

Period   C 

"      A 

4,604 

12,255 

6,618 

2,957 

23 

-3,507 

64  23 

Period  D 

"      A 

5,637 

3,530 

62  62 

Average 

64.41 

426 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Metabolizable 
Energy,  Cals. 

Gain,  Lying, 
Cals. 

Net  Availability, 
Per  Cent. 

Period  B 

9,482 
6,618 

1,157 
-1,046 

"      A 

2,864 

11,222 
6,618 

2,203 

2,136 
-1,046 

76  92 

Period   C 

"      A 

4,604 

12,255 
6,618 

3,182 

2,743 
-1,046 

69   12 

Period  D 

"      A 

5,637 

3,789 

67  22 

71  08 

The  results  are  likewise  shown  graphically  on  the  accompanying 
diagram,  in  which  the  full  line  represents  the  average  availability 


7000 


000  9000  10000  11000 

METABOLIZABLE  ENERGY,  CALS. 


NET  AVAILABLE  ENERGY— MAINTENANCE. 


427 


observed  and  the  broken  lines  that  computed  respectively  for 
standing  and  lying,  while  the  points  indicate  the  results  for  each 
period.  As  computed  standing,  the  results  are  all  practically  at  or 
below  the  maintenance  point,  and  their  fairly  close  agreement  with 
each  other  and  with  those  actually  observed  indicates  that  the  net 
availability  of  the  metabolizable  energy  of  this  sample  of  timothy 
hay  was  between  63  and  65  per  cent. 

Summary. — The  foregoing  data  as  to  availability  are  sum- 
marized in  the  following  table,  those  experiments  in  which  the  total 
ration  was  less  than  the  maintenance  requirement  being  separated 
from  those  in  which  more  or  less  gain  by  the  body  took  place : 


Experiments  on  Car> 
Below  Maintenance 
Proteids  : 

•1 
•1 

rORA. 

Per  Cent. 
115.4 

89.6 
r    93.7 
95.2 
97.0 
61.8 
.    70.1 
83.4 
87.7 

86.1 

Experiments  on  Herbiv 
Below  Maintenance. 

Fat: 
v.  Knieriem 

Cane-sugar  : 

ORA. 

Rubner 

Magnus-Levy  ...... 

Fat: 

Per  Cent. 
.     95  2 

Rubner 

Starch  : 

Pettenkofer  &  Voit. 

100.0 

33.8 
92.6 

87.8 
91.1 

71.3 

'104.9 
98.6 

Rubner 

Magnus-Levy  (rice). 

Dextrose  : 

Rubner 

Cane-sugar  : 

;i 

I     86.8 
■<     87  2 

Rubner 

Starch  and  Cane-sugar . 
Rubner 

•  "j     93.6 
[113.2 

. .     95.0 

v.  Knieriem .  . 

I  102.5 
50.5 

428 


PRINCIPLES  OF  ANIMAL  NUTRITION. 


Pentoses  : 


-    74.0 

(?) 

103.0 

110.0 

101.0 

79.0 

80.0 

121.0 

88.0 

Crude  Fiber : 

v.  Knieriem - 

121.3 
126.5 

Timothy  Hay: 
Armsby  &  Fries 

63-65 

Above  Maintenance. 
Proteids  : 

Pettenkofer  &  Voit 

Per  Cent. 
.     81.5 
.     79.0 
.     84.0 

Above  Maintenance. 

Fat: 

Pettenkofer  &  Voit 

t    59.6 

.  {    93.7 

'    73.8 

/    98.6 

.  \    94.9 

(   89.1 

(    98.5 

'1    97.0 

Fat: 

Rubner 

Per  Cent. 
.     98.2 

Magnus-Levy 

Starch  : 

Pettenkofer  &  Voit 

(   72.0 

.  \   63.4 

I   79.1 

Magnus-Levy  (rice) 

i    89.1 
'  (    92.8 

Dextrose  : 

Pettenkofer  &  Voit 

Rubner 

..     89.7 
..     83.7 

Cane-sugar : 

Rubner 

..    112.0 

Cane-sugar : 
v.  Knieriem 

.     82.  t 

Cane-sugar  and  Starch  : 
Rubner 

..     95.0 

NET  AVAILABLE  ENERGY— MAINTENANCE.  429 

It  scarcely  seems  possible  to  draw  any  well-founded  conclusions 
regarding  the  net  availability  of  the  several  nutrients  from  such 
widely  divergent  results  as  those  tabulated  above,  even  if  the  ex- 
treme and  obviously  incorrect  figures  be  discarded.  Two  things, 
however,  seem  worthy  of  remark. 

First,  in  but  few  cases  does  the  net  availability  of  the  food  reach 
100  per  cent.,  and  most  of  those  results  relate  to  cane-sugar  or  rham- 
nose;  that  is,  to  cases  in  which  some  of  the  gain  of  carbon  which  is 
computed  as  fat  may  have  been  in  the  form  of  a  carbohydrate.  It 
would  seem  fairly  safe  to  conclude,  therefore,  that  no  such  complete 
substitution  of  the  heat  resulting  from  digestive  work  for  that  re- 
sulting from  the  general  metabolism  took  place  as  Rubner's  hypoth- 
esis supposes.  Apparently,  under  the  conditions  of  these  experi- 
ments, there  was,  in  most  cases  at  least,  a  material  loss  of  energy 
in  digestive  work. 

Second,  there  is  no  clear  indication  of  a  smaller  loss  of  energy 
below  than  above  the  maintenance  ration,  although  the  wide  range 
of  the  results  renders  a  definite  conclusion  upon  this  point  hazardous. 
This  question,  however,  may  be  more  properly  considered  in  con- 
nection with  a  study  of  the  utilization  of  the  net  available  energy  of 
the  food. 

Finally,  it  is  to  be  said  that  if  the  validity  of  the  conception  of 
a  critical  amount  of  food,  as  developed  on  p.  409,  be  admitted — 
that  is,  of  an  amount  of  food  below  which  the  heat  resulting  from 
the  work  of  digestion  and  assimilation  is  substituted  for  that  pro- 
duced by  the  general  metabolism,  while  above  it  no  such  substitu- 
tion takes  place — a  very  important  element  is  lacking  for  the 
interpretation  of  the  above  experiments,  except,  perhaps,  those  on 
timothy  hay,  in  which  the  uniformity  of  the  results  with  varying 
amounts  seems  to  show  clearly  that  all  the  rations  supplied  more 
than  the  critical  amount  of  food.  If  that  conception  is  correct, 
to  determine  the  real  availability  of  the  energy  of  a  food  it  is 
necessary  to  compare  the  effects  of  two  quantities  both  of  which 
are  greater  than  the  critical  amount.  On  the  other  hand,  the 
complete  substitution  of  energy  supposed  by  Rubner  could  only 
be  demonstrated  by  comparing  quantities  less  than  the  critical 
amount,  while  a  comparison  of  quantities  below  the  latter 
amount  (including,   of    course,   fasting)  with   those   exceeding    it 


43°  PRINCIPLES   OF  ANIMAL   NUTRITION. 

can  give  only  mixed   results  varying  with   the    quantities   com- 
pared. 

It  seems  tolerably  clear,  then,  that  the  whole  subject  of  the  net 
availability  of  foods  and  nutrients  needs  reinvestigation  by  more 
rigorous  methods  and  with  due  regard  to  the  amounts  of  the  food 
materials  compared  and  to  the  thermal  environment  of  the  animals 
experimented  upon. 

Discussion  of  Results. 

For  the  reasons  just  stated,  any  strict  quantitative  discussion 
of  the  above  results  seems  impossible.  At  the  same  time,  certain 
general  conclusions  may  be  at  least  tentatively  deduced  from  them 
which,  even  though  to  a  considerable  extent  speculative,  may  at 
least  serve  provisionally  as  a  connecting  thread  between  the  known 
facts. 

Influence  of  Amount  of  Food  on  Availability. — In  the  fore- 
going paragraphs  it  has  been  tacitly  assumed  that  the  amount  of 
food  eaten  has  no  influence  on  its  availability,  or,  to  state  it  in 
another  way,  that  the  expenditure  of  energy  in  digestion  and  assimi- 
lation is  proportional  to  the  quantity  of  food.  To  express  the  same 
thing  in  mathematical  terms,  we  have  assumed,  in  constructing  the 
diagram  on  p.  410,  that  the  net  available  energy  is  a  linear  function 
of  the  metabolizable  energy. 

While  it  seems  highly  probable  that  such  is  the  case  the  only  ex- 
periments bearing  specifically  upon  this  question  of  which  the  writer 
is  aware  are  those  upon  timothy  hay  just  cited.  An  examination 
of  the  graphic  representation  of  the  results  strongly  supports  the 
hypothesis  that  the  net  availability  of  the  food  is  independent  of 
its  amount,  but  the  evidence  of  so  few  experiments  must  naturally 
be  accepted  with  some  reserve.  The  other  recorded  results,  as 
computed  above,  apart  from  the  possible  source  of  uncertainty 
pointed  out  on  p.  429,  show  such  considerable  variations  in  indi- 
vidual cases  that  it  scarcely  seems  possible  to  reach  any  definite 
conclusions  from  them  regarding  the  influence  of  quantity  of  food. 
As  will  appear  in  the  next  chapter,  the  extensive  respiration  exper- 
iments made  in  recent  years  at  the  Mockern  Experiment  Station  by 
G.  Kiihn  and  O.  Kellner  upon  fattening  cattle  indicate  that  the 


NET  AVAILABLE  ENERGY— MAINTENANCE.  43 1 

actual  gain  obtained  (expressed  in  terms  of  energy),  at  least  within 
certain  limits,  is  proportional  to  the  amount  of  metabolizable  energy 
supplied  in  excess  of  maintenance.  This  would  mean  that  above 
the  maintenance  ration  the  energy  required  for  digestion  and 
assimilation  plus  that  consumed  in  the  chemical  changes  incident 
to  the  formation  of  new  tissue  (compare  p.  396)  is  proportional 
to  the  amount  of  food.  If  this  be  true  it  seems  more  reasonable 
to  conclude  that  each  of  these  forms  of  work  separately  is  propor- 
tional to  the  amount  of  food  than  to  assume  a  compensation  between 
the  two,  and  granting  this,  we  should  have  every  reason  to  suppose 
that  the  same  proportionality  would  hold  good  for  the  work  of 
digestion  and  assimilation  below  the  maintenance  requirement. 

Character  of  Food. — The  investigations  of  Zuntz  &  Hagemann 
(pp.  385-393)  have  shown  that,  in  the  case  of  the  horse  at  least 
and  doubtless  with  other  animals  also,  the  work  of  digestion  and 
assimilation  varies  with  the  kind  of  food,  a  result  which  is  entirely 
in  accordance  with  what  we  should  expect.  For  reasons  stated  in 
describing  their  experiments,  their  results  are  to  be  regarded  as 
qualitative  rather  than  quantitative,  but  they  suffice  to  demon- 
strate the  very  marked  difference  as  regards  availability  which 
exists  between  the  relatively  pure  nutrients  employed  in  the  exper- 
iments of  Pettenkofer  &  Voit,  Magnus-Levy,  Rubner,  and  others 
and  the  feeding-stuffs  consumed  by  our  herbivorous  domestic 
animals,  and  to  show  the  fallacy  involved  in  applying  the  results 
of  the  former  experiments  directly  to  the  latter  case.  The  same 
conclusion  is  also  indicated  by  the  few  results  upon  timothy  hay 
on  p.  424. 

Unfortunately  no  other  direct  determinations  of  the  availability 
of  the  food  of  herbivorous  animals  in  amounts  below  the  mainte- 
nance ration  are  on  record,  so  that  we  are  unable  to  compare  either 
different  feeding-stuffs  or  different  species  of  animals  in  this  respect. 
The  extensive  investigations  of  the  Mockern  Experiment  Station 
mentioned  in  the  previous  paragraph  show  how  large  a  proportion 
of  the  metabolizable  energy  of  the  food  of  fattening  animals  becomes 
economically  waste  energy,  thus  fully  confirming  the  conclusions 
drawn  from  Zuntz  &  Hagemann's  experiments  upon  the  horse,  but 
they  afford  no  means  of  distinguishing  between  the  work  of  diges- 


432  PRINCIPLES   OF  ANIMAL    NUTRITION. 

tion  and  assimilation  and  the  energy  expended  in  converting  the 
resorbed  material  into  permanent  tissue. 

The  Maintenance  Ration. — As  already  defined,  net  available 
energy  is  that  portion  of  the  metabolizable  energy  of  the  food 
which  serves  to  make  good  the  losses  of  potential  energy  arising 
from  the  internal  work  plus  the  work  of  digestion  and  assimilation, 
or,  in  other  words,  which  contributes  towards  the  maintenance  of 
the  stored-up  capital  of  energy.  We  may,  therefore,  appropriately 
consider  the  bearings  of  the  known  facts  regarding  availability 
upon  the  amount  of  food  required  for  maintenance. 

Relations  to  Availability. — Not  a  little  effort  has  been 
expended  in  determining  the  maintenance  requirements  of  farm 
animals  on  the  more  or  less  tacit  assumption  that  this  quantity  is 
a  constant  for  the  same  animal,  and  the  same  assumption  has  even 
more  largely  controlled  in  computations  based  on  the  experimental 
data  obtained. 

By  the  maintenance  ration,  of  course,  we  understand  a  ration 
just  sufficient  to  prevent  any  loss  of  tissue — that  is,  of  potential 
energy — by  the  animal.  To  accomplish  this  we  must  give  a  ration 
containing  net  available  energy  equal  in  amount  to  the  potential 
energy  lost  when  no  food  is  given.  Expressed  thus  in  terms  of  net 
available  energy,  the  maintenance  requirement  under  given  condi- 
tions is  a  constant  and  is  equal  to  the  energy  of  the  fasting  metabo- 
lism. 

The  maintenance  requirement,  however,  particularly  in  the  case 
of  farm  animals,  has  not  usually  been  expressed  thus,  since  the 
necessary  data  are  lacking,  but  in  terms  of  total  digestible  matter 
or  of  real  or  supposed  metabolizable  energy.  When  thus  expressed, 
however,  it  is  apparent  in  the  light  of  the  foregoing  discussion  that 
the  maintenance  requirement  must  be  a  variable,  depending  upon 
the  availability  of  the  metabolizable  energy  of  the  food.  Referring 
again  to  the  graphic  representation  on  p.  410,  it  is  evident  that, 
under  the  conditions  there  represented,  with  an  availability  ex- 
pressed by  tan  DAC,  the  amount  of  metabolizable  energy  required 
for  maintenance  will  be  equal  to  OS.  Furthermore,  it  is  equally 
evident  that  as  the  availability  decreases  and  the  angle  DAC  con- 
sequently becomes  more  acute  OS  will  increase.  Only  when  the 
critical  amount  of  food,  OM,  is  greater  than  the  fasting  metabolism 


NET  AVAILABLE  ENERGY— MAINTENANCE.  433 

and  the  point  K  falls  above  the  axis  OX  will  there  be  an  apparent 
exception  to  this  law.  In  that  case,  since  the  energy  expended  in 
digestion  and  assimilation  seems  to  be  indirectly  utilized,  the  ap- 
parent availability  will  be  100  per  cent,  and  the  metabolizable 
energy  required  for  maintenance  will  be  constant  and  equal  to  the 
energy  of  the  fasting  metabolism. 

This  case  might  and  perhaps  does  occur  with  animals  whose  food 
consumes  little  energy  in  digestion,  such  as  the  carnivora.  As  was 
pointed  out  on  p.  412,  however,  an  increase  in  the  work  of  digestion 
tends  to  reduce  the  critical  amount  of  food  and  there  would  appear 
to  be  good  reason  for  believing  that,  in  ruminants  at  least  if  not  in 
the  horse,  it  lies  considerably  below  the  point  of  maintenance. 

Relative  Value  of  Grain  and  Coarse  Fodder. — We  know 
from  the  investigations  of  Zuntz  &  Hagemann  (pp.  385-393)  that 
the  work  expended  in  the  digestion  of  coarse  fodders  is,  in  the  horse 
and  presumably  therefore  in  other  animals,  materially  greater  than 
that  caused  by  grain.  It  follows,  then,  that  a  unit  of  digestible 
matter  or  of  metabolizable  energy  should  have  more  value  for 
maintenance  in  the  latter  than  in  the  former. 

That  such  is  the  case  with  cattle  is  rendered  probable  by  experi- 
ments by  the  writer.*  In  the  absence  of  a  respiration  apparatus 
the  nutritive  effect  of  the  rations  was  judged  of  from  the  live  weight 
and  the  proteid  metabolism  during  relatively  long  periods  and  the 
methane  production  was  computed  from  the  carbohydrates  digested. 
A  ration  in  which  only  about  24  per  cent,  of  the  digested  organic 
matter  was  derived  from  coarse  fodder,  as  compared  with  rations 
consisting  exclusively  of  coarse  fodder,  gave  the  following  results 
for  the  metabolizable  energy  of  the  maintenance  ration  per  day 
and  500  kgs.  live  weight: 

Exclusive  coarse  fodder,  12  experiments.  . . .   12,771  Cals. 
Largely  grain,  3  experiments 11,023     " 

Such  determinations  of  the  maintenance  requirements  of  the 
horse  as  have  been  made  tend  to  confirm  the  results  obtained  with 
ruminants.  Wolff,  in  his  investigations  upon  work  production 
described  in  the  following  chapter,  has  computed  the  maintenance 
requirements  of  the  horse  in  the  manner  there  explained  both  from 

*  Penna.  Expt.  Station,  Bull.  42,  p.  159. 


434 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


his  own  experiments  and  from  those  of  Grandeau  and  LeClerc, 
with  the  following  results  per  500  kgs.  live  weight : 

Total  Digestible  Nutrients,* 
Grms. 

On  hay  alone 4586 

About  equal  parts  hay  and  grain 4190 

About  |  grain  and  \  hay  (Grandeau) 3626 

Zuntz  &  Hagemann,t  from  the  results  of  a  respiration  experi- 
ment with  the  horse,  make  a  still  lower  estimate  of  the  maintenance 
requirement,  viz.,  3265  grams  total  nutrients  per  500  kgs.  live 
weight  on  a  ration  of  which  about  four  sevenths  was  grain,  but 
after  allowing  for  the  differences  in  crude  fiber  content  compute  a 
satisfactory  agreement  between  their  results  and  Wolff's.  Since 
their  estimate  for  the  work  of  digestion  of  crude  fiber  is  really 
based  on  the  difference  in  digestive  work  required  by  coarse  fodder 
and  by  grain  this  is  equivalent  to  showing  that  the  latter  is  more 
valuable  for  maintenance  than  the  former. 

On  the  other  hand,  Grandeau  and  LeClerc  %  in  later  experiments 
on  exclusive  hay  feeding  found  that  the  live  weight  was  almost 
exactly  maintained  for  a  month  on  8  kgs.  of  hay  per  day,  the  total 
digested  nutrients  being  as  follows: 


Animal. 

Live  Weight, 
Kgs. 

Total  Digestible  Nutrients. 

Per  Head, 
Grms. 

Per  500  Kgs., 
Grms. 

No.   1 

395 
419 
413 

2892 
3036 
3058 

3660 
3622 
3701 

"      2 

"     3 

These  figures  do  not  materially  exceed  the  average  computed  by 
Wolff  from  their  previous  experiments  on  heavy  grain  rations.  The 
horses  had  a  half-hour's  walking  exercise  daily,  so  that  the  ration 
seems  to  have  been  amply  sufficient  for  maintenance,  and  no  reason 
for  the  divergent  result  is  obvious. 

While  none  of   these  comparisons  have  the  conclusiveness  of 

*  Including  fat  X  2.4. 

t  Loc.  cit.,  pp.  422-4. 

%  L'alimentation  du  Cheval  du  Trait,  3d  memoir,  pp.  23-31. 


NET  AVAILABLE  ENERGY— MAINTENANCE.  435 

complete  metabolism  experiments,  their  results  as  a  whole  indicate 
clearly  that  the  metabolizable  energy  of  the  grains  is  more  valu- 
able for  maintenance  than  that  of  the  coarse  fodders,  a  fact  un- 
doubtedly due  to  the  greater  expenditure  of  energy  in  the  digestion 
and  assimilation  of  the  latter. 

The  maintenance  ration  of  horses,  cattle,  and  sheep,  then,  as 
ordinarily  expressed  (i.e.,  in  units  of  digestible  matter  or  of  metabo- 
lizable energy)  is  not  a  constant  but  a  variable,  depending  on  the 
availability  of  the  metabolizable  energy,  and  such  a  statement  of  it, 
to  be  definite,  must  be  accompanied  by  a  statement  of  the  kind  of 
feed  used. 

No  similar  experiments  upon  swine  appear  to  have  been  made. 
The  ordinary  feed  of  this  animal,  however,  probably  varies  less  in 
availability  than  that  of  ruminants,  and  it  may  be  presumed  that 
no  such  striking  differences  would  be  found. 

Value  of  Crude  Fiber. — As  a  result  of  Wolff's  conclusions  con- 
cerning the  apparent  worthlessness  of  crude  fiber  for  work  production, 
as  discussed  in  the  succeeding  chapter,  and  of  Zuntz  &  Hagemann's 
estimates  regarding  its  digestive  work  (p.  389),  there  has  been  a 
tendency  to  ascribe  the  difference  between  grain  and  coarse  fodders 
to  the  greater  amount  of  crude  fiber  in  the  latter,  forgetting  that 
what  these  investigators  have  actually  shown  is  simply  the  lower 
value  of  the  digestible  matter  from  coarse  fodders,  and  that  their 
conclusions  regarding  crude  fiber  are  deductions  from  the  observed 
facts.  Kellner's  more  recent  experiments  (see  p.  182  and  Chapter 
XIII,  §  1)  have  demonstrated  that  at  least  one  form  of  crude  fiber 
is  nearly  as  efficient  in  producing  a  gain  of  fat  by  cattle  as  is 
starch.  A  fortiori,  therefore,  it  should  be  equally  valuable  for 
maintenance.  We  have  as  yet  no  sufficient  evidence  to  justify  us 
in  ascribing  the  difference  between  grain  and  coarse  fodder  to  the 
crude  fiber  as  such  aside  from  its  influence  on  the  mechanical 
structure  of  the  material. 

Influence  of  Thermal  Environment. — It  has  been  not 
uncommonly  assumed  that  the  maintenance  requirement  of  an 
animal  is  affected  by  changes  in  the  temperature  and  other  external 
factors  which  combine  to  determine  the  refrigerating  effect  of  the 
environment;  in  other  words,  the  heat  production  of  the  animal 
has  been  looked  upon  more  or  less  distinctly  as  an  end  in  itself. 


436  PRINCIPLES   OF  ANIMAL   NUTRITION. 

We  have  already  seen  reason  to  believe  that  this  is  the  case  to  a 
very  limited  extent  only,  even  in  the  fasting  animal,  and  to  a  still  less 
degree  in  one  consuming  food.  If  we  are  justified  in  thinking  that 
the  critical  amount  of  food  fOr  herbivorous  animals  is  ordinarily 
less  than  the  maintenance  requirement,  it  follows  that  the  heat 
production  on  a  maintenance  ration  is  in  excess  of  the  actual  needs 
of  the  organism  for  heat  by  an  amount  depending  upon  the  avail- 
ability of  the  metabolizable  energy  of  the  food,  and  that  this  excess 
of  heat  is  disposed  of  by  "  physical "  regulation.  That  such  is  the 
case  appears  to  be  clearly  indicated  by  the  writer's  experiments 
upon  timothy  hay  (p.  424),  since  there  was  obviously  no  such  in- 
direct utilization  of  the  heat  resulting  from  the  work  of  digestion 
and  assimilation  as  takes  place,  according  to  Rubner's  theory, 
below  the  critical  amount  of  food.  If,  now,  the  temperature  to 
which  such  an  animal  is  exposed  falls,  it  is  in  accord  with  all  that 
we  know  regarding  the  regulative  processes  in  the  body  to  suppose 
that  the  additional  draft  on  it  for  heat  will  be  compensated  for  by 
a  fall  in  the  emission  constant  rather  than  by  an  increased  produc- 
tion of  heat,  or,  to  put  it  in  another  way,  that  some  of  the  heat 
resulting  from  digestive  work  will  be  utilized  to  maintain  the  tem- 
perature of  the  animal  instead  of  being  at  once  dissipated. 

No  exact  experiments  upon  the  influence  of  external  tempera- 
ture on  the  maintenance  requirement  appear  to  have  been  made, 
but  Kern,  Wattenberg  &  Pfeiffer  *  have  investigated  the  influence 
of  the  greater  exposure  to  cold  caused  by  shearing  upon  the  metabo- 
lism of  sheep  consuming  a  maintenance  ration.  A  slight  decrease 
in  the  proteid  metabolism  was  found  to  result,  due,  as  Pfeiffer  con- 
jectures, to  a  more  rapid  growth  of  wool  after  shearing,  but  the 
corresponding  difference  in  the  metabolism  of  energy  is  insignificant. 
The  removal  of  a  nine-months  fleece  appears  to  have  caused  at  first 
an  increased  excretion  of  carbon  dioxide,  but  this  practically  dis- 
appeared within  four  or  five  days  and  is  probably  to  be  attributed 
to  greater  muscular  activity  on  the  part  of  the  shorn  animals. 
Comparing  the  results  before  shearing  with  those  obtained  from 
five  to  sixteen  days  after,  we  have  the  following  averages,  the 
amount  of  water-vapor  given  off  being  only  an  approximate  esti- 
mate: 

*  Jour.  f.  Landw.,  39,  1. 


NET  AVAILABLE  ENERGY— MAINTENANCE.  437 


Carbon  dioxide 

per  Day, 

Grms. 

Estimated 

Water-vapor 

per  Day, 

Grms. 

Before  shearing  (4  experiments) .  .  . 
After         "          (4           "          )... 

719.6 
725.1 

1939 
434 

The  total  metabolism,  as  indicated  by  the  excretion  of  carbon 
dioxide,  shows  scarcely  any  increase  as  a  result  of  the  shearing,  and 
if  we  accept  Pfeiffer's  suggestion  that  the  result  for  the  first  of  the 
four  days  (736  grams)  may  have  been  slightly  affected  by  the  stimu- 
lation of  movement  above  noted,  the  difference  becomes  still  less. 
On  the  other  hand,  the  difference  in  the  amount  of  water-vapor 
given  off  is  very  striking  and  apparently  admits  of  but  one  con- 
clusion, viz.,  that  drawn  by  Pfeiffer,  that  the  unshorn  animals  upon 
a  maintenance  ration  produced  an  excess  of  heat  which  was  gotten 
rid  of  by  evaporation  of  water,  while  the  shorn  animals,  instead  of 
meeting  the  greater  refrigerating  effect  of  their  surroundings  by  an 
increased  metabolism,  simply  evaporated  less  water  and  thus  com- 
pensated for  the  increased  loss  of  heat  by  radiation  and  conduction. 

Even  in  the  case  of  man,  where  the  digestive  work  is  much 
less  than  in  the  herbivora,  the  heat  production  on  a  mainte- 
nance ration  may  be  in  excess,  and  even  largely  in  excess,  of  the 
minimum  requirement,  it  being  simply  a  question  of  clothing, 
temperature,  etc.  This  has  been  most  strikingly  demonstrated 
by  Ranke,*  who  shows  that  with  relatively  high  temperature  and 
humidity  the  heat  production  on  a  maintenance  ration  may  be  so 
great  as  to  even  produce  pathological  effects  and  that  under  such 
circumstances  the  consumption  of  food  is  instinctively  reduced 
below  the  maintenance  requirement. 

Sanborn,f  in  experiments  upon  the  maintenance  ration  of  swine, 
found  the  amount  of  middlings  required,  per  hundred  pounds  of  live 
weight,  to  be  as  tabulated  on  the  next  page.  The  second  summer 
experiment  is  not  comparable  with  the  others,  since  the  smaller 
animal  would  require  a  relatively  greater  maintenance  ration. 
The  remaining  experiments  seem  to  show  a  lower  requirement  for 
maintenance  in  winter  than  in  summer. 

*  Einfluss  des  Tropenklimas  auf  die  Ernahrung  des  Menschen,  and  Zeit. 
f.  Biol.,  40,  288. 

t  Mo.  State  Agr.  Coll.,  Bull.  28,  pp.  5  and  6. 


433 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Live  Weight, 
Lbs. 

Maintenance    j 

Requirement. 

per  100  Pounds, 

Lbs. 

Winter  (temp,  about  40°  F.) I 

Summer  (  "           "      80°  F.)  ....  -j 

173.5 

171.6 

173.6 

48.3 

1.65 
1.89 
2.02 
2.07 

On  the  other  hand,  Cooke,*  in  a  series  of  experiments  on  swine 
at  the  Colorado  Station,  found  the  following  amounts  of  computed 
digestible  matter  required  for  maintenance  per  hundred  pounds  live 
weight  of  animals  weighing  from  85  to  182  pounds  per  head : 

In  hot  weather 0.93  lbs. 

In  moderate  weather 1 .  25     " 

In  cold  weather 1 .41     " 

Consumption  of  Water. — A  not  inconsiderable  amount  of  energy 
is  usually  required  to  raise  the  ingesta  to  the  temperature  of  the 
body.  This  is  particularly  true  of  the  water  consumed,  especially 
in  case  of  the  herbivora,  both  by  reason  of  its  relatively  large  amount 
as  compared  with  the  dry  matter  of  the  food  and  on  account  of  its 
high  specific  heat.  At  first  thought  it  might  seem  that  the  warming 
of  the  ingesta  is  part  of  the  work  of  digestion,  since  it  is  an  expendi- 
ture of  energy  in  preparing  the  food  for  assimilation.  This  same 
matter  or  its  equivalent,  however,  finally  leaves  the  body,  in  the 
form  of  various  excreta,  at  body  temperature,  thus  carrying  off  as 
sensible  heat  substantially  the  same  amount  of  energy  which  was 
imparted  to  it  when  its  temperature  was  raised,  and  this  heat  it 
imparts  in  cooling  to  the  environment  of  the  animal.  It  would 
seem,  then,  that  the  warming  of  the  ingesta  may  be  more  logically 
regarded  as  a  part  of  the  general  draft  for  heat  which  the  surround- 
ings make  upon  the  animal,  the  process  being  simply  a  little  less 
direct  than  the  loss  of  heat  by  radiation  and  conduction  through 
the  skin. 

From  this  point  of  view  the  influence  of  the  consumption  of  cold 

food  and  particularly  of  cold   water  will  be  subject  to  the  same 

general  laws  as  the  other  forms  of  the  demand  for  heat.     On  a 

ration  supplying  less  than  the    critical  amount  of  metabolizable 

*  Private  communication. 


NET  AVAILABLE  ENERGY-MAINTENANCE.  439 

energy  any  increase  in  the  consumption  of  water  (taking  this  as  the 
typical  case)  will  increase  the  metabolism  by  an  amount  sufficient 
to  warm  the  water  to  the  body  temperature.  Above  the  critical 
amount  of  food  the  excess  of  heat  arising  from  the  digestive  work 
will,  we  may  reasonably  suppose,  be  applied  to  the  warming  of  the 
additional  water  consumed,  and  only  when  this  is  insufficient  will 
an  increased  metabolism  be  required  to  make  up  the  deficit.  In 
case  of  farm  animals,  however,  it  would  appear  that  the  waste  heat 
even  on  a  maintenance  ration  is  ordinarily  sufficient,  and  more 
than  sufficient,  to  supply  all  the  energy  needed  for  warming  the 
ingesta. 

The  Time  Element. — One  important  factor  in  modifying  the 
results  of  the  demand  for  heat,  particularly  with  relation  to  the 
water  consumption,  is  what  we  may  call  the  time  element.  Hitherto 
it  has  been  tacitly  assumed  that  all  the  factors  making  up  the 
demand  for  heat  act  at  a  uniform  rate.  As  a  matter  of  fact  this  is  at 
best  only  partially  true.  Ordinarily  a  farm  animal  is  watered  but 
once  or  twice  per  day  and  then  consumes  a  relatively  large  amount 
in  a  few  minutes.  A  sudden  demand  for  heat  is  thus  set  up,  since 
this  water  must  be  raised  to  body  temperature  within  a  compara- 
tively short  time.  It  is  quite  conceivable,  therefore,  that  the  demand 
for  heat  may  temporarily  exceed  the  supply,  requiring  the  deficit 
to  be  made  up  by  an  increased  metabolism,  while  if  the  same  water 
consumption  were  distributed  uniformly  over  the  twelve  or  twenty- 
four  hours  no  such  effect  would  be  produced.  Such  a  temporary 
increase  in  the  heat  production,  however,  cannot  be  made  up  for 
later  when  the  heat  production  is  in  excess,  but  is  a  permanent  loss. 
Once  converted  into  heat,  the  energy  of  food  or  tissue  has,  so  to 
speak,  escaped  from  the  grasp  of  the  organism,  which  appears  to 
have  no  power  to  reconvert  it  into  any  other  form  of  energy.  We 
may  plausibly  suppose  that  these  considerations  constitute  a  partial 
explanation  of  the  advantages  observed  in  practice  from  the  warm- 
ing of  drinking-water  and  the  installation  of  self-watering  devices 
in  the  stable. 

What  is  true  in  regard  to  the  consumption  of  water  is  of  course 
equally  true  of  other  forms  of  the  demand  for  heat.  The  time  ele- 
ment is  an  important  factor.  Thus  an  exposure  of  an  hour  or  two 
in  a  cold  yard  or  to  a  cold  rain  may  cause  an  increased  metabolism 


440  PRINCIPLES   OF  ANIMAL   NUTRITION. 

and  heat  production  although  the  average  conditions  for  the  twenty- 
four  hours  may  be  such  that  the  necessary  production  of  heat  by 
the  internal  work  and  the  work  of  digestion  and  assimilation  would 
be  more  than  sufficient  for  the  needs  of  the  animal. 

Influence  of  Size  of  Animal. — The  discussion  of  the  heat 
production  of  the  fasting  animal  in  Chapter  XI  led  us  to  the  con- 
clusion that  under  comparable  conditions,  at  least  for  the  same 
species  of  animal,  the  internal  work  is  probably  approximately 
proportional  to  the  surface  of  the  body.  This,  however,  is  equiva- 
lent to  saying  that  the  quantity  of  net  available  energy  required 
for  maintenance  is  proportional  to  the  body  surface.  Furthermore, 
if  we  are  right  in  supposing  that  the  available  energy  is  a  linear 
function  of  the  metabolizable  energy,  the  amount  of  the  latter 
required  for  maintenance  will  also  be  proportional  to  the  surface  of 
the  body.  Referring  once  more  to  the  diagram  on  p.  410,  if  OA  is 
proportional  to  the  body  surface,  then  OS,  which  for  a  given  food 
bears  a  fixed  ratio  to  OA,  must  also  be  proportional  to  the  surface. 
If  the  critical  point,  K,  lies  above  the  maintenance  requirement, 
then  the  metabolizable  energy  required  for  maintenance  will  equal 
the  fasting  metabolism,  and  this,  as  shown  on  pp.  359-363,  is  pro- 
portional to  the  surface. 

Apparently,  then,  we  are  justified  in  concluding  that  the  mainte- 
nance requirements  of  different  normal  animals  of  the  same  species 
are  proportional  to  their  body  surface,  or,  for  approximate  computa- 
tions, to  the  two-thirds  power  of  their  live  weights.  It  must  not  be 
overlooked,  however,  that  the  results  upon  which  this  conclusion  is 
based  were  obtained  largely  with  the  dog,  an  animal  which  when  at 
rest  lies  down,  and  which,  therefore,  in  these  experiments  was  in  a 
state  of  almost  complete  muscular  relaxation.  Our  common  farm 
animals,  on  the  contrary,  pass  a  considerable  portion  of  their  time 
standing,  which  involves  an  expenditure  of  energy  in  muscular 
work.  This  expenditure  we  should  naturally  assume  to  be  pro- 
portional to  the  mass  to  be  sustained  rather  than  to  its  surface, 
and  if  this  be  true  we  have  here  a  second  determining  factor  in  the 
maintenance  requirement.  How  important  this  factor  is  it  is  diffi- 
cult to  say,  although  the  writer's  results  with  a  steer  (p.  343)  in- 
dicate that  it  is  a  large  one.  Its  tendency  would  be  to  make  the 
maintenance  requirement  increase  more  rapidly  than  the  surface. 


NET  AVAILABLE  ENERGY— MAINTENANCE.  441 

Moreover,  so  far  as  we  can  judge  from  the  accounts  of  Rubner's 
experiments,  it  would  seem  likely  that  what  were  designated  on 
p.  342  as  incidental  muscular  movements  are  a  more  important 
factor  in  determining  the  maintenance  requirements  of  farm  ani- 
mals than  they  are  in  fixing  that  of  the  dog. 

While,  therefore,  we  are  probably  justified  in  retaining  pro- 
visionally the  computation  of  the  maintenance  requirement  in 
proportion  to  the  real  or  estimated  surface,  it  should  be  with  a  clear 
understanding  that  it  is  at  present  a  deduction  from  experiments 
on  other  species  and  under  more  or  less  different  conditions. 

Effect  of  Fattening  on  Maintenance  Requirement. — An  interesting 
question,'  and  one  of  practical  importance,  is  what  effect  the  pro- 
gressive change  in  weight  of  the  same  animal  as  it  is  fattened  has 
upon  its  maintenance  requirement.  We  can  hardly  suppose  that 
the  internal  work  of  the  body  will  be  materially  increased  by  such  a 
gain.  The  increased  mass  of  tissue  must  involve,  of  course,  some 
increase  in  metabolism,  but  all  that  we  know  of  metabolism  of  adi- 
pose tissue  indicates  that  it  is  very  sluggish.  The  most  important 
effect  might  be  anticipated  to  be  an  increase  in  the  muscular  ex- 
ertion required  in  standing,  perhaps  counterbalanced  to  a  greater  or 
less  extent  by  the  tendency  of  the  fat  animal  to  pass  more  of  its 
time  in  a  recumbent  position. 

Zuntz  &  Hagemann  *  have  investigated  the  effect  of  a  load 
carried  on  the  back  upon  the  metabolism  of  the  horse,  and  have 
found  the  latter  to  be  proportional  to  the  total  mass  (horse  plus 
load),  but  the  applicability  of  this  result  to  another  species  of  ani- 
mal and  to  an  increase  of  weight  caused  by  fattening  may  perhaps 
be  questioned.  The  only  experiments  upon  cattle  bearing  on  this 
point  are  those  of  Kellner,f  who  has  compared  the  maintenance 
requirements  of  fattened  and  unfattened  cattle.  It  being  impossible 
to  hit  upon  exactly  the  maintenance  ration,  it  is  computed  from  the 
actual  results.  In  case  there  was  a  loss  of  tissue  the  maintenance 
requirement  of  the  animal  is  computed  by  subtracting  the  poten- 
tial energy  of  the  excreta  from  the  potential  energy  of  food  plus 
tissue  lost;  in  other  words,  the  replacement  of  energy  claimed  by 
Rubner  is  assumed  to  occur.     When  there  was  a  gain  of  tissue,  on 

*  Landw.  Jahrb.,  27,  Supp.  Ill,  269. 
f  Landw.  Vers.  Stat.,  50,  245;  53,  14. 


442 


PRINCIPLES  OF  ANIMAL    NUTRITION. 


the  other  hand,  the  amount  of  metabolizable  energy  required  to 
produce  it  is  computed  on  the  basis  of  the  results  upon  utilization 
obtained  in  other  experiments,  this  larger  amount  being  added  to 
the  energy  of  the  excreta  and  the  sum  of  the  two  subtracted  from 
the  potential  energy  of  the  food;  that  is  the  energy  of  digestion 
and  assimilation  above  the  maintenance  ration  is  assumed  to  be 
waste  energy. 

Computed  in  this  way,  and  assuming  further  that  the  mainte- 
nance requirements  of  different  animals  are  substantially  propor- 
tional to  the  two-thirds  powers  of  their  live  weights,  the  results  are 
as  follows: 


No.  of 
Animals. 

Live 

Weight, 

Kgs. 

Stable 
Tem- 
perature, 
Deg.  C. 

Main- 
tenance 
Require- 
ment, 
Cals. 

Observed  : 

7 
3 

7 
3 

632 

785 

800 
800 

15.2 
15.7 

15.2 
15.7 

13,470 
19,671 

15,760 
19,920 

Computed  to  same  live  weight : 

Fattened  

Kellner  concludes  from  these  figures  that  the  maintenance  re- 
quirements of  fattened  animals  are  greater  per  unit  of  surface  than 
those  of  unfattened  ones. 

These  experiments,  it  is  true,  were  on  different  animals  and  the 
individuality  of  the  animal  is  an  important  factor  in  determining 
the  maintenance  requirement.  The  results  on  the  seven  unfattened 
animals,  when  computed  to  600  kgs.  live  weight,  show  a  range  of 
1760  Cals.,  or  13.54  per  cent,  of  the  average,  while  the  three  results 
on  fattened  animals,  computed  to  800  kgs.  live  weight,  show  a 
range  of  2420  Cals.,  or  12.16  per  cent,  of  the  average.  Moreover, 
in  making  up  the  average  of  the  unfattened  animals,  one  animal 
was  excluded  on  the  ground  that  the  results  were  probably  abnor- 
mally high,  but  the  same  animal  is  subsequently  included  among 
the  three  fattened  animals  the  results  on  which  are  averaged. 

Even  after  making  all  allowances  for  these  facts,  however,  the 
results  for  the  fattened  animals    are    decidedly  higher   relatively 


NET  AVAILABLE  ENERGY-MAINTENANCE. 


443 


than  for  the  unfattened,  but  how  much  higher  can  hardly  be  deter- 
mined from  such  averages. 

Comparing  the  results  on  the  one  animal   common  to  the  two 
series  of  experiments  we  have — 


Live  Weight, 
Kgs. 

Maintenance, 
Cals. 

Observed: 

611.5 

750 

800 
800 

16,835.6 
18,959.6 

20,140 
19,800 

Computed  to  800  kgs.: 

According  to  the  above  figures  the  maintenance  ration  of  this 
animal  was  practically  proportional  to  the  two-thirds  power  of  its 
live  weight.  On  the  other  hand,  however,  its  maintenance  require- 
ment in  the  unfattened  state  was  much  higher  than  the  average 
for  the  seven  unfattened  animals,  while  after  fattening  it  did  not 
differ  materially  from  the  average  for  the  three  fattened  animals. 
If,  then,  we  are  to  regard  the  above  result  as  correct  we  must 
assume  that  by  chance  all  three  of  the  fattened  animals  had  a 
higher  normal  rate  of  metabolism  than  the  seven  unfattened  ones, 
which  is  not  exactly  probable.  Although  this  leaves  the  question 
in  a  rather  unsatisfactory  state,  it  would  seem  that  we  must  be 
content  to  let  it  rest  there  pending  further  comparative  experi- 
ments on  identical  animals  in  different  stages  of  fattening. 


CHAPTER  XIII. 
THE   UTILIZATION    OF   ENERGY. 

According  to  the  conceptions  discussed  in  the  preceding  chap- 
ter a  certain  portion  of  the  metabolizable  energy  of  the  food  is 
consumed  in  what  has  been  called  in  a  broad  sense  the  work  of 
digestion  and  assimilation,  while  the  remainder  constitutes  net 
available  energy  and  contributes  to  the  maintenance  of  the  store  of 
potential  energy  in  the  body.  If  the  food  is  sufficient  to  supply 
net  available  energy  equal  to  that  dispensed  by  the  internal  work 
of  the  body,  the  balance  between  income  and  expenditure  of  energy 
is  just  maintained.  If  we  increase  the  food  beyond  this  maintenance 
requirement  we  supply  the  body  with  an  excess  of  net  available 
energy.  In  general  terms  we  can  say  that  this  excess  may  be 
disposed  of  in  two  ways:  it  may  be  utilized  for  the  peformance  of 
external  work,  or  it  may  give  rise  to  a  storage  of  potential  energy 
in  the  body  in  the  form  of  new  tissue,*  particularly  of  fat  tissue. 
It  appears  probable,  however,  that  neither  of  these  processes  takes 
place  without  more  or  less  loss  of  energy  in  the  form  of  heat. 
This  is  certainly  true  of  the  performance  of  muscular  work,  as  has 
already  been  mentioned  (p.  189)  and  as  will  be  shown  in  detail 
on  subsequent  pages.  Out  of  the  total  potential  energy  of  the 
material  metabolized  rather  more  than  one  third,  in  the  most  favor- 
able case,  is  actually  recovered  in  the  form  of  external  work,  the 
remainder  taking  the  form  of  heat.  In  this  case,  then,  we  might 
speak  of  the  coefficient  of  utilization  of  the  energy  as  being  about 
one  third. 

In  the  utilization  of  surplus  energy  by  storage  of  tissue  it 
appears  likely  that  there  must  be  also  a  loss  of  energy,  although, 

*  From  this  point  of  view  the  production  of  milk  is  to  be  regarded  as 
the  formation  of  new  tissue. 


THE  UTILIZATION   OF  ENERGY.  445 

as  will  appear  later,  we  are  not  yet  in  a  position  to  make  any  such 
definite  statements  regarding  its  amount  as  in  the  case  of  muscular 
work,  and  although  the  writer's  few  results  on  timothy  hay  cited 
on  p.  424  afford  no  indication  of  such  a  loss,  the  utilization  of  the 
metabolizable  energy  for  the  production  of  gain  seeming  to  have 
been  practically  equal  to  its  net  availability.  It  is  obvious,  how- 
ever, that  the  conversion  of  the  resorbed  nutrients  of  the  food  into 
the  ingredients  of  tissue  involves  profound  chemical  changes,  and 
we  can  hardly  suppose  that  these  take  place  without  some  evolution 
of  heat.  As  a  good  illustration  we  may  take  the  case  of  a  carbo- 
hydrate. As  resorbed  into  the  blood  it  appears  to  be  in  the  form 
of  a  sugar,  and  it  would  seem  that  this  sugar  can  serve,  without  any 
very  extensive  chemical  changes,  to  sustain  the  metabolism  incident 
to  the  internal  work  of  the  body;  that  is,  that  it  is  oxidized  more 
or  less  directly  in  the  various  tissues  to  supply  energy  for  their 
physiological  work.  When,  however,  a  surplus  of  a  carbohydrate 
is  to  be  utilized  for  the  storage  of  energy  in  the  form  of  fat,  the  case 
is  different.  The  formation  of  fat  from  a  carbohydrate  is  chemi- 
cally a  process  of  reduction,  and  the  oxygen  which  is  removed 
from  the  carbohydrate  must  unite  with  the  carbon  and  hydrogen 
either  of  other  molecules  of  the  carbohydrate  or  of  other  in- 
gredients of  food  or  tissue,  in  either  case  giving  rise  to  an  evolu- 
tion of  heat.  If  we  suppose  the  transformation  to  take  place 
according  to  the  equation  given  in  Chapter  II  (p.  24),  the  re- 
sulting fat  would  contain  about  87  per  cent,  of  the  energy  of 
the  dextrose.  Whether  this  percentage  expresses  the  actual  facts 
of  the  case  or  not,  it  is  very  improbable  that  this  or  any  similar 
synthetic  process  takes  place  in  the  body  without  the  evolution 
of  some  heat. 

Provisionally,  then,  we  seem  justified  in  assuming  that  only  a 
part  of  the  net  available  energy  supplied  to  the  organism  above 
the  maintenance  requirement  can  be  utilized  to  increase  the  store 
of  potential  energy  in  the  body,  and  we  may  speak  in  this  case,  as 
in  that  of  muscular  work,  of  the  coefficient  of  utilization.  Repro- 
ducing here  the  essential  parts  of  the  graphic  representation 
on  p.  410,  we  may  now  complete  it  so  as  to  represent  in  a  general 
and  qualitative  way  the  relations  indicated  above,  assuming  pro- 
visionally that  the  effects  are  linear  functions  of   the  food.     As 


446 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


before,  OG  represents  the  fasting  metabolism  at  a  temperature 
below  the  critical  point  and  OM  the  critical  amount  of  food  at  this 
temperature.  Then  the  line  GKS  represents  the  availability  of 
the  food,  HLS'  the  heat  production,  and  OS  the  maintenance 
requirement.  Beyond  the  point  *S  we  may  assume  that  the  net 
availability  of  the  food  remains  the  same,  represented  by  the  line 
ST.    But  a  fraction  of  this  net  available  energy,  however,  can  be 


recovered  as  mechanical  work,  and  its  utilization  will  therefore  be 
represented  by  some  such  line  as  £7,  while  the  heat  production  will 
be  correspondingly  increased  as  represented  by  S'V.  Similarly 
the  proportion  of  the  net  available  energy  which  in  the  quiescent 
animal  is  stored  up  in  the  form  of  new  tissue  may  be  expressed  by 
a  line  SU  and  the  corresponding  heat  production  by  S'U'.  What 
the  relation  between  the  proportions  utilized  in  the  two  cases  is  we 
do  not  know,  and  the  diagram  is  intended  to  be  simply  schematic; 


THE   UTILIZATION  OF  ENERGY.  447 

but  we  do  know  that  the  proportion  is  materially  greater  in  the 
latter  case,  since  the  heat  production  of  a  fattening  animal  is  ob- 
viously much  less  than  that  of  a  working  animal  utilizing  the  same 
amount  of  food. 

In  the  following  pages  the  attempt  has  been  made  to  bring 
together  the  more  important  experimental  evidence  bearing  upon 
the  utilization  of  food  energy  for  the  production  of  tissue  and  of 
work.  Before,  however,  proceeding  to  a  consideration  of  our  present 
knowledge  upon  the  subject,  attention  should  be  called  once  more 
to  the  fact  that  we  are  here  dealing  with  it  from  the  statistical  point 
of  view  of  the  balance  between  income  and  expenditure  of  energy 
of  the  body. 

In  an  animal  performing  work,  each  muscular  contraction 
metabolizes  a  certain  quantity  of  energy,  part  of  which  finally 
appears  as  heat  and  part  as  mechanical  work.  Besides  this,  how- 
ever, a  secondary  result  is  an  increase  in  the  activity  of  the  organs 
of  circulation  and  respiration  which  requires  the  expenditure  of  a 
certain  amount  of  energy,  this  energy  ultimately  taking  the  form 
of  heat  and  being  added  to  that  resulting  directly  from  the  activity 
of  the  skeletal  muscles.  When  we  compare  the  actual  external 
work  done  with  the  total  energy  metabolized  for  its  performance, 
and  so  compute  the  coefficient  of  utilization,  we  group  all  these 
sources  of  heat  production  and  regard  them  as,  from  the  economic 
standpoint,  a  waste  of  energy,  just  as  in  a  heat  engine  the  energy 
which  escapes  conversion  into  work  is  regarded  as  waste  energy  not- 
withstanding the  fact  that  the  loss  is  inevitable.  So,  too,  in  the  pro- 
duction of  new  tissue  we  look  upon  total  gain  of  potential  energy 
by  the  body  as  constituting  the  net  useful  result  of  the  feeding, 
and  the  coefficient  of  utilization  in  this  case,  as  in  that  of  muscular 
work,  would  express  the  relation  which  this  bears  to  the  net  avail- 
able energy  supplied  in  the  food.  That  the  effect  of  abundant 
food  may  be  in  some  cases  to  stimulate  the  metabolism  of  tissue  or 
the  "  incidental "  muscular  work  (p.  342)  is  rendered  probable  by 
Zuntz  &  Hagemann's  results  with  the  horse  (see  p.  376).  All  these 
effects  are  part  of  the  necessary  expenditure  of  energy  by  the  body, 
and  however  interesting  physiologically  are  statistically  sources  of 


448  PRINCIPLES   OF  ANIMAL   NUTRITION. 

§  i.  Utilization  for  Tissue  Building. 

Under  this  head  we  have  to  consider  almost  exclusively  ex- 
periments upon  the  fattening  of  mature  animals.  While  the  growth 
of  young  animals  and  the  production  of  milk  are  both  forms  of 
tissue  building,  the  experimental  data  available  seem  too  scanty 
to  justify  including  them  in  the  scope  of  the  present  work.  For 
convenience  we  may  first  bring  together  the  recorded  results  and 
later  discuss  them  in  their  more  general  bearings. 

One  difficulty,  however,  is  encountered  at  the  outset  in  our 
inadequate  knowledge  of  the  net  availability  of  nutrients  and 
feeding-stuffs,  as  pointed  out  in  the  foregoing  chapter.  Until  this 
gap  is  filled  it  is  of  course  impossible  to  compare  the  gain  of  energy 
by  the  body  with  the  supply  of  net  available  energy.  Accordingly 
the  results  of  the  experiments  upon  productive  feeding  can  at  present 
be  utilized  only  to  determine  what  proportion  of  the  metabolizable 
energy  of  the  food  is  recovered  in  the  gain  of  tissue,  and  the  experi- 
ments cited  in  the  following  paragraphs  will  be  considered  from 
this  point  of  view. 

Experimental  Results. 
Experiments  on  Carnivora. — In  connection  with  the  dis- 
cussion of  net  availability  in  the  preceding  chapter  a  number  of 
experiments  were  cited  (p.  428)  in  which  more  or  less  gain  was 
made  by  the  animals.  In  addition  to  these  Rubner  *  has  made  a 
preliminary  report  of  investigations  upon  the  effect  of  abundant 
feeding  on  the  heat  production  A  dog  weighing  25  kgs  received 
successively  isodynamic  amounts  of  lean  meat,  fat,  and  carbo- 
hydrates (kind  not  stated)  equivalent  to  155  per  cent,  of  its  fasting 
metabolism,  a  two-days'  fast  intervening  in  each  case  between  the 
different  rations.  Few  details  are  given,  but  presumably  the 
methods  were  those  of  Rubner's  other  experiments  already  de- 
scribed (compare  p.  253).  In  a  second  experiment  the  effects  of 
two  different  amounts  of  meat  were  also  compared  In  the  follow- 
ing table  the  results  of  these  experiments  have  been  put  into  the 
same  form  as  those  on  net  availability  in  the  preceding  chapter,  the 
data  given  being  per  day  and  head : 

*  Sitzungsber.  k.  bayer.  Akad.  der  Wiss.,  Math  -phys.  Classe,  15,  452 


THE  UTILIZATION  OF  ENERGY. 


449 


Metabolizable 

Energy  of 

Food. 

Cals. 

Total  Gain. 
Cals. 

Gain  Over  Fasting 
Metabolism. 

Total, 
Cals. 

Per  Cent,  of 

Energy  of 

Food. 

Nothing 

Fat 

Carbohydrates 

Meat ■] 

0 
1549 
1549 
1549 
1463 
2181 

-944 
+  540 
+  509 
+  418 
+  332 
+  805 

1484 
1453 
1362 
1276 
1749 

95.8 
93.8 
87.9 
87.2 
80.2 

Experiments  by  Gruber  *  upon  the  formation  of  fat  from  pro- 
teids  (see  p.  112)  afforded  the  following  results,  computed  f  by 
the  use  of  the  factors  given  on  p.  414: 


Metabolizable 

Energy  of 

Food 

Cals. 

Total  Gain 
Cals. 

Gain  Over  Fasting 
Metabolism. 

Total 
Cals. 

Per  Cent,  of 

Energy  of 

Food. 

Nothing 

0 

1325 
1325 

1325 

-743 

250 
296 

273 

993 

1039 

1016 

1500  grms  meat: 

1st  series 

2d  series 

Average 

74.9 

78.4 

76.7 

The  difficulty  in  interpreting  these  results,  as  well  as  those  tabu- 
lated on  p.  428  r  as  already  stated,  lies  in  our  imperfect  data  regard- 
ing the  net  availability  of  the  materials  below  the  point  of  mainte- 
nance. Rubner,  in  discussing  his  results,  assumes  an  availability 
of  100  per  cent.,  or  in  other  words  that  the  fasting  metabolism  is 
the  measure  of  the  amount  of  metabolizable  energy  required  for 
maintenance.  He  accordingly  subtracts  this  amount  from  the 
total  metabolizable  energy  of  the  food  and  regards  the  remainder 
as  excess  food,  which  may  be  utilized  for  the  storage  of  energy. 
The  percentage  utilization  of  this  excess  was  as  shown  in  the  follow 
ing  table,  to  which  Gruber's  results,  computed  by  the  writer  in  the 
same  way,  have  been  added: 

*  Zeit  f   Biol ,  42,  409. 

f  From  the  last  two  complete  days  of  each  series 


45° 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Metab 
ohzable 
Energy  of 
Food 
Cais. 

Mainte- 
nance lie 
quirement 
(Fasting 
Metab- 
olism), 
Cals. 

Excess 
Food 
Cals. 

Gain 
Cals. 

Percent 

1  tihza 
tion. 

Fat 

1549 
1549 

1549 
14G3 
2181 
1325 
1325 

944 

944 

944 
944 
944 
743 
743 

605 
605 

605 
519 
1237 

582 
582 

540 
509 

418 
332 
805 
250 
296 

89.3 

84.1 

Meat: 

69.1 
63.9 

Gruber   -j 

65.1 
43.0 
50.9 

As  was  shown  in  the  preceding  chapter,  however,  while  the 
recorded  determinations  of  net  availability  are  far  from  satisfactory 
they  show  with  a  considerable  degree  of  probability  that  there  is 
some  loss  of  energy  below  the  maintenance  point  and  that  100  per 
cent,  of  net  availability  is  at  least  not  ordinarily  reached.  A  lower 
net  availabilty,  however,  means  a  larger  maintenance  requirement, 
and  this  in  turn  results  in  a  larger  computed  percentage  utilization 
of  the  excess  food. 

In  the  following  table  the  latter  percentage  has  been  computed 
by  the  writer  for  most  of  the  experiments  tabulated  on  p.  428,  as 
well  as  for  those  of  Rubner  and  Gruber  just  cited,  on  the  assump- 
tion that  the  net  availability  below  the  maintenance  requirement 
was: 

Meat 85  per  cent. 

Fat 98    "       " 

Starch 90   "       " 

Cane  sugar 96 

The  factor  for  meat  is  the  average  of  all  the  results  on  p.  427; 
that  for  fat  is  based  on  Magnus-Levy's  results  upon  digestive  work ; 
those  for  starch  and  cane-sugar  are  the  averages  of  Rubner's  re- 
sults, omitting  those  which  exceed  100  per  cent.  By  dividing  the 
fasting  metabolism  by  the  above  percentages  we  may  compute  the 
amount  of  metabolizable  energy  required  for  maintenance  on  the 
above  assumption,  while  subtracting  this  from  the  metabolizable 
energy  of  the  food  leaves  the  amount  of  excess  food,  which  can  be 
compared  with  the  observed  gain. 


THE  UTILIZATION  OF  ENERGY. 


451 


Fasting 
Metab- 
olism. 
Cals. 


Metab- 
olizable 
Energy 

of 
Food, 
Cals. 


Com- 
puted 
Main- 
tenance, 

Excess 
Food, 
Cals. 

Gain, 
Cals. 

Cals. 

1225 

100 

38 

307 

40 

13 

1169 

380 

418 

1169 

294 

332 

1169 

1012 

805 

920 

405 

250 

920 

405 

296 

1108 

2190 

878 

565 

377 

329 

565 

1319 

837 

671 

1067 

1016 

476 

466 

428 

266 

82 

49 

963 

586 

540 

1220 

795 

353 

1220 

1856 

853 

616 

258 

137 

1049 

500 

509 

470 

102 

190 

325 

377 

365 

Per- 
centage 
Utiliza- 
tion. 


Proteids  (meat) : 

Pettenkofer  &  Voit 

( 

Rubner ■{ 

I 
Gruber ] 

Fat: 

Pettenhofer  &  Voit -j 

Rubner 

Starch  : 

Pettenkofer  &  Voit 

Rubner  ("  carbohydrates",  as- 
sumed to  be  starch) 

Cane-sugar  : 

Rubner    

Cane-sugar  and  Starch   (93  per 
cent,  availability): 
Rubner 


1041 
261 
944 
944 
944 
743 
743 


1325 
347 
1549 
1463 
2181 
1325 
1325 


1086  3298 
554*   942 


554* 

658 

466 

261 

944 


1098 

1098 

554* 

944 


451 


302 


1884 

1738 

942 

348 

1549 


2015 
3076 

874 

1549 


572 


702 


38.0 
32.5 
110.0 
112.9 
79.6 
61.7 
73.1 


40.1 
87.3 
63.5 
95.2 
91.8 
59.8 
92.1 


44.4 
46.0 
53.1 

101.8 


186.3 


While  as  a  whole  the  results  of  the  computation  would  seem  to 
indicate  that  the  percentage  utilization  for  tissue  building  is  less 
than  the  percentage  availability,  the  remarkable  range  of  the 
figures  and  the  uncertain  basis  upon  which  they  are  computed  do 
not  encourage  any  attempt  at  a  critical  discussion. 

Experiments  on  Man. — The  only  respiration  experiments  upon 
man  which  the  writer  has  been  able  to  find  in  which  any  large 
amount  of  excess  food  was  given  are  those  of  Johansson,  Lander- 
gren,  Sonden  &  Tigerstedt  f  already  cited  on  p.  383  in  their  bearing 
on  the  subject  of  digestive  work.     If  we  assume,  on  the  basis  of 

*  Loss  on  basal  ration. 

t  Skand.  Arch.  f.  Physiol.,  7,  29. 


452 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


Magnus-Levy's  results,  that  the  work  of  digestion  in  man  equals 
about  9  per  cent,  of  the  metabolizable  energy  of  the  food,  the 
average  results  of  the  experiments  are  as  follows: 

Fasting  metabolism 2022.4  Cals. 

Metabolizable  energy  of  food 4193.4     " 

Computed  maintenance  requirement 2222.5     " 

Excess  food 1970.9     " 

Gain 1676.0     " 

Percentage  utilization 85 . 0  per  cent. 

The  computation  gives  a  somewhat  lower  percentage  for  the 
utilization  of  the  excess  food  than  that  assumed  for  the  availability 
of  the  maintenance  food. 

Experiments  on  Swine. — Meissl,  Strohmer  &  Lorenz  *  in  their 
investigation  upon  the  sources  of  animal  fat  made  six  respiration 
experiments  with  swine,  the  results  of  which  afford  some  data  as  to 
the  utilization  of  their  food  by  these  animals.  In  Experiments  V 
and  VI,  made  on  two  different  animals,  no  food  was  given,  and  the 
following  results  were  obtained,  the  energy  equivalent  to  the  loss 
of  tissue  being  computed  as  in  Rubner's  experiments  in  the  pre- 
vious chapter: 


Experi- 
ment. 

Tempera- 
ture. 
Deg.  C. 

Hours 

Since 

Last 

Feeding. 

Live 

Weight. 
Kgs. 

Loss  of 

Nitrogen. 

Grms. 

Loss  of 
Carbon, 
Grms. 

Total 
Metab- 
olism, 
Cals. 

Metab- 
olism per 
100  Kgs. 

Live 

Weight  ,t 

Cals. 

V 

VX....J 

20 

20 

20.4 

24 
12 

72 

140 
120 
120 

9.80 
9.55 
6.77 

224.51 
375.78 
194.93 

2607 
2291 

2083 
2029 

The  experiment  begun  only  twelve  hours  after  the  last  feeding 
obviously  gave  too  high  results,  owing  to  the  presence  of  food  in 
the  digestive  canal.  That  this  source  of  error  was  substantially 
eliminated  after  twenty -four  hours  appears  probable  from  the  close 
agreement  of  the  results  with  those  obtained  after  seventy-two 
hours.  The  average  fasting  metabolism  per  100  kgs.  live  weight 
is  2056  Cals.  and  this  average  has  been  made  the  basis  of  the  com- 
putations which  follow,  except  in  Experiment  I.     This  experiment 

*  Zeit.  f.  Biol.,  22,  63. 

t  Assumed  to  be  proportional  to  the  two-thirds  power  of  the  weight. 


THE  UTILIZATION  OF  ENERGY.  453 

having  been  made  on  the  same  individual  as  Experiment  V,  the 
result  of  the  latter  is  used  directly. 

In  Experiments  I  and  II  the  ration  consisted  of  rice,  in  Experi- 
ment III  of  barley,  and  in  Experiment  IV  of  rice,  flesh-meal,  and 
whey.  In  all  cases  large  amounts  of  food  were  consumed  and  a 
rapid  production  of  fat  was  observed.  The  digestibility  of  the  food 
was  determined.  Its  metabolizable  energy  has  been  computed  by 
the  writer  from  the  results  of  the  digestion  experiments  by  the  use 
of  the  following  factors:* 

1  gram  digestible  protein 4.1  Cals. 

1      "  "        nitrogen-free  extract. .  .  4.2     " 

1      "  "        crude  fiber 3.5     " 

1      "  "         ether  extract 8.8     " 

No  attempt  was  made  in  these  experiments  to  determine  the 
methane,  if  any  existed,  in  the  respiratory  products.  The  results 
per  day  and  head  were  as  follows: 


Experiment. 

Tem- 
perature, 
Deg.  C. 

Live 

Weight, 

Kgs. 

Computed 
Fasting     l 
Metabolism, 
Cals. 

Metab- 
olizable 

Energy. 
Cals. 

Energy  of 
Gain,  Cals. 

Nutritive 
Ratio. 

I.. 

II 

Ill 

IV 

18.0 
18.5 
19.3 
16.7 

140 

70 

125 

104 

2607 
1621 
2386 
2111 

7157 
7167 
5125  t 
6129 

3464 
4048 
1774 
2556 

1 :15.4 
1  :14.1 
1  :    9.3 
1  :    2.4 

No  determinations  were  made  of  the  actual  requirements  for 
maintenance  as  distinguished  from  the  fasting  metabolism,  and 
hence  the  data  are  lacking  for  a  computation  of  the  net  availability 
of  the  metabolizable  energy  of  the  food  on  the  one  hand  and  the 
percentage  utilization  of  the  excess  food  on  the  other.  Cooke's  re- 
sults mentioned  on  p.  438,  however,  seem  to  give  some  indication 
that  the  maintenance  demand  of  swine  may  not  be  greatly  in  excess 
of  the  fasting  metabolism.  If  in  these  experiments  we  assume  the 
same  net  availability  as  that  just  assumed  in  the  case  of  man,  viz., 
91  per  cent.,  we  obtain  the  following  figures: 

*  Compare  p.  332. 

t  In  this  experiment  the  ether  extract  of  the  feces  exceeded  that  of  the 
food  by  23.95  grms.  This  excess  has  been  assumed  to  have  a  heat  value 
of  4.2  Cals.  per  grm.  and  a  corresponding  amount  deducted  from  the  com- 
puted energy  of  the  other  digested  nutrients 


454 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


a 
S 

s 

•c 
i 

M 

w 

Food. 

Nutri- 
tive 
Ratio. 
1- 

Com- 
puted 
Fasting 
Metab- 
olism, 

Cals. 

Metab- 
olizable 
Energy 

of 
Food, 
Cals. 

Com- 
puted 

Main- 
tenance 

Re- 
quire- 
ment , 
Cals. 

Excess 
Food, 
Cals. 

Gain, 
Cals. 

Per- 

cent- 

Utifiza- 
tion. 

T 

15.4 

14.1 

9.3 

2.4 

2607 
1621 
2386 

2111 

7157 
7167 
5125 

6129 

2865 
1781 
2622 

2320 

4292      3464 
5386      4048 

80.7 

TT 

a 

75.2 

TTT 

2503 
3809 

1774 
2556 

70.9 

IV 

Rice,     flesh-meal, 
and  whey 

67.1 

It  is  interesting  to  note  that  the  utilization  as  thus  computed 
diminishes  as  the  proportion  of  protein  in  the  ration  increases,  a 
result  which  the  low  average  figures  obtained  on  pp.  427  and  450 
for  the  availability  and  the  percentage  utilization  of  the  proteids 
would  lead  us  to  expect.  Obviously,  however,  too  much  value 
should  not  be  attached  to  such  computations  as  the  above. 

Kornauth  &  Arche  *  in  an  investigation  on  the  feeding  value  of 
cockle  have  also  made  respiration  experiments  with  a  swine.  The 
food  consisted  in  Period  II  of  cockle,  barley,  and  maize,  and  irn 
Period  III  of  rape-cake,  barley,  and  maize,  the  amounts  of  the 
several  nutrients  actually  digested  being  nearly  the  same  in  the 
two  periods.  In  each  period  two  respiration  experiments  were 
made  which  gave  concordant  results.  The  following  table  contains 
the  average  results  for  each  period  computed  on  the  same  basis  as  in 
the  experiments  of  Meissl,  Strohmer  &  Lorenz.  No  fasting  experi- 
ments having  been  made,  the  average  results  of  the  experiments  by 
the  last-named  authors  have  been  used,  the  average  live  weight  of 
50  kgs.  being  taken  as  the  basis. 


Com- 

Esti- 

Metab- 

puted 

Main- 

1 

Nutri- 

mated 

olizable 

Per- 

tive 

Fasting 

Energy 

tenance 

Excess 

Gain, 

centage 

i 

Ratio. 

Metab- 

of 

Re- 

Cals. 

1. 

olism. 

Food, 

quire- 

Cals. 

Cals. 

ment, 

Cals. 

ii 

Cockle,  barlev,  and 

6.7 

1296 

3057 

1424 

1633 

1170 

71.7 

in 

Rape-cake,  barley, 

and  maize 

6.2 

1296 

3101 

1424 

1677 

1095 

65.3 

*  Landw.  Vers.  Stat.,  40,  177. 


THE   UTILIZATION  OF  ENERGY.  455 

The  percentages  as  thus  computed  are  seen  to  agree  fairly  well 
with  the  ones  computed  for  those  of  Meissl,  Strohmer  &  Lorenz's 
experiments  in  which  the  proportion  of  protein  in  the  food  was 
similar. 

Experiments  on  Ruminants. — Experiments  upon  ruminants 
necessarily  differ  in  one  important  respect  from  those  hitherto  con- 
sidered. With  carnivora  and  with  swine  it  is  possible  to  determine 
the  fasting  metabolism,  or,  in  other  words,  to  trace  the  line  repre- 
senting the  net  availability  or  the  utilization  throughout  its  entire 
extent.  With  herbivora,  and  particularly  with  ruminants,  this  is 
practically  impossible,  for  obvious  reasons,  and  the  course  of  the 
lower  portion  of  the  line  is  imaginary.  This,  however,  is  no  obsta- 
cle to  a  determination  of  the  net  availability  or  percentage  utiliza- 
tion of  the  food  within  the  limits  as  to  amount  prescribed  by  the 
nature  of  the  animals.  As  is  clear  from  the  graphic  discussion  of 
the  problem  on  pp.  410  and  446,  all  that  is  necessary  is  to  determine 
the  gain  or  loss  of  energy  by  the  body  corresponding  to  two  different 
amounts  of  food  above  or  below  maintenance.  A  simple  com- 
parison of  differences  then  gives  in  the  one  case  the  percentage 
utilization  and  in  the  other  the  net  availability  of  the  energy  of  the 
food  added.  The  method  is  the  same  principle  as  that  already 
employed  in  computing  the  metabolizable  energy  of  the  added 
food. 

The  Mockern  Experiments. — The  very  extensive  and  elabo- 
rate investigations  upon  cattle  at  the  Mockern  Experiment  Station 
by  G.  Kiihn  and  Kellner,*  which  have  already  been  discussed  in 
relation  to  the  metabolizable  energy  of  the  food,  are  also  our  chief 
source  of  knowledge  regarding  the  utilization  of  this  energy  by 
ruminants  and  will  necessarily  constitute  the  principal  basis  of  the 
present  discussion. 

These  experiments  were  chiefly  upon  the  fattening  of  mature 
cattle,  various  additions  being  made  to  basal  rations  which  were 
themselves  in  almost  every  case  more  than  sufficient  for  maintenance. 
The  actual  gain  of  carbon  and  nitrogen  by  the  animals,  both  on  the 
basal  and  the  augmented  rations,  was  accurately  determined,  and 
from  the  data  thus  obtained  the  gain  of  proteids  and  fat  and  of 
energy  was  computed  in  the  usual  way.  By  a  comparison  of  the 
*Landw.  Vers.  Stat.,  44,  257;  47,275;  50,245;  53,1. 


456  PRINCIPLES   OF  ANIMAL   NUTRITION. 

gains  on  the  basal  and  on  the  augmented  ration,  then,  we  may  deter- 
mine what  proportion  of  the  metabolizable  energy  of  the  added 
food  was  stored  in  the  gain  of  tissue.  In  other  words,  we  may 
determine  two  points  on  the  line  SU  m  the  figure  on  p.  446, 
thereby  determining  the  line  if  it  is  a  straight  line. 

If  the  added  metabolizable  energy  of  the  larger  ration  were  de- 
rived solely  from  the  material  added,  the  result  would  show  the  utili- 
zation of  the  energy  of  that  material.  As  we  have  seen,  however,  in 
connection  with  the  discussion  of  the  metabolizable  energy  of  the  food 
in  Chapter  X,  this  is  rarely  if  ever  the  case  with  herbivorous  animals. 
The  difference  in  metabolizable  energy  between  two  rations  usually 
includes,  in  addition  to  the  real  metabolizable  energy  of  the  added 
food,  differences  in  the  digestibility  of  the  original  ration  and  in 
the  losses  in  urine  and  methane.  Accordingly,  we  are  here  con- 
fronted with  the  same  alternative  as  before,  viz.,  whether  to  attempt 
to  eliminate  these  secondary  effects  and  base  our  computations 
on  the  real  metabolizable  energy  of  the  feeding-stuff  under  experi- 
ment or  to  take  the  apparent  metabolizable  energy  as  representing 
the  actual  amount  of  energy  contributed  to  the  metabolism  of  the 
body.  In  the  one  case,  if  successful,  we  shall  obtain  a  result  which 
will  be  physiologically  correct  but  which  when  applied  in  practice 
will  require  modification  for  the  secondary  effects  just  mentioned. 
In  the  other  case  we  shall  have  a  summary  expression  including 
all  these  results,  but  with  the  disadvantage  of  being  more  or  less 
empirical  in  its  nature.  Either  method  has  its  advantages  and 
disadvantages.  In  the  present  case  we  shall  use  the  apparent 
metabolizable  energy  of  feeding-stuffs  as  computed  on  pp.  285-297 
and  in  Tables  I-VI  of  the  Appendix  as  the  basis  of  computation. 
This  does  not,  of  course,  affect  the  absolute  amount  of  energy 
utilized  from  a  unit  weight  of  the  material,  but  only  the  percentage 
calculated  upon  the  metabolizable  energy. 

Sources  of  Uncertainty  in  Computation. — While  the  computation 
of  the  energy  utilized  from  feeding-stuffs  in  the  manner  just  indi- 
cated is  in  principle  very  simple,  certain  complications  arise  in  its 
execution  from  the  impossibility  of  securing  exactly  comparable 
conditions  of  experiment.  Two  of  these  in  particular  require 
consideration  here. 

Differences  in  Organic  Matter  Consumed. — As  was  noted  in  the 


THE   UTILIZATION  OF  ENERGY.  457 

discussion  of  the  metabolizable  energy  of  feeding-stuffs,  the  un- 
avoidable slight  variations  in  the  moisture-content  of  the  latter 
in  the  Mockern  experiments  resulted  in  slight  differences  in  the 
amounts  of  organic  matter  of  the  basal  ration  consumed  in  the 
several  periods.  A  comparison,  then,  between  two  periods,  as  re- 
gards metabolizable  energy  and  resulting  gain,  shows  the  effect  of 
the  added  feeding-stuff  plus  the  effect  of  this  small  difference. 
For  the  metabolizable  energy  an  approximate  correction  was  com- 
puted. In  order  to  make  a  similar  correction  in  the  resulting  gain 
of  tissue,  however,  it  is  necessary  to  know  to  what  extent  this 
difference  in  metabolizable  energy  contributed  to  the  observed 
gain;  that  is,  to  know  the  percentage  utilization  of  the  basal  ration. 
No  direct  determinations  of  this  factor,  however — that  is,  no  com- 
parisons of  the  results  of  feeding  different  amounts  of  the  basal 
ration — were  made.  In  his  discussion  of  the  results  Kellner  virtually 
assumes  a  percentage  of  utilization  by  subtracting  from  the  total 
metabolizable  energy  of  the  food  the  average  amount  required  for 
maintenance  as  determined  by  his  own  experiments  and  then  com- 
paring the  energy  in  excess  of  the  maintenance  requirement  with 
the  resulting  gain. 

Differences  in  Live  Weight. — The  live  weights  of  the  animals  in 
the  Mockern  experiments  differed  considerably  in  the  different 
periods.  This  would  probably  result  in  differences  in  the  require- 
ments for  maintenance,  although  the  data  at  hand  seem  insufficient 
to  satisfactorily  determine  the  relation  between  live  weight  and 
maintenance  (see  p.  441).  Kellner  assumes  that  the  maintenance 
ration  is  in  proportion  to  the  two-third  power  of  the  live  weight,  a 
result  which  has  already  been  shown  to  correspond  fairly  well  with 
the  results  upon  Ox  B,  although  in  apparent  conflict  with  the  aver- 
age results  obtained  on  other  animals. 

Utilization  of  Basal  Ration. — In  order  to  be  able  to  correct  the 
results  for  differences  in  organic  matter  consumed  and  differences 
in  live  weight,  it  is  necessary,  as  has  just  been  pointed  out,  to  know 
the  percentage  utilization  of  the  basal  ration.  This  Kellner  assumes 
in  assuming  a  maintenance  ration.  There  are,  however,  serious  ob- 
jections to  this  method  of  procedure.  First,  the  maintenance  ration 
used  by  Kellner  is  an  average,  based  on  results  which  were  obtained 
with  a  number  of  animals,  not  including  all  those  used  in  the  fatten- 


458  PRINCIPLES   OF  ANIMAL   NUTRITION. 

ing  experiments,  and  which  show  a  range  of  13.5  per  cent,  of  the 
average.  Second,  the  computed  maintenance  ration  is  based  upon 
experiments  with  coarse  fodder  only.  We  have  seen  reason  to  be- 
lieve, however  (pp.  388-391)  that  the  net  availability  of  the  metabo- 
lizable  energy  in  coarse  fodders  is  decidedly  less  than  in  case  of  con- 
centrated feeds,  and  that  consequently  more  metabolizable  energy 
would  be  required  for  maintenance  on  a  ration  composed  of  coarse 
fodder  than  on  one  containing  concentrated  feeds,  as  did  Kellner's 
basal  rations.  In  other  words,  Kellner's  assumed  maintenance 
ration  is  probably  somewhat  too  large  and  his  computed  utiliza- 
tion of  the  basal  ration,  therefore,  also  somewhat  too  high.  Third, 
it  is  by  no  means  demonstrated  that  the  maintenance  ration  of 
fattened  as  compared  with  unfattened  animals  is,  as  Kellner  as- 
sumes, in  proportion  to  the  two-third  power  of  the  live  weight. 

In  the  absence  of  any  direct  determinations  of  the  utilization 
of  the  basal  rations,  however,  there  seems  to  be  no  course  open  but 
to  follow  substantially  Kellner's  method  of  computation  and  assume 
a  maintenance  ration  for  each  of  the  animals  in  proportion  to  the 
two-thirds  power  of  its  live  weight  during  the  period  under  con- 
sideration. 

Computation  of  Results. — The  method  of  computing  the  correc- 
tions for  the  differences  in  live  weight  and  in  the  amount  of  the 
basal  ration  consumed  may  be  illustrated  by  the  same  two  periods 
which  were  used  on  pages  288-9  to  exemplify  the  computation  of 
metabolizable  energy,  viz.,  Periods  4  and  7  with  Ox  H,  on  meadow 
hay.  In  Period  4,  on  the  basal  ration,  the  live  weight  was  668.9 
kgs.,  the  computed  maintenance  requirement  13,989.1  Cals.,  and 
the  gain  by  the  animal  2003.2  Cals.  The  percentage  utilization 
therefore  was  as  follows : 

Metabolizable  energy  of  ration 17,388.8  Cals. 

Computed  maintenance  requirement  .  .  13,989.1     " 

Excess  food 3399.7  Cals. 

Gain 2003.2     " 

Percentage  utilization 58.9  <f0 

In  Period  7  the  total  metabolizable  energy  of  the  ration  was 
26,013.0  Cals.  and  the  gain  5643.2  Cals.     Of  the  excess  of  8624.2 


THE  UTILIZATION  OF  ENERGY. 


459 


Cals.  over  Period  4,  however,  it  was  computed  that  119.4  Cals.  were 
due  to  an  increased  consumption  of  the  ingredients  of  the  basal 
ration,  leaving  8504.8  Cals.  as  the  metabolizable  energy  of  the 
added  hay.  This  119.4  Cals.,  however,  contributed  to  the  increased 
gain  of  3640.0  Cals.  made  by  the  animal.  If  we  assume  the  per- 
centage 58.9  just  computed  to  apply  to  it,  the  corresponding  gain 
would  be  119.4  X  0.589  or  70.3  Cals.,  leaving  3569.7  Cals.  as  the 
gain  produced  by  the  8504.8  Cals.  of  metabolizable  energy  derived 
from  the  meadow  hay. 

In  Period  7,  however,  the  animal  weighed  736.0  kgs.,  and  his 
computed  maintenance  requirement  was  therefore  14,909.6  Cals.  of 
metabolizable  energy,  or  920.5  Cals.  more  than  in  Period  4.  In 
other  words,  if  he  had  weighed  no  more  in  Period  7  than  in  Period 
4,  there  would  have  been  920.5  Cals.  more  metabolizable  energy 
which  could  have  served  to  produce  a  gain  of  tissue.  Assuming,  as 
before,  that  58.9  #  of  this  would  be  stored  in  the  body,  the  result- 
ing gain  would  have  been  920.5  X  0.589  or  542.2  Cals.  Adding  this 
to  the  gain  of  3569.7  Cals.  just  computed  makes  a  total  of  4111.9 
Cals.  as  the  computed  gain  to  be  credited  to  8504.8  Cals.  of  metab- 
olizable energy  in  the  hay  added,  which  is  equivalent  to  a  per- 
centage utilization  of  48.4  per  cent.  Expressed  in  tabular  form, 
the  results  of  these  comparisons  are  as  follows : — 


3 

§ 

< 
H 

II 

— ' 
o 

7 
4 

<  > 
3 

Metabo- 
lizable 

Energy, 
Cals. 

Com- 
puted 
Mainte- 
nance, 
Cals. 

Excess 
over 
Mainte- 
nance, 
Cals. 

Energy 
of  Gain 

(Cor- 
rected), 

Cals. 

Meadow  Hay,  VI: 

Basal  ration  +  hay 

Correction  for  organic  matter 

736.0 
668.9 

26.013.0 
-119.4 

14,909.6 
13,989.1 

11,103.4 
-119.4 

5,643.2 
-70.3 

Correction  for  live  weight . .  . 

25,893.6 

17,388.8 

10,984.0 
+  920.5 

5.572.9 

+  542.2 

11,904.5 
3,399.7 

6.115.1 
2.003.2 

58.9 

8,504.8 

8,504.8 

4,111.9 

48.4 

Table  VII  of  the  Appendix  contains  the  details  of  the  computa- 
tions of  percentage  utilization  according  to  the  above  method. 
The  results   differ   somewhat  from  those  reported    by  Kellner,* 
*  Loc.  cit.,  pp.  63,  133,  226,  and  334. 


460  PRINCIPLES  OF  ANIMAL   NUTRITION. 

first,  because  they  include  a  correction  for  the  differences  in  or- 
ganic matter  consumed,  and  second,  because  the  energy  of  the  gain 
has  been  corrected  for  the  amount  of  nitrogen  retained  in  the  body 
in  the  same  manner  as  the  energy  of  the  urine  (compare  p.  285), 
viz.,  by  deducting  7.45  Cals.  per  gram  of  nitrogen.  In  most  cases 
the  metabolizable  energy  is  that  already  computed  in  Tables  I  to 
VI  of  the  Appendix,  being  based  on  actual  calorimetric  determina- 
tions in  food  and  feces.  In  two  instances  (distinguished  by  being 
bracketed)  the  metabolizable  energy  has  been  computed  by  the 
writer  from  such  data  as  are  available.* 

In  Table  VII  the  final  results  are  expressed  as  percentages  of 
the  metabolizable  energy  utilized.  By  combining  them  with  the 
results  contained  in  the  six  preceding  tables  of  the  Appendix  they 
may  likewise  be  expressed  as  percentages  of  the  gross  energy  of 
the  several  materials  and  also  as  energy  utilized  per  gram  of  total 
organic  matter.  The  summary  on  pp.  461-2  contains  the  results 
expressed  in  all  of  these  ways. 

Earlier  Experiments. — The  earlier  respiration  experiments 
of  Henneberg  &  Stohmann  f  on  oxen,  in  1865,  while  made  in  accord- 
ance with  the  experience  then  available,  are  now  known  to  be  de- 
fective in  several  respects.  The  respiratory  products  were  deter- 
mined for  twelve  hours  only,  while  the  same  authors  subsequently 
showed  that  twenty-four  hours  was  the  minimum  time  necessary. 
The  food  consumed  on  the  respiration  day  was  less  than  the  average 
for  the  whole  experiment,  but  how  much  less  does  not  appear,  and 
finally  the  methods  used  for  the  determination  of  the  hydrocarbon 
gases  excreted  have  subsequently  been  shown  to  give  too  tow  re- 
sults. It  seems  useless  therefore  to  enter  into  an  elaborate  com- 
putation of  the  results.  In  the  later  experiments  of  the  same 
authors  %  with  sheep,  these  sources  of  inaccuracy  were  largely  re- 

*  The  data  used  in  these  computations  are  as  follows: 

For  Ox  IV  the  average  results  for  Periods  la  and  \b  have  been  com- 
puted on  the  assumption  that  the  heat  values  of  food  and  excreta  per  gram 
in  Period  16  were  the  same  as  those  determined  in  Period  \a 

For  Ox  V  the  metabolizable  energy  in  Period  3  has  been  computed  by 
adding  to  that  in  Period  2a  3  345  Cals.  for  each  gram  of  organic  matter  in 
the  starch  added,  this  being  the  metabolizable  energy  computed  for  the 
starch  in  Period  2a. 

t  Neue  Beitrage,  p.  287.  J  hoc.  cit ,  p  68. 


THE   UTILIZATION  OF  ENERGY. 


461 


ENERGY  UTILIZED. 

Per  Cent,  of 

Metabo- 

lizable 

Energy. 

Per  Cent,  of 
Gross 
Energy. 

Per  Grm. 

Total 
Organic 
Matter. 

Meadow  Hay  : 

Sample  V.OxF 

40.4 
36.2 

16.5 
15.9 

0.780 

"       V,  "    G 

0.756 

38.3 

50.4 

48.4 
34.8 

16.2 

26.5 
26.1 
18.5 

0.768 

Sample  VI,  Ox  H,  Period  2 

1.266 

"       VI,  "    H,       "       7 

1.247 

VI,   "     J 

0.883 

44.5 
41.4 

38.8 
33.4 

23.7 
20.0 

14.2 
11.7 

1.132 

0.950 

Oat  Straw  : 
Ox  F.   .              

0.682 

"    G 

0.564 

36.1 

10.8 
24.0 

12.9 

3.2 

7.8 

0.623 

Wheat  Straw  : 

Ox  H 

0.153 

"    J 

0.373 

17.4 

67.3 
58.6 

5.5 

51.6 
43.6 

0.263 

Extracted  Rye  Straw : 

OxH 

2.194 

"     J 

1.854 

63.0 

58.5 
83.4 
50.2 

47.6 

41.6 
65.9 
36.5 

2.024 

eet  Molasses : 
Sample    I,  Ox  F 

1.700 

"       II,  "  H 

2.760 

"       II,  "     J 

1.529 

66.8 

50.0 
49.2 

51.2 

35.6 
31.3 

2.145 

Starch — Kuhn's  Experiments : 

Sample  I.  Ox  III 

1.514 

I,   "     IV 

1  331 

Average 

Sample  II,  Ox    V,  Period  2a   

"       II.   "     V,       "      26 

49.6 

53.2 

53.7 
59.7 
48.1 
46.6 

33.5 

42.0 
40.1 

34.3 
32.6 

1.423 

1.779 

1.699 

"       II.  "VI,       "      26 

"       II.   "    VI,       "      3 

1.452 
1.380 

50.4 
50.0 

37.3 
35.4 

1.578 

Average  I  and  II 

1.501 

462 


PRINCIPLES  OF  ANIMAL   NUTRITION. 
ENERGY  UTILIZED  (Continued). 


Per  Cent,  of 
Metabo- 
lizable 
Energy. 


Per  Cent,  of 
Gross 
Energy. 


PerGrm 

Total 
Organic 
Matter 


Starch — Kellner's  Experiments  : 

Sample  I  and  II,  Ox  B 

"       I     "    II,   "     C 

Average 

Sample  III,  Ox  D 

Ill,   "     F 

"       III,   "    G 

Average. 

Sample  IV,  Ox  H 

"       IV,   "     J 

Average 

Average  III  and  IV 

Wheat  Gluten — Kiihn's  Experiments : 

Ox  III,  Period  3 

"    III,       "       4 

Average. 

Ox  IV 

Wheat  Gluten — Kellner's  Experiments 

Sample  I,  Ox  B,  Period  1 

"       I.    "  B,       "      3 

"       I,    "    C 

Average   

Sample  II,  Ox  D 

Average  of  I  and  II 

Peanut  Oil . 

Sample    I.  Ox  D 

"       II.  "    F 

"       II,  "    G 

Average 


65.4 
57.6 


61.5 

53.7 

64.8 
65.8 


61.4 


56.0 
54.8 


55.4 

58.4 


45.3 
48.0 


46.7 
58.2 


49.7 
43.2 


43.3 
37.3 
40.3 


51.6 
65.1 
69.4 


67.3 


31.8 
28.0 


29.9 

36.1 

46.2 
50.9 


44.4 


44.4 
39.5 


42.0 
43.2 


37.0 
35.8 


36.4 
58.9 


19.6 
32.6 
30.9 


27.7 
26.1 
26.9 


40.1 
34.2 
41.2 


1.325 
1.168 


1.247 

1.500 
1.922 
2.116 


1.846 


1.855 
1.652 


1.754 
1.800 


2.289 
2.213 


2.251 
3.645 


1.115 
1.849 
1.850 


37.7 


1.605 
1.516 
1.561 


3.811 
3.238 
3.903 

3.571 


THE  UTILIZATION  OF  ENERGY.  463 

moved,  but  the  experiments  were  upon  maintenance  feeding  only 
and  afford  no  data  for  a  computation  of  utilization. 

A  series  of  respiration  experiments  on  sheep  was  made  by 
Kern  &  Wattenberg  at  the  Gottingen-Weende  Experiment  Station 
in  1879,  the  results  of  which  were  reported  after  Kern's  death  by 
Henneberg  &  Pfeiffer.*  Varying  quantities  of  nearly  pure  proteids 
(conglutin  or  flesh-meal)  were  added  to  a  basal  ration  of  hay  and 
barley  meal,  the  amount  of  proteids  in  the  ration  being  regularly 
increased  by  about  50  grams  in  each  of  four  successive  periods  and 
then  similarly  diminished  through  three  more  periods. 

The  experiments  suffered  from  some  defects  in  technique  which 
later  experience  has  remedied,  the  results  most  strikingly  affected 
being  those  for  the  amount  of  methane  excreted.  For  the  first 
two  periods  no  results  are  reported;  for  the  remaining  periods  they 
are  quite  variable,  and  those  on  different  days  of  the  same  period 
differ  widely.  The  authors  consider  that  their  figures  represent 
the  minimum  amount  present,  and  in  their  final  computations  use 
the  average  of  all  the  five  periods  as  the  basis  for  estimating  the 
quantity  of  carbon  excreted  in  this  form.  The  amounts  as  actually 
determined  showed  a  considerable  diminution  in  the  periods  in 
which  most  proteids  were  fed,  contrary  to  Kuhn's  results,  but  it  is 
worthy  of  note  that  the  average  proportion  of  carbon  dioxide  to 
methane  was  not  much  different  from  that  found  by  the  latter. 
The  determinations  of  carbon  dioxide  in  the  respiratory  products 
likewise  showed  considerable  fluctuations  from  day  to  day,  but  as 
the  results  are  mostly  the  average  of  three  or  four  trials  of  twenty- 
four  hours  each  it  may  be  assumed  that  these  variations  are  more 
or  less  compensated  for.  The  respiratory  products  were  determined 
for  both  animals  together,  although  ail  the  other  data  were  secured 
for  each  individual.  The  results  given  on  the  following  pages, 
therefore,  are  the  totals  for  both  animals. 

It  is  stated  that  addition  of  proteids  to  the  ration  resulted  in 
the  diminution,  and  final  disappearance  in  the  middle  period,  of  the 
hippuric  acid  of  the  urine,  but  the  actual  amounts  present  are  re- 
ported only  for  the  first  and  last  periods.  It  is  not  possible,  there- 
fore, to  make  any  satisfactory  computation  of  the  energy  of  the 
urine  or  of  the  proper  factor  for  the  metabolizable  energy  of  the 
digested  proteids  of  the  total  ration.  By  another  method  of  com- 
*  Jour.  f.  Landw.,  38.  215. 


464 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


putation,  however,  it  seems  possible  to  secure  an  approximate  idea 
of  the  relation  of  added  food  to  gain. 

By  subtracting  from  the  food  digested  in  Periods  II-VI  the 
average  amount  digested  in  Periods  I  and  VII,  on  the  basal 
ration,  we  find  the  amounts  of  added  food,  consisting  chiefly  of 
proteids.  Reckoning  the  metabolizable  energy  of  the  added  pro- 
teids  at  4.958  Cals.  per  gram  (compare  p.  317),  that  of  the  crude 
fiber  and  nitrogen-free  extract  at  3.674  Cals.,  and  that  of  the  ether 
extract  at  8.322  Cals.,  we  get  the  approximate  metabolizable  energy 
of  the  added  food,  and  can  compare  it  with  the  energy  of  the  corre- 
sponding gain.     Thus  for  Period  II  we  have  the  following: 

DIGESTED. 


Protein,* 
Grms. 

Crude 
Fiber, 
Grms. 

Nitrogen- 
free 
Extract, 
Grms. 

Ether 
Extract 
Grms. 

Period  II 

211.33 

280.77 
277.91 

643.22 

633.12 

20.88 

101.05 

21.60 

110.28 

Cals. 
546.8 

2.86 

10.10 

-0.72 

Equivalent     metabolizable 

12 

Cals. 

47.6 

.96 

Cals. 

-6 

*  Protein  of  basal  ration  and  of  feces  equals  N  X  6.25;   that  of  conglutin 
or  flesh-meal  equals  its  total  organic  matter. 

GAIN. 


Protein   Grms. 

Fat.  Grms. 

Period  11 

15.00 
6.85 

69.27 
19.66 

Periods  I  and  VII.  .  . 

Difference 

Equivalent  energy 

8.15 

Cals. 

46.3 

49.61 
Cals. 
471.5 

The  figures  for  the  gain  are  those  given  by  the  authors,  based 
on  the  assumption  of  a  uniform  excretion  of  methane  throughout 
the  experiments;  the  gain  of  protein  includes  that  contained  in  the 
wool  produced.  The  animals  gained  slightly  in  weight,  in  addition 
to  the  growth  of  wool.  Computed  on  this  basis,  the  percentage  of 
the  energy  of  the  added  food  which  was  utilized  was  as  follows: 


THE   UTILIZATION  OF  ENERGY. 


465 


Period. 

Metabolizable 

Energy  of 

Added  Food, 

Cals. 

Energy  of 

Resulting  Gain. 

Cals. 

Per  Cent. 
Utilized. 

I      II 

588.4 
1100.3 
1639.2 
1131.7 

454.9 

517.8 
741.8 
1106.8 
672.5 
315.7 

88.00 

Consrlutin:    <   III 

67.42 

g             1    IV 

67.51 

•p,    ,          ,    1      V 

59.41 

±lesh-meal:  j    yj 

69.39 

A  computation  based  on  the  observed  amounts  of  methane 
would  affect  the  above  figures  in  two  ways.  First,  if  the  added 
proteids  diminished  the  production  of  methane,  this  was  equivalent 
to  an  increase  in  the  apparent  metabolizable  energy  of  the  food, 
and  the  figures  for  the  latter  must  be  correspondingly  increased. 
Second,  the  gain  of  fat  will  also  appear  relatively  greater  in  the 
intermediate  periods,  II-VI,  and  the  figures  for  the  energy  of  the 
gain  must  also  be  increased.  Computed  on  this  basis  the  results 
are: 


Period. 

Energy  of 

Added  Food, 

Cals. 

Energy  of 

Resulting  Gain, 

Cals. 

Per  Cent. 
Utilized. 

(       II 

Conglutin:    1   III 

715.4 
1245.8 
1902.3 

1288.2 
582.1 

605.7 
842.4 
1288.8 
780.7 
403.6 

84.68 
67  63 

(   iv 

™    .          ,    I      V 

67.76 
60.59 

b  lesn-meal :  -j    y  j 

69.33 

No  obvious  explanation  of  the  exceptionally  high  results  ob- 
tained in  Period  II  presents  itself.  Those  of  the  remaining 
periods  do  not  seem  to  indicate  any  considerable  differences  in  the 
utilization  of  different  quantities.  The  figures  are  notably  higher 
than  those  computed  from  the  Mockern  experiments,  but  in  view 
of  the  uncertainties  attaching  to  them  too  much  stress  should  not 
be  laid  on  this  fact. 


Discussion  of  Results. 

As  was  pointed  out  at  the  beginning  of  this  section,  and  as  was 
further  apparent  in  considering  the  results  of  experiments  upon 
carnivora,  our  knowledge  of  the  net  availability  of  the  energy  of 
feeding-stuffs  and  nutrients  is  too  imperfect  to  permit  the  experi- 


466  PRINCIPLES   OF  ANIMAL    NUTRITION. 

mental  results  above  detailed  to  be  discussed  from  the  standpoint 
of  the  percentage  utilization  of  the  net  available  energy. 

Furthermore,  even  confining  ourselves  to  a  consideration  of  the 
utilization  of  the  metabolizable  energy  of  the  food,  we  have  already 
seen  that  the  recorded  results  upon  carnivorous  animals  show  such 
wide  divergencies  as  to  render  it  difficult  if  not  impossible  to  draw 
any  quantitative  conclusions  from  them. 

For  the  present,  accordingly,  our  discussion  of  the  utilization 
of  energy  must  be  confined  chiefly  to  the  results  which  have  been 
reached  with  herbivora.  and  in  the  main  to  the  Mockern  experi- 
ments, and  we  must  content  ourselves  with  an  attempt  to  trace  the 
relations  between  metabolizable  energy  and  energy  utilized,  or,  to 
look  at  the  subject  from  the  other  point  of  view,  with  determining 
the  proportion  of  the  metabolizable  energy  of  the  food  which  is 
expended  in  the  combined  work  of  digestion,  assimilation,  and 
tissue  building.  From  the  practical  standpoint  this  is  of  course 
the  important  thing,  since  either  form  of  expenditure  of  energy 
constitutes,  in  the  economic  aspect  of  the  matter,  a  waste,  but  it 
is  nevertheless  to  be  regretted  that  it  is  at  present  impossible  to 
further  analyze  this  waste. 

Influence  of  Amount  of  Food. — As  in  the  discussion  of  net 
availability  in  Chapter  XII,  we  have  thus  far  assumed  the  energy 
utilized  to  be  a  linear  function  of  the  net  available  or  of  the  metabo- 
lizable energy  of  the  food.  Before  proceeding  further  it  becomes 
important  to  consider  how  far  this  assumption  is  justified  by  the 
facts  on  record. 

Carnivora. — Of  the  experiments  upon  carnivora  recorded  on 
preceding  pages,  those  of  Rubner  with  different  amounts  of  meat, 
when  computed  by  his  method  (that  is,  assuming  an  availability  of 
100  per  cent,  below  the  maintenance  point,  as  on  p.  450).  appear  to 
indicate  that  the  utilization  above  that  point  is  constant.  If,  how- 
ever, a  lower  percentage  of  availability  is  assumed,  as  on  p.  451, 
this  constancy  disappears.  None  of  the  other  results  there  sum- 
marized seem  suitable  for  discussion  from  this  point  of  view. 

Swine. — If  in  the  experiments  of  Meissl,  Strohmer  &  Lorenz,  as 
computed  on  p.  454,  we  express  the  estimated  metabolizable  energy 
of  the  excess  food  as  a  percentage  of  the  fasting  metabolism,  we 
have  the  following  comparison  of  the  percentage  utilization  with 


THE   UTILIZATION  OF  ENERGY. 


467 


the  relative  amount  of  excess  food,  to  which  may  be  added  Kor- 
nauth  &  Arche's  results  similarly  computed: 


Excess  Over 

Fasting 

Metabolism, 

Per  Cent. 

Percentage 
Utilization. 

Meissl : 

133 

250 

74 

180 

126 
129 

80.7 
75.2 
70.9 
67.1 

71.7 
65.3 

experiment    £•••■■••; 

"             III 

"             IV 

Kornauth  &  Arche : 

experiment   ^. ........ 

While  there  is  some  variation  in  the  percentage  utilization,  as 
would  naturally  be  expected  in  experiments  with  different  animals, 
the  range  in  the  relative  amount  of  excess  food  is  much  greater 
and  there  is  no  indication  of  a  connection  between  the  two. 

Ruminants. — The  earlier  Mockern  experiments  by  G.  Kiihn 
include  one  upon  wheat  gluten  and  two  upon  starch  in  which  two 
different  quantities  were  added  to  the  basal  ration  of  the  same 
animal.    The  final  results  were  as  follows: 


Animal. 

Period 

Added  to 

Basal  Ration, 

Kgs 

Percentage 
Utilization 
of  Metaboliz- 
able  Energy. 

Wheat  gluten ....  -J 

Starch J 

I 

Ill 

111 

V 

V 

V 

VI 

VI 

3 

4 

2a 

2b 

3 

2b 

3 

0.68 

1.36 

2.0 

2.0 

3.5 

2.0 

3.5 

45.3 
48.0 
53.2 
53.7 
59.7 
48.1 
46.6 

These  results  do  not  indicate  that  any  material  effect  is  exerted 
upon  the  utilization  of  the  metabolizable  energy  of  the  food  by 
the  amount  consumed,  since  the  differences  are  small  in  themselves 
and  in  both  directions. 

The  results,  reported  by  Pfeiffer,  of  experiments  upon  the  addi- 
tion of  varying  amounts  of  proteids  to  a  basal  ration,  as  computed 
by  the  writer  (p.  465),  likewise  show  a  fairly  constant  percentage 


468 


PRINCIPLES  OF  ANIMAL  NUTRITION. 


utilization  of  the  energy  of  the  proteids  used,  with  the  exception 
of  the  strikingly  higher  result  of  the  first  period. 

A  similar  conclusion  may  be  drawn  from  a  study  of  the  Mockern 
results  as  a  whole,  as  recorded  in  Table  VII  of  the  Appendix. 
While  the  computed  percentages  in  each  series  vary  more  or  less 
in  the  different  experiments,  the  differences  are  in  most  cases  not 
large  and  appear  to  bear  no  relation  either  to  the  total  quantity 
of  food  given  or  to  the  amount  of  the  particular  food  under  experi- 
ment which  was  added  to  the  basal  ration,  but  to  be  due  rather 
to  individual  differences  in  the  animals.  This  is  strikingly  shown 
in  the  following  table,  in  which  the  results  upon  hay,  wheat  gluten, 
and  starch  are  arranged  in  the  order  of  the  percentage  utilization : 


Metaboliz- 

Total  Excess 

Percentage 

able  Energy 

Over  Com- 

Utilization 

Feeding-stuff. 

Animal. 

Period. 

of  Added 

puted  Main- 

of Metabo- 

Food, 

tenance, 

lizable 

Cals. 

Cals. 

Energy. 

f 

J 

2 

7875 

12,192 

34.8 

G 

2 

5726 

9,780 

36.2 

Meadow  hay < 

F 

1 

5506 

10,184 

40.4 

H 

7 

8505 

11,905 

48.4 

I 

H 

2 

7875 

11.275 

50.4 

B 

1 

4483 

15,129 

36.9 

D 

4 

5713 

17,373 

37.3 

C 

3 

6033 

19,635 

43.2 

Wheat  gluten * 

in 

3 

2913 

8,982 

45.3 

in 

4 

5332 

11,401 

48.0 

B 

3 

5507 

16,153 

49.7 

IV 

3 

3645 

7,132 

58.2 

VI 

3 

8264 

12,364 

46.6 

VI 

26 

5038 

9,138 

48.1 

Starch — Kilhn'sexpts. . 

IV 

III 

2 
2 

4350 
4998 

3,411 
6,592 

49.2 
50.0 

V 

2a 

5425 

8,821 

53.2 

V 

3 

9658 

13,054 

59.7 

D 

2 

4420 

16,080 

53.7 

J 

3 

4826 

9,142 

54.8 

H 

3 

6668 

10,068 

56.0 

Starch — Kellner's  expts  - 

c 

2 

3027 

16,829 

57.6 

F 

4 

5009 

9,686 

64.8 

B 

2 

3291 

13,937 

65.4 

G 

4 

5387 

9,441 

65.8 

But  while  this  is  true  of  each  series  by  itself,  a  comparison  of 
the  two  series  upon  starch  leads  to  a  different  conclusion.  In 
Kiihn's  experiments  the  basal  rations  consisted  largely  or  exclu- 
sively of  coarse  fodder.     In  Kellner's  experiments  the  starch  was 


THE  UTILIZATION  OF  ENERGY.  469 

added  to  a  materially  heavier  basal  ration  containing  considerable 
grain  and  therefore  already  tolerably  rich  in  starch  and  other  carbo- 
hydrates. In  spite  of  the  smaller  average  amounts  of  starch  added, 
then,  Kellner's  results  in  a  sense  represent  the  percentage  utiliza- 
tion of  larger  quantities  of  starch  than  do  Kuhn's;  that  is,  they 
represent  the  utilization  of  starch  at  a  greater  distance  above  the 
maintenance  ration.     The  average  utilization  (pp.  461-2)  was — 

Kuhn's  experiments 50 . 0  per  cent. 

Kellner's  experiments,  moderate  rations  ...   58.4   "       " 
"  heavy  rations 61.5    "       " 

It  would  appear,  then,  from  these  figures  that  the  metaboliz- 
able  energy  of  starch  was  more  fully  utilized  in  rations  containing  a 
relatively  large  quantity  of  it.  At  least  a  partial  explanation  of 
this  seems  to  be  afforded  by  the  variations  in  the  production  of 
hydrocarbons  (methane).  As  was  mentioned  in  discussing  the 
metabolizable  energy  of  starch,  the  conditions  in  Kuhn's  experi- 
ments were  such  as  to  permit  a  considerable  proportion  of  the 
starch  to  undergo  the  methane  fermentation,  while  the  more  abun- 
dant supply  of  it  in  Kellner's  experiments  resulted  in  reducing,  or 
in  some  cases  wholly  suppressing,  this  fermentation  of  the  starch. 
The  effect  of  this,  as  there  pointed  out.  was  to  make  the  metaboliz- 
able energy  per  gram  greater  in  Kellner's  than  in  Kuhn's  experi- 
ments, but  it  has  also  another  result.  As  we  have  seen,  the  methane 
fermentation  constitutes  part  of  the  work  of  digestion,  in  the  general 
sense  in  which  that  term  is  here  employed,  the  amount  of  the  latter 
being  measured  by  the  heat  evolved.  This  amount  being  less  in 
Kellner's  than  in  Kuhn's  experiments,  the  net  availability  of  the 
metabolizable  energy  of  the  starch  should  be  greater,  and,  other 
things  being  equal,  the  storage  of  energy  (gain  of  tissue)  should  also 
be  greater. 

Kellner  *  computes  that  for  each  100  grams  of  starch  digested 
there  was  produced,  on  the  average,  methane  corresponding  to  the 
following  amounts  of  carbon: 

In  Kuhn's  experiments 3.0  grams 

In  Kellner's  experiments 2.3      " 

*  hoc.  tit.,  p.  423. 


47o  PRINCIPLES   OF  ANIMAL   NUTRITION. 

An  approximate  computation  of  the  probable  differences  in  the 
heat  evolved  by  the  fermentation,  based  on  such  data  as  are  avail- 
able, gives  as  a  result  0.159  Cal.  per  gram  of  starch,  or  somewhat 
more  than  one-half  the  difference  in  average  utilized  energy,  viz., 
0.265  Cal.  per  gram.  The  data  on  which  the  computation  is  based, 
however,  are  too  uncertain  to  allow  us  to  attach  very  much  value 
to  the  results,  except  perhaps  as  an  indication  that  the  supposed 
cause  of  the  difference  in  the  utilization  of  the  energy  is  insuffi- 
cient to  fully  account  for  the  effect. 

Conclusions. — It  cannot  be  claimed  that  the  above  results  are 
sufficiently  extensive  or  exact  to  permit  final  conclusions  to  bs 
drawn,  but  their  general  tendency  seems  to  be  in  favor  of  the  hy- 
pothesis that  the  proportion  of  energy  utilized  is  substantially  inde- 
pendent of  the  quantity  of  food,  provided  that  the  changes  in  the 
latter  are  not  so  great  as  to  modify  the  course  of  the  fermentations 
•  in  the  digestive  tract.  The  results  upon  starch  just  considered 
seem  to  indicate  that  if  the  variations  in  quantity  or  make-up  of 
the  ration  are  pushed  beyond  that  point,  a  difference  in  the  pro- 
portion of  the  energy  utilized  may  be  caused  by  a  difference  in  the 
digestive  work;  in  other  words,  that  it  is  the  availability  that  is 
modified  rather  than  the  proportion  of  the  available  energy  which  is 
recovered  as  gain.  While  not  denying  that  the  latter  function  may 
be  also  modified,  either  directly  as  the  effect  of  varying  amounts 
of  food,  or  indirectly  by  changes  in  the  chemical  nature  of  the  sub- 
stances resorbed  from  the  digestive  tract  under  varying  conditions 
of  fermentation,  it  seems  probable  that  the  main  effect  is  that  upon 
availability. 

It  is  to  be  observed  that  the  rations  used  in  these  experiments, 
while  not  heavy  fattening  rations,  still  produced  very  fair  gains. 
The  experimental  periods  were  comparatively  short  and  hence 
the  testimony  of  the  live  weight  itself  is  liable  to  be  misleading. 
Taking  the  actual  gains  of  fat  and  proteids  as  shown  by  the  respi- 
ration experiments,  however,  and  comparing  them  with  the  compo- 
sition of  the  increase  of  live  weight  in  fattening  as  determined  by 
Lawes  &  Gilbert,  it  appears  that  the  total  gain  per  day  was  equiva- 
lent to  from  0.9  to  2.5  pounds  gain  in  live  weight  per  day  in  the  ex- 
periments on  coarse  fodder,  while  in  those  upon  concentrated  feeds 
the  corresponding  range  is  from  1  to  3  pounds. 


THE   UTILIZATION   CF  ENERGY.  47 x 

It  may  be  remarked  further  that  the  rations  in  Kiihn's  experi- 
ments differed  materially  from  those  ordinarily  used  in  practice, 
both  as  to  their  make-up  and  their  very  wide  nutritive  ratio,  so 
that  the  conditions  may  fairly  be  regarded  as  in  a  sense  abnormal. 
Kellner's  rations  represent  more  nearly  normal  conditions,  and 
they  fail,  as  we  have  seen,  to  give  any  clear  indications  of  an  in- 
fluence of  amount  of  food  upon  the  proportion  of  energy  utilized. 
Whether  other  feeding  materials  show  a  behavior  analogous  to  that 
of  starch,  future  investigations  must  decide.  In  the  meantime 
we  are  apparently  justified  in  discussing  such  results  as  are  now 
on  record  upon  the  provisional  hypothesis  that,  within  reasonable 
limits,  the  utilization  of  energy  is  independent  of  the  amount  of 
food,  or,  in  other  words,  is  a  linear  function. 

Influence  of  Thermal  Environment.  —  The  influence  of  the 
thermal  environment  of  the  animal  upon  its  heat  production  and 
upon  the  net  availability  of  the  energy  of  the  food  has  already  been 
fully  discussed  in  previous  pages  and  needs  only  a  brief  consider- 
ation here. 

Ruminants. — We  have  already  found  reason  to  think  that  in 
ruminants  the  heat  production  on  the  ordinary  maintenance  ration 
is  in  excess  of  the  needs  of  the  body.  Kiihn's  and  Kellner's  results 
show  us  that  from  25  to  72  per  cent,  of  the  metabolizable  energy 
of  the  food  supplied  in  excess  of  the  maintenance  requirement  was 
converted  into  heat,  so  that  the  heat  production  was  frequently 
increased  40  or  50  per  cent,  above  that  which  was  observed  on  the 
maintenance  ration.  Under  these  circumstances  we  can  hardly 
suppose  that  any  moderate  changes  in  the  thermal  environment 
would  sensibly  affect  either  the  availability  of  the  food  energy  or  its 
percentage  utilization. 

The  writer  is  not  aware  of  any  exact  determinations  of  the 
influence  of  the  thermal  environment  upon  the  heat  production  of 
fattening  ruminants,  but  the  above  conclusion  is  in  harmony  with 
the  practical  experience  of  many  feeders  that  moderate  exposure 
to  cold  is  no  disadvantage,  but  rather  an  advantage  in  maintaining 
the  health  and  appetite  of  the  animals,  and  it  appears  also  to  have 
the  support  of  not  a  few  practical  feeding  trials.* 

*  Compare  Henry,  "  Feeds  and  Feeding,"  second  edition,  p.  364,  and 
Waters,  Bulletin  Mo.  Bd.  Agr.,  September,  1901,  p.  23. 


472  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Naturally  this  can  be  true  only  within  limits,  and  exposure  to 
very  low  temperatures,  especially  in  a  damp  climate,  and  particu- 
larly to  cold  rains,  causing  a  large  expenditure  of  heat  in  the  evapo- 
ration of  water  from  the  surface  of  the  body,  may  very  well  pass 
the  limit  and  cause  an  increase  in  the  metabolism  simply  to  main- 
tain the  temperature  of  the  body.  Finally,  the  time  element,  as 
pointed  out  on  p.  439,  is  one  to  be  taken  into  consideration. 

Swine. — As  was  remarked  on  p.  435,  the  work  of  digestion  is 
doubtless  less  with  the  swine  than  in  ruminants,  on  account  of  the 
more  concentrated  nature  of  his  food,  and  as  was  shown  on  p.  438, 
the  maintenance  requirement  appears  to  be  affected  by  the  thermal 
environment.  The  same  reason  would  tend  to  make  fattening 
swine  more  susceptible  to  this  influence  than  fattening  ruminants. 
This  conclusion  is  borne  out  by  the  experiments  of  Shelton  *  at  the 
Kansas  Agricultural  College,  who  found  that  swine  kept  in  an  open 
yard  during  rather  severe  weather  required  25  per  cent,  more  corn 
to  make  a  given  gain  than  those  sheltered  from  extreme  cold. 

Influence  of  Character  of  Food. — Attention  was  called  in  the 
previous  chapter  to  the  fact  that  the  expenditure  of  energy  in  the 
digestion  and  assimilation  of  the  food  is  largely  dependent  upon  the 
nature  of  that  food,  but  as  was  there  pointed  out,  we  have  few 
quantitative  determinations  of  the  differences.  Experiments  of 
the  class  now  under  consideration  show  marked  variations  in  the 
proportion  of  the  metabolizable  energy  of  different  foods  which 
is  utilized,  and  we  should  naturally  be  inclined  to  ascribe  these 
variations  to  differences  in  the  work  of  digestion  and  assimilation 
rather  than  to  differences  in  the  physiological  processes  involved 
in  tissue  production. 

The  data  recorded  in  the  foregoing  pages  constitute  only  a 
beginning  of  the  study  of  the  utilization  of  the  energy  of  feeding- 
stuffs,  but  a  brief  consideration  of  the  main  results  will  prove  at 
least  suggestive. 

Concentrated  Feeding-stuffs. — As  we  saw  in  connection 
with  the  discussion  of  the  metabolizable  energy  of  feeding-stuffs  in 
Chapter  X,  the  Mockern  experiments,  to  which  we  owe  the  larger 
share  of  our  present  knowledge  regarding  the  metabolism  of  energy 
in  farm  animals,  were  made  for  the  purpose  of  comparing  the 
*  Rep.  Prof,  of  Agriculture,  1883. 


THE   UTILIZATION  OF  ENERGY.  473 

principal  classes  of  nutrients  rather  than  commercial  feeding-stuffs. 
Accordingly  such  representative  materials  as  starch,  oil,  and  glu- 
ten were  largely  used,  and  we  have  as  yet  but  few  determinations 
either  of  the  metabolizable  energy  of  ordinary  concentrated  feeding- 
stuffs  or  of  its  percentage  utilization.  We  have  already  considered 
to  some  extent  the  advantages  and  disadvantages  resulting  from 
making  the  pure  nutrients,  on  the  one  hand,  or  actual  feeding-stuffs, 
on  the  other,  the  starting-point  for  investigations.  Passing  over 
this  question  for  the  present,  we  may  conveniently  group  together 
here  such  results  as  are  on  record  for  materials  other  than  coarse 
fodders. 

Starch. — Starch,  as  a  representative  of  the  readily  digested  car- 
bohydrates, has,  as  we  have  seen,  received  a  large  share  of  atten- 
tion. The  results  obtained  are  tabulated  in  the  Appendix,  and 
have  already  been  partially  considered  in  their  bearings  upon  the 
influence  of  amount  of  food.  It  was  there  noted  that  the  earlier 
series  of  experiments  by  Kuhn,  in  which  the  starch  was  added  to 
a  ration  of  coarse  fodder  only,  gave  results  differing  decidedly  from 
those  obtained  later  by  Kellner  from  the  addition  of  starch  to  a 
mixed  fattening  ration.  Among  the  latter  experiments,  more- 
over, were  two  (animals  B  and  C)  which  were  exceptional  in  that 
very  large  total  amounts  of  starch  were  contained  in  the  ration, 
relatively  large  amounts  escaping  digestion,  while  none  of  the  added 
starch  underwent  the  methane  fermentation. 

A  clear  image  of  the  fate  of  the  total  potential  energy  supplied 
to  the  organism  in  the  starch  is  best  obtained  by  a  study  of  its  per- 
centage distribution  among  the  several  excreta,  the  work  of  digestion, 
assimilation,  and  tissue  building,  and  the  gain  secured,  as  in  the 
table  on  page  474,  in  which  each  of  the  three  sets  of  experiments 
indicated  above  is  given  separately.  The  figures  for  the  work  of 
digestion,  etc.,  are,  of  course,  obtained  by  difference. 

As  pointed  out  in  the  discussion  of  metabolizable  energy,  the 
percentage  of  the  gross  energy  carried  off  in  the  feces  includes,  as 
here  computed,  not  only  the  energy  of  the  undigested  portion  of  the 
starch  itself,  but  also  that  of  the  portion  of  the  basal  ration  which 
escaped  digestion  under  the  influence  of  the  starch.  This  is  espe- 
cially true  of  Kellner's  experiments  with  moderate  rations,  in  which 
little  or  no  starch  could  be  detected  in  the  feces.     Similarly,  the 


474 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


PERCENTAGE  DISTRIBUTION  OF  CROSS   ENERGY   OF  STARCH. 


Work  of 

Diges- 

tion, 

1 

In 
Feces. 

In 
Urine. 

§ 

Assimi- 
lation, 
and 

In 
Gain. 

o 

V 

Tissue 

'3 

Build- 

< 

fi 

~ 

ing. 

f 

III 

2 

20.02 

-1.29 

10.06 

35.61 

35.60 

IV 

2 

25.29 

-1.01 

12.01 

32.41 

31.30 

Kiihn's  experiments \ 

V 
V 

2a 
26 

8.82 
15.73 

1.03 

-0.27 

11.20 
9.86 

36.95 
34.58 

42.00 
40.10 

VI 

20 

22.49 

-2.61 

8.86 

36.96 

34.30 

I 

VI 

3 

19.03 

-0.88 

11.87 

37.38 

32.60 

f 

D 

2 

29.99 

-3.27 

6.08 

31.10 

36.10 

Kellner's  experiments: 
Moderate  rations \ 

F 

4 

16.42 

0.73 

11.41 

25.24 

46.20 

G 

4 

13.35 

0.35 

8.98 

26.42 

50.90 

1 

H 

3 

15.72 

-2.32 

7.38 

34.82 

44.40 

t 

J 

3 

14.85 

1.14 

11.85 

32.66 

39.50 

Kellner's  experiments: 

Heaw  rations \ 

B 
C 

2 
2 

59.60 
52.22 

-3.25 
-0.89 

-4.96 
-0.01 

16.82 
26.68 

31.80 

y                                   ( 

28.00 

Averages : 

Kiihn's  experiments 

19.59 

-0.92 

10.74 

35.19 

35.40 

Kellner's  experiments: 

Moderate  rations 

17.61 
55.91 

-0.66 
-2.07 

9.21 
-2.49 

30.64 
18.75 

43.20 

Heavy  rations 

29.90 

negative  losses  in  the  urine  and,  in  two  cases,  in  the  methane 
mean,  of  course,  that  under  the  influence  of  starch  the  metabolic 
or  other  processes  were  so  modified  that  less  of  the  potential  energy 
of  the  basal  ration  was  lost  through  these  channels.  The  starch, 
so  to  speak,  borrowed  energy  from  the  basal  ration.  In  brief,  the 
figures  of  the  table  give  us  a  picture  of  the  aggregate  net  results  of 
supplying  100  units  of  additional  potential  energy  in  the  form  of 
starch,  or  in  other  words,  of  the  "apparent"  utilization. 

As  between  Kiihn's  results  and  those  of  Kellner  upon  moderate 
rations,  the  chief  difference,  as  already  noted,  is  the  less  evolution 
of  methane  in  the  latter  and,  apparently  as,  in  part,  a  consequence 
of  this,  the  smaller  expenditure  of  energy  in  the  work  of  digestion, 
etc.  Combined  with  the  slightly  smaller  loss  in  the  feces,  this 
results  in  making  the  energy  utilized  a  much  larger  percentage  of 
the  gross  energy.  Apparently  Kellner's  figures  correspond  most 
nearly  to  normal  conditions  of  feeding  and  may  be  taken  to  repre- 
sent the  average  utilization  of  starch  under  these  circumstances. 


THE   UTILIZATION  OF  ENERGY. 


475 


In  Kellner's  two  experiments  on  heavy  rations  the  enormous  losses 
in  the  feces  cut  down  the  percentage  utilization  to  a  very  low 
figure  and  thus  render  difficult  a  direct  comparison  with  the 
other  averages. 

While  the  above  form  of  stating  the  results  appears  the  simplest 
and  most  direct,  it  is  of  interest  also  to  eliminate  the  influence  of 
varying  digestibility  by  computing  the  percentage  distribution 
of  the  gross  energy  of  the  apparently  digested  portion  of  the  starch. 
This  is  particularly  the  case  since  Kellner's  computations  of  his 
experiments  are  made  in  a  somewhat  similar  way.  Combining 
the  data  given  on  p.  461,  regarding  the  percentages  of  metaboliz- 
able  energy  utilized,  with  those  on  p.  301  for  the  energy  of  the 
apparently  digested  matter,  we  have  the  following: 

DISTRIBUTION  OF  ENERGY  OF  APPARENTLY  DIGESTED  STARCH. 


In  Urine. 
Per  Cent. 


In  Methane. 
Per  Cent. 


Work  of 
Digestion, 
Assimilation, 
and  Tissue 
Building. 
Per  Cent, 


In  Gain. 
Per  Cent. 


Kuhn's  experiments  . 
Kellner's  experiments 

Moderate  rations .  . 

Heavy  rations  .... 


-1.19 


-0.92 
•4.95 


13.42 


11.12 
-6.15 


43. 


37.36 

42.77 


43.88 


52.44 
68.33 


Kellner's  computations  are  made  in  a  different  manner.*  Omit- 
ting in  the  computation  of  metabolizable  energy  the  correction  for 
nitrogen  gained  or  lost,  he  compares  the  period  in  which  starch  was 
fed  with  that  on  the  basal  ration  substantially  as  has  been  done 
above.  He  then,  however,  introduces  a  correction  for  the  influence 
of  the  starch  upon  the  digestibility  of  the  basal  ration.  For  ex- 
ample, comparing  Periods  3  and  4  on  Ox  H,  he  finds  in  the  manner 
shown  on  p.  307,  Chapter  X,  that  the  equivalent  of  820  Cals.  less 
of  the  basal  ration  was  digested  in  the  period  in  which  starch  was 
added  to  it,  while  there  is  a  further  correction  of  112  Cals.  to  be 
made  for  the  less  amount  of  organic  matter  of  the  basal  ration  con- 
sumed in  Period  3,  making  a  total  difference  of  932  Cals.  Of  the 
gross  energy  of  the  basal  ration,  79.9  per  cent,  was  found  to  be  met- 


*  Compare  Landw.  Vers.  Stat.,  53,  450. 


476 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


abolizable,  so  that  the  above  difference  in  gross  energy  would  corre- 
spond to  745  Cals.  of  metabolizable  energy.  Of  the  metabolizable 
energy  of  the  basal  ration  in  excess  of  maintenance,  59.6  per  cent, 
was  recovered  in  the  gain.  If,  then,  the  differences  in  organic  matter 
consumed  and  in  the  digestibility  of  the  basal  ration  had  not  offset 
some  of  the  effect  of  the  starch  in  Period  3,  there  would  have  been 
745  Cals.  more  of  metabolizable  energy  disposable  from  the  basal 
ration,  and  presumably  the  gain  resulting  from  this  would  have  been 
59.6  per  cent,  of  745  Cals.,  or  444  Cals.  We  have,  then,  by  this 
method  the  following: 


Metabolizable 

Energy  Above 

Maintenance, 

Cals. 


Energy  of 
Gain, 
Cals. 


Period  3  minus  Period  4..  . 
Correction  tor  live  weight. 


Correction  for  organic  matter  and  for  decreased 
digestibility 


Percentage  utilization 


3752 
40 


3712 
444 


4156 
56.6* 


Kellner's  results,  then,  assuming  that  the  corrections  are  accu- 
rate, represent  respectively  the  metabolizable  and  the  utilizable 
energy  of  the  digested  matter  of  the  starch  itself,  while  the  results 
as  computed  on  the  preceding  pages  represent,  as  was  there  pointed 
out,  a  balance  between  the  various  negative  and  positive  effects  of 
the  addition  of  starch.  In  other  words,  Kellner  attempts  to  com- 
pute the  real  as  distinguished  from  the  apparent  utilization  of  the 
energy  of  the  starch.  The  comparison  on  the  opposite  page  of  the 
percentages  obtained  in  this  way  with  those  computed  on  p.  461 
will  therefore  be  of  interest. 

Kellner  also  computes  by  his  method  the  distribution  of  the 
gross  energy  of  the  digested  starch  in  Kiihn's  experiments  and  in  his 
own  experiments  on  moderate  rations.  As  calculated  in  Chapter  X, 
pp.  325-6,  the  average  loss  of  potential  energy  in  methane  was  12.7 
per  cent,  in  Kiihn's  experiments,  and  10.11  per  cent,  in  Kellner's, 
while  none  of  the  potential  energy  of  the  digested  starch  passed 


THE   UTILIZATION  OF  ENERGY. 


477 


UTILIZATION   OF  METABOLIZABLE  ENERGY  OF  STARCH. 


Animal. 

Period. 

Real  Utiliza- 
tion as 

Computed  by 
Kellner. 
'  w  Cent. 

Apparent 

Utilization  as 

Computed  on 

p.  461. 

Per  Cent. 

Kiihn's  experiments - 

Kellner's  experiments  : 

Ill 
IV 
V 
V 
VI 
VI 

D 
F 
G 
H 
J 
B 
C 

2 

2 

2a 

26 

2b 

3 

2 
4 
4 
3 
3 
4 
2 

46.2 
49.0 
51.3 
52.6 
48.0 
46.8 

54.2 
63.2 
65.2 
56.2 
55.2 
61.4 
56.4 

49.0 

58.9 
58.9 

50.0 
49.2 
53.2 
53.7 
48.1 
46.6 

53.7 
64.8 
65  8 

Heavy  rations ■] 

Averages ; 

56.0 
54.8 
65.4 
57.6 

50  0 

Kellner's  experiments: 

58  4 

61  5 

into  the  urine.  In  the  two  cases,  then,  87.30  per  cent,  and  89.89 
per  cent,  respectively  of  the  potential  energy  of  the  digested  starch 
was  metabolizable.  Of  this  metabolizable  energy  49.0  per  cent, 
and  58.9  per  cent,  respectively  was  recovered  in  the  gain.  Com- 
bining these  figures  we  have — 

DISTRIBUTION  OF  ENERGY  OF  DIGESTED  STARCH. 


In  Urine 
Per  Cent. 


In  Methane 
Per  Cent 


Work  of 
Digestion. 
Assimilation, 
and  Tissue 
Building 
Per  Cent. 


In  Gain 
Per  Cent. 


Kiihn's  experiments . . . 

Kellner's  experiments; 

Moderate  rations 


12.70 
10.11 


44.52 
36.95 


42.78 
52.94 


The  final  results  for  the  energy  recovered  in  the  gain  of  tissue, 
whether  expressed  as  a  percentage  of  metabolizable  energy  or  of 
energy  of  digested  matter,  are  substantially  the  same  numer- 
ically as  those  reached  by  the  former  method  of  computation,  but 
this  agreement  is  purely  accidental,  and  the    significance  of  the 


478 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


figures  is  essentially  different,  as  already  explained.  From  the  re- 
sults last  given,  assuming  the  gain  of  energy  to  have  been  entirely 
in  the  form  of  fat,  Kellner  *  computes  that  the  conversion  of  starch 
into  fat  in  cattle  takes  place  according  to  the  following  scheme: 

Starch 100.00  grams 

+  Oxygen 38.69 

Yield: 

Methane 

Water 

Carbon  dioxide 

Fat 


3.17 

grams 

23.40 

it 

88.78 

<< 

23.34 

<< 

138.69  grams         138.69      " 

Oil. — Applying  to  Kellner's  three  experiments  upon  the  addition 
of  oil  to  a  basal  ration  the  same  method  of  computation  which  was 
used  for  the  starch — that  is.  computing  the  apparent  utilization — 
we  have  the  results  shown  in  the  two  following  tables : 

DISTRIBUTION  OF  GROSS  ENERGY  OF  OIL. 


In  Feces. 
Per  Cent. 


In  Urine. 
Per  Cent. 


In 

Methane. 
Per  Cent. 


Work  of 
Digestion 
Assimila- 
tion  and 

Tissue 
Building. 
Per  Cent. 


In  Gain. 
Per  Cent. 


Sample    I 

"    = j 

Average  of  Sample  II 


24.34 

64.77 
41.00 
52.89 


-1.08 
-1.19 
1.37 
0.09 


1.02 

16.10 

1.76 

8.93 


37.66 
18.32 
18.19 
18.25 


40.10 
34.20 
41.20 
37.70 


DISTRIBUTION   OF   ENERGY    OF  APPARENTLY  DIGESTED  OIL. 


Animal. 

Period. 

In  Urine. 
Per  Cent. 

In 
Methane 
Per  Cent. 

Work  of 
Digestion 
Assimila- 
tion   and 

Tissue 
Building 
Per  Cent. 

In  Gain. 
Per  Cent. 

D 

V 

G 

3 
5 
5 

-1.42 

-3.38 

2.32 

-0.53 

-  1.34 

-45.69 

-  3.01 
-24.35 

49.76 
52.01 
30.83 
41.42 

53.00 

"  " i 

Average  for  Sample  II 

97.06 
69.86 
83.46 

*  Loc.  cit.,  53,  452. 


THE   UTILIZATION   OF  ENERGY. 


479 


As  was  noted  in  the  discussion  of  metabolizable  energy  in 
Chapter  X,  the  results  on  Ox  F  appear  to  be  exceptional,  but  those 
upon  the  other  two  show  considerable  differences,  and  it  is  evident 
that  further  investigation  will  be  necessary  to  obtain  satisfactory 
data  upon  the  effect  of  oil  fed  in  this  way. 

Kellner's  method  of  computation,  based  upon  the  provisional 
conclusion  on  p.  323,  Chapter  X,  that  oil  has  substantially  no  effect 
upon  the  loss  of  energy  in  urine  and  methane  under  normal  condi- 
tions, gives  the  following  results: 


PERCENTAGE   OF  METABOLIZABLE   ENERGY  UTILIZED. 


As  Computed 
by  Kellner. 

As  Computed 
on  p.  462. 

Ox  D 

"    F 

52.2 

51.6 
65.1 
69.4 

"   G 

59.4 

DISTRIBUTION  OF  ENERGY  OF  DIGESTED  OIL. 


Ani- 
mal. 

Period. 

In  Urine, 
Per  Cent. 

In  Methane, 
Per  Cent. 

Work  of 
Digestion, 
Assimilation 
and  Tissue 
Building, 
Per  Cent. 

In  Gain, 
Per  Cent. 

Sample  I 

"       II 

D 
G 

3 
5 

0 
0 

0 
0 

47.8 
40.6 

52.2 
59.4 

0 
0.5 

0 
-2.2 

44.2 
40.3 

55  8 

Average  computed 

61  4 

The  numerical  results  of  these  experiments  show  more  clearly 
than  was  the  case  with  the  starch  the  difference  in  the  two  methods  of 
computation.  Both  methods  agree,  however  in  showing  that  the 
combined  expenditure  of  energy  in  the  digestion  and  assimilation  of 
the  oil  and  in  tissue  building  is  very  considerable.  We  have  already 
seen  that  the  expenditure  of  energy  in  the  digestion  of  fat  by  cai- 
nivora  and  by  man  is  comparatively  small.  If  we  are  justified  in 
assuming  that  the  same  thing  is  true  of  ruminants,  the  result  just 
reached  signifies  that  the  digested  fat  undeigoes  extensive  trans- 
formations before  being  finally  deposited  in  the  adipose  tissue. 


480 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


Until,  however,  we  have  satisfactory  determinations  of  the  per- 
centage utilization  of  fat  by  carnivora,  or  of  its  net  availability 
in  ruminants,  or  both,  no  final  conclusion  on  this  point  is  possible. 
Wheat  Gluten. — The  three  samples  of  this  feeding-stuff  experi- 
mented upon  contained  respectively  87.88,  83.45,  and  82.67  per 
cent,  of  crude  protein  in  the  dry  matter,  the  remainder  being 
chiefly  starch,  with  the  exception  of  2.22  per  cent,  of  ether 
extract  in  the  first  lot.  A  reference  to  the  results  obtained  for 
the  metabolizable  energy  will  show  that  they  were  variable  and 
also  that,  especially  in  the  earlier  experiments,  the  incidental 
effects  were  large.  Tabulating  the  results  as  in  case  of  starch 
and  oil  we  have  the  results  contained  in  the  tables  on  this  and 
the  opposite  pages. 


DISTRIBUTION  OF  GROSS  ENERGY  OF  WHEAT  GLUTEN. 


■a 

| 

< 

T3 
O 

Ah 

In  Feces. 
Per  Cent. 

In 

Urine. 

PerCent. 

In 

Methane. 
Per  Cent. 

Work  of 
Diges- 
tion. 
Assimila- 
tion, and 
Tissue 
Building. 
Per  Cent. 

In  Gain. 
Per  Cent. 

f 

Kilhn's  experiments. .  j 

Kellner's  experiments: 

Sample  I <; 

I 

III 
111 

Av. 
IV 

B 
B 
C 

Av. 
D 

3 

4 

1 
3 
3 

'4' 

-10.38 
-    1.28 

17.85 

21.71 

10.81 

5.08 

44.72 
38.69 

37.00 
35.80 

-   5.83 
-16.17 

30.16 

22.55 
20.89 

19.78 
16.18 

16.58 
13.52 
11.19 

7.95 
-1.26 

0.08 
-1.62 
-3.69 

41.70 
42.35 

33.58 
32.95 
40.71 

36.40 
58.90 

19.60 
32.60 
30.90 

24.53 
15.80 
20.16 

13.76 
12.39 
13.08 

-1.74 
1.91 
0.08 

35.75 
43.  SO 
39.78 

27.70 
26.10 

26.90 

The  exceptionally  small  loss  of  energy  in  the  urine  in  the  case 
of  Ox  IV,  Period  3,  and  the  total  suppression  of  the  methane  fer- 
mentation, as  well  as  the  fact  that  the  metabolizable  energy  was 
apparently  greater  than  the  gross  energy,  seem  to  justify  exclud- 
ing this  experiment  from  the  average,  although  there  was  appar- 
ently nothing  abnormal  in  the  ration  fed.  In  the  experiment 
with  Ox  D,  Period  4,  the  nutritive  ratio  was  very  narrow 
(1 ';  3.3),  and  Kellner  considers  this  a  probable  explanation  of  the 


THE   UTILIZATION  OF  ENERGY. 


48: 


DISTRIBUTION  OF  ENERGY  OF  APPARENTLY   DIGESTED  MATTER. 


Work  of 

Digestion, 

1 

•c 

In  Urine. 
Per  Cent. 

In 
Methane. 
Per  Cent. 

Assimila- 
tion, and 
Tissue 

In  Gain. 
Per  Cent. 

< 

a, 

Building. 
Per  Cent. 

f 

III 

3 

16.17 

9.79 

40.50 

33.54 

1 

111 

4 

21.44 

5.02 

38.24 

35.30 

Kiihn's  experiments 

j 

I 

Av. 

18.81 

7.39 

39.38 

34.42 

1 

IV 

3 

13.92 

-1.07 

36.44 

50.71 

Kellner's  experiments: 

f 

B 

1 

23.74 

0.11 

48.06 

28.09 

1 

B 

3 

17.46 

-2.10 

42.57 

42.07 

Sample  I  

■1 

1 
1 

C 
Av 

3 

14.15 

-4.67 

51.46 

39.06 

18.45 

-2.22 

47.35 

36.42 

1) 

4 

14.72 

2.27 

52  01 

31  00 

16.59 

0.02 

49.68 

33  71 

relatively  small  utilization  of  the  protein  as  computed  by  his 
method.  (See  below.)  An  unexpected  result  is  that  while  the 
earlier  sample  of  gluten  seems  to  have  increased  the  methane 
fermentation,  the  later  samples,  although  containing  more  starch, 
caused  a  decrease  in  the  methane  production  except  in  case  of 
OxD. 

Digestible  Protein. — Kellner  does  not  attempt  to  compute  the 
energy  utilized  from  the  wheat  gluten  as  a  whole  by  his  method,  but 
uses  the  results  as  a  basis  for  computing  the  utilization  of  the  energy 
of  the  digested  protein.  He  finds  that  of  the  metabolizable  energy 
of  the  latter,  computed  in  the  manner  described  in  Chapter  X 
(p.  316);  the  following  percentages  were  recovered  in  the  gain: 

OxB 45.0  per  cent. 

OxC 42.7    "      " 

Ox  III 45.1    '•      " 

Ox  IV 48.8    "      " 

Average 45 . 2    "      " 

OxD 32.9    "      " 

The  average  loss  of  energy  in  the  urine  was  found  (p.  317)  to  be 
19.3  per  cent,  of  the  gross  energy  of  the  digested  protein.     Applying 


482 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


this  average  to  the  above  figures,  and  assuming  with  Kellner  that 
the  protein  does  not  take  part  in  the  methane  fermentation,  we 
have  the  following: 


DISTRIBUTION  OF  ENERGY  OF  DIGESTED  PROTEIN, 


Animal. 

In  Urine. 
Per  Cent. 

In  Methane. 
Per  Cent. 

Woik  of 
Digestion. 
Assimilation 
and  Tissue 
Building. 
Per  Cent 

Jn  Gain. 
Per  Cent 

15 

19.30 

0        - 

44.38 
46.24 
44.30 
41.32 

36.32 

c 

34.46 

Ill 

36.40 

IV 

39.28 

Average  .... 
D 

44.07 
54.15 

36.63 
26.55 

There  is  a  wide  discrepancy  between  these  results  and  those 
computed  on  p.  465  from  the  experiments  of  Kern  &  Wattenberg 
upon  sheep  with  conglutin  and  flesh-meal.  Omitting  the  apparently- 
exceptional  result  of  Period  II,  we  have  the  following  as  the  per- 
centages of  the  (computed)  metabolizable  energy  of  the  digested 
proteids  which  was  utilized  in  those  experiments: 


Conglutin. 

Average . 
Flesh-meal , 

Average . 


Ill 


V 
VI 


67.63 
67.76 


67  70 


60.59 
69.33 


64.96 


While  the  gain  in  these  cases  includes  a  considerable  growth 
of  wool,  it  seems  difficult  to  suppose  that  this  alone  can  have  made 
the  conditions  so  much  more  favorable  for  the  storing  up  of  the 
added  protein  as  to  account  for  the  great  difference  between  these 
results  and  Kellner's,  and  it  must  apparently  be  left  to  further 
investigation  to  clear  up  the  matter. 


THE   UTILIZATION   OF  ENERGY. 


483 


It  need  hardly  be  added  that  none  of  these  results  are  directly 
comparable  with  those  computed  above,  after  another  method,  for 
the  wheat  gluten  as  a  whole. 

Beet  Molasses.— The  results  of  the  three  experiments  upon  beet 
molasses  show  such  great  differences,  as  was  noted  in  Chapter  X 
and  as  is  further  apparent  from  the  following  table,  that  any  dis- 
cussion of  them  would  evidently  be  premature : 

DISTRIBUTION  OF  GROSS  ENERGY  OF  BEET  MOLASSES. 


I 

a 

< 

■6 
.2 

In  Feces. 
Per  Cent. 

In  Urine. 
Per  Cent. 

In 
Methane. 
Per  Cent. 

Work  of 
Digestion, 
Assimilation, 
and  Tissue 
Building. 
Per  Cent. 

In  Gain. 
Per  Cent. 

Sample  I 

"       11 j 

F 

H 
J 

6 
6 
6 

26.87 

5.40 

14.45 

3.92 
3.16 

2.67 

-1.95 
12.44 
10.18 

29.56 

13.10 
36.20 

41.60 
65.90 
36.50 

9.92 

2.92 

11.31 

24.65 

51.20 

ffice. — The  two  experiments  upon  swine  by  Meissl,  Strohmer  & 
Lorenz,  when  computed  as  on  p.  454,  show  that  of  the  (estimated) 
metabolizable  energy  of  the  food  approximately  the  following  per- 
centages were  recovered  in  the  gain: 

Period  1 80. 7  per  cent. 

"      II 75.2    "      " 

Average 78 . 0 

These  results  are  notably  higher  than  any  obtained  in  experi- 
ments on  ruminants.  Like  the  results  on  barley  and  cockle  below 
they  are  the  expression  in  another  form  of  the  well-known  supe- 
riority of  the  swine  as  an  economical  producer  of  meat. 

Barley— -For  the  utilization  of  the  energy  of  this  grain  the 
single  experiment  by  Meissl,  Strohmer  &  Lorenz  gives  70.9  per  cent, 
of  the  (estimated)  metabolizable  energy. 

Mixed  Grains.— For  mixed  grains  Kornauth  &  Arche's  results 
on  swine  give  figures  which  do  not  differ  materially  from  the  result 
just  computed  for  barley,  viz.: 

Experiment  II 71.7  per  cent. 

HI 65.3    "      " 


484 


PRINCIPLES  OF  ANIMAL  NUTRITION. 


Coarse  Fodders. — Kellner's  results  upon  hay,  straw,  and  ex- 
tracted straw  are  the  only  data  regarding  the  utilization  of  the 
energy  of  this  class  of  feeding-stuffs  which  we  as  yet  possess.  Only 
those  experiments  in  which  coarse  fodder  was  added  to  a  mixed 
basal  ration  are  available  for  a  computation  of  this  sort. 

Meadow  Hay. — The  two  kinds  of  meadow  hay  (V  and  VI)  used 
in  Kellner's  experiments  gave  the  following  results  for  the  distri- 
bution of  their  energy,  computed  as  in  previous  instances : 

DISTRIBUTION  OF  GROSS  ENERGY  OF  MEADOW  HAY. 


•a 

< 

1 

In 

Feces. 

Per  Cent. 

In 

Urine. 

Per  Cent. 

In 
Methane. 
Per  Cent. 

Work  of 
Digestion, 
Assimila- 
tion, and 

Tissue 
Building. 
Per  Cent. 

In  Gain. 
Per  Cent. 

Sample  V J 

F 
G 

Av. 

II 
H 
J 

Av. 

1 

2 

2 

7 

2 

49.81 
44.80 

4.32 
4.26 

5.12 
6.94 

24.25 
28.10 

16.50 
15.90 

I 

Sample  VI j 

47.30 

37.07 
34.78 
34.30 

4.29 

5.24 

5.00 
6.33 

6.03 

4.87 
6.15 
6.13 

26.18 

26.32 

27.97 
34.74 

16.20 

26.50 
26.10 
18.50 

[ 

35.38 
41.34 

5.52 
4.91 

5.72 

5.87 

29.68 
27.93 

23.70 
19.95 

DISTRIBUTION  OF  ENERGY  OF  APPARENTLY  DIGESTED  MATTER. 


Sample  V. 


Sample  VI j 

Average  of  V  and  VI.  . 


In  Urine. 
Per  Cent. 


8.61 
7.72 


8.17 

8.32 
7.66 
9.64 


8.54 
8.34 


In 
Methane. 
Per  Cent. 


10.20 
12.58 


11.39 

7.74 
9.43 
9.33 


8.83 
10.78 


Work  of 
Digestion, 
Assimila- 
tion, and 

Tissue 
Building. 
Per  Cent. 


48.39 
50.85 


49.62 

41.63 

42.77 
52.83 


45.75 
49.08 


In  Gain. 
Per  Cent. 


32.80 

28.85 


30.82 

42.31 
40.14 
28.20 


36.88 
31.80 


THE  UTILIZATION  OF  ENERGY. 


485 


Computed  by  Kellner's  method,  the  percentage  of  the  metabo- 
lizable  energy  of  the  hay  which  was  recovered  as  gain  of  tissue  was 
as  follows,  as  compared  with  the  results  obtained  by  the  writer's 
method : 


PERCENTAGE  OF  METABOLIZABLE  ENERGY  RECOVERED. 


Computed  by 
Kellner's 
Method. 

Computed  by 

the  Writer's 

Method. 

f 

Ox  F 

42.8 
37.7 

40  4 

Hav  V 

"    G 

36.2 

Average 

I 

40.2 

I      49.9      | 

35.8 

38  3 

Hay  VI ■( 

Ox  H,  Period  2.... 
"    H,       "       7  ... 
"     J 

50.4 
48.4 
34  8 

1 

Average 

I 
Average  of  V  and  VI ...  . 

42.8 
41.5 

44.5 
41  4 

Computing  the  results  upon  the  gross  energy  of  the  digested 
matter  of  the  hay,  Kellner  obtains  the  following: 

DISTRIBUTION  OF  ENERGY  OF  DIGESTED  MATTER. 


In  T"rine. 
Per  Cent. 

In  Methane. 
Per  Cent 

Work  of 
Digestion. 
Assimilation, 
and  Tissue 
Building. 
Per  Cent. 

In  Gain. 
Per  Cent. 

Hay  V 

8.2 
8.8 

11.5 
9.0 

48.00 
48.10 

32.3 
34.1 

"  VI 

Average 

8.5 

10.3 

48.00 

33.2 

As  in  some  previous  cases,  the  numerical  results  of  the  two 
methods  of  computation  do  not  vary  greatly,  but  their  essentially 
different  significance  should  not  be  forgotten. 

Oat  Straw. — For  the  single  sample  of  this  feeding-stuff  experi- 
mented on,  the  results,  arranged  in  the  same  order  as  before,  were 
as  follows: 


486 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


DISTIBUTION  OF  GROSS   ENERGY  OF  OAT  STRAW. 


Animal. 

Period. 

In  Feces. 
Per  Cent. 

In  Urine. 
Per  Cent. 

In 
Methane. 
Per  Cent. 

Work  of 
Digestion, 
Assimila- 
tion, and 

Tissue 
Building. 
Per  Cent. 

In  Gain. 
Per  Cent. 

F 

2 

1 

56.77 
56.86 

2.29 
1.86 

4.40 
6.23 

22.34 
23.35 

14.20 

G 

11.70 

56.81 

2.08 

5.31 

22.85 

12.95 

DISTRIBUTION  OF  ENERGY  OF  APPARENTLY  DIGESTED  MATTER. 


Animal. 

Period. 

In  Urine. 
Per  Cent. 

In  Methane. 
Per  Cent. 

Work  of 
Digestion, 
Assimilation, 
and  Tissue 
Building. 
Per  Cent. 

In  Gain. 
Per  Cent. 

F 

2 

1 

5.30 
4.32 

10.17 
14.42 

51.73 
54.12 

32.80 

G 

27.14 

4.81 

12.30 

52.92 

29.97 

PERCENTAGE  OF  METABOLIZABLE   ENERGY  RECOVERED. 


Computed  by 
Kellner's  Method. 

Computed  by  the 
Writer's  Method. 

Ox  F 

39.9 
35.3 

38.8 

33.4 

"   G 

Average 

37.6 

36.1 

DISTRIBUTION  OF  ENERGY  OF  DIGESTED  MATTER  (KELLNER). 


In  Urine. 
Per  Cent. 


In  Methane. 
Per  Cent. 


Work  of 
Digestion, 
Assimilation, 
and  Tissue 
Building. 
Per  Cent. 


In  Gain. 
Per  Cent. 


Average  F  and  G. 


4.7 


12.2 


51.! 


31.2 


THE  UTILIZATION  OF  ENERGY. 


487 


Wheat  Straw. — Tabulating  the  results  upon  wheat  straw  in  the 
same  manner  as  those  for  oat  straw  we  have — 


DISTRIBUTION    OF    GROSS    ENERGY    OF    WHEAT    STRAW. 


Animal. 

Period. 

In  Feces. 
Per  Cent. 

In  Urine. 
Per  Cent. 

In 
Methane. 
Per  Cent. 

Work  of 
Digestion, 
Assimila- 
tion, and 

Tissue 
Building. 
Per  Cent. 

In  Gain. 
PerCent. 

H 

J 

1 
1 

60.41 

56.03 

1.88 
2.85 

7.96 
8.65 

26.55 
24.67 

3.20 
7.80 

58.21 

2.37 

8.31 

25.61 

5.50 

DISTRIBUTION    OF    ENERGY    OF   APPARENTLY    DIGESTED    MATTER. 


Animal. 

Period. 

In  Urine. 
Per  Cent. 

In  Methane. 
Per  Cent. 

Work  of 
Digestion, 
Assimilation, 
and  Tissue 
Building. 
Per  Cent. 

In  Gain. 
Per  Cent. 

H 

1 
1 

4.75 
6.49 

20.11 
19.67 

67.03 
56.12 

8.11 

J 

17.72 

5.62 

19.89 

61.57 

12.92 

PERCENTAGE    OF    METABOLIZABLE    ENERGY    RECOVERED. 


Computed  by 
Kellner's  Method. 

Computed  by  the 
Writer's  Method. 

Ox  H 

11.2 
24.3 

10.8 
24.0 

"     J 

Average 

17.8 

17.4 

DISTRIBUTION    OF   ENERGY    OF    DIGESTED    MATTER    (KELLNER). 

Average  of 
H  and  J. 

In  urine 5.6 

In  methane  20 . 0 

Work  of  digestion,  assimilation,  and  tissue  building.  61 .2 
In  gain 13.2 


100.0 


488 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


Extracted  Straw. — As  previously  noted  in  another  connection, 
this  material  consisted  of  rye  straw  which  had  been  treated  with  an 
alkaline  liquid  under  pressure,  substantially  as  in  the  manufacture 
of  straw  paper.  It  contained  in  the  water-free  state  76.78  per  cent. 
of  crude  fiber  and  19.96  per  cent,  of  nitrogen-free  extract.  Con- 
siderable interest  attaches  to  the  results  obtained  upon  this  sub- 
stance as  representing  to  a  degree  the  crude  fiber  of  the  food  of 
herbivorous  animals.     Computed  as  before, these  results  were: 

DISTRIBUTION    OF    GROSS    ENERGY    OF   EXTRACTED    STRAW. 


Animal. 

Period. 

In  Feces. 
Per  Cent. 

In  Urine. 
Per  Cent. 

In 
Methane. 
Per  Cent. 

Work  of 
Digestion, 
Assimila- 
tion, and 

Tissue 
Building. 
Per  Cent. 

In  Gain. 
Per  Cent. 

H 

5 
5 

11.35 
14.14 

-0.46 
-1.11 

12.40 
12.52 

25.11 
30.85 

51.60 

J 

43.60 

12.75 

-0.79 

12.46 

27.98 

47.60 

DISTRIBUTION    OF  ENERGY    OF    APPARENTLY    DIGESTED    MATTER. 


Animal. 

Period. 

In  Urine. 
Per  Cent. 

In  Methane. 
Per  Cent. 

Work  of 
Digestion, 
Assimilation, 
and  Tissue 
Building. 
Per  Cent. 

In  Gain. 
Per  Cent. 

H 

5 

5 

-0.52 
-1.29 

13.99 
14.58 

28.29 

35.89 

58.24 

J 

50.82 

-0.91 

14.29 

32.09 

54.53 

PERCENTAGE    OF   METABOLIZABLE    ENERGY    RECOVERED. 


Computed  by 
Kellner's  Method. 

Computed  by  the 
Writer's  Method. 

OxH 

67.5 
58.7 

67.3 
58.6 

"   J 

Average 

63.1 

63.0 

THE   UTILIZATION  OF  ENERGY.  489 


DISTRIBUTION    OF   ENERGY    OF    DIGESTED    MATTER    (KELLNER). 

Average  of 
H  and  J. 

In  urine 0.0 

In  methane 14.0 

Work  of  digestion,  assimilation,  and  tissue  building.  31 . 7 

In  gain 54 . 3 


100.0 


As  was  noted  in  discussing  the  results  upon  metabolizable 
energy,  the  treatment  to  which  the  straw  was  submitted  left 
it  in  a  condition  in  which  its  digestibility,  and  consequently  its 
percentage  of  metabolizable  energy,  compared  favorably  with  that 
of  starch.  As  we  now  see,  this  analogy  extends  also  to  its  effect 
in  producing  gain,  the  figures  showing  in  this  respect  a  slight 
superiority  on  the  part  of  the  extracted  straw,  as  appears  from 
the  following  summary: 

RECOVERED    IN    GAIN. 


Starch  (Kellner's 

Experiments  on 

Moderate 

Rations). 


Extracted  Straw. 


Per  cent,  of  gross  energy 

"       "       "  apparently  digested  energy .  . 
"       "       "  metabolizable  energy 


43.4 
53.1 

59.0 


47.6 
54.5 
63.0 


The  reason  for  this  strikingly  high  value  of  the  extracted  straw 
as  compared  with  the  low  value  indicated  for  crude  fiber  by  the 
results  of  Zuntz  and  Wolff  will  be  considered  in  a  subsequent  para- 
graph. 

Summary. — For  convenience  of  reference  the  foregoing  results 
may  be  summarized  in  the  tables  on  pages  490  and  491, 
showing  respectively  the  percentage  distribution  of  the  gross 
energy  of  the  feeding-stuffs,  that  of  the  energy  of  the  appar- 
ently digested  organic  matter,  and  the  percentage  utilization  of 


49° 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


the  metabolizable  energy  according  to  the  two  methods  of  com- 
putation adopted: 

DISTRIBUTION    OF   GROSS   ENERGY. 


Concentrated  Feeding-stuffs  : 
Starch,  Kiihn's  experiments.  . 
"      Kellner's  experiments 

moderate  rations 
heavy  rations..  . 

Oil,  average,  Sample  II 

Wheat  gluten,  Kellner's  expts.  . 
Beet  molasses,  Sample  II 

Coarse  Fodders  : 

Meadow  hay 

Oat  straw 

Wheat  straw 

Extracted  straw 


Work  of 

Diges- 

In 

In 

In 

tion,  As- 

In 

Urine. 

Methane. 

simila- 

Gain. 

Per  Cent. 

Per  Cent. 

Per  Cent. 

tion,  and 

Tissue 
Building. 
Per  Cent. 

Per  Cent. 

19.59 

-0.92 

10.74 

35.19 

35.40 

17.61 

-0.66 

9.21 

30.64 

43.20 

55.91 

-2.07 

-2.49 

18.75 

29.90 

52.89 

0.09 

-8.93 

18.25 

37.70 

20.17 

13.08 

0.08 

39.78 

26.90 

9.92 

2.92 

11.31 

24.65 

51.20 

41.34 

4.91 

5.87 

27.93 

19.95 

56.81 

2.08 

5.31 

22.85 

12.95 

58.21 

2.37 

8.31 

25.61 

5.50 

12.75 

-0.79 

12.46 

27.98 

47.60 

DISTRIBUTION    OF   ENERGY    OF   APPARENTLY    DIGESTED    MATTER. 


Work  of 

Digestion, 

Urine. 

Methane. 

Assimila- 

Per  Cent. 

Per  Cent,. 

Tissue 
Building. 
Per  Cent. 

-1.19 

13.42 

43.99 

-0.92 

11.12 

37.36 

-4.95 

-   6.15 

42.77 

-0.53 

-24.35 

41.42 

16.59 

0.02 

49.02 

8.34 

10.78 

49.08 

4.81 

12.30 

52.92 

5.62 

19.89 

61.57 

-0.91 

14.29 

32.09 

In 
Gain. 

Per 
Cent. 


Concentrated  Feeding-stuffs  : 

Starch,  Kiihn's  experiments 

"       Kellner's    experiments,    moder- 
ate rations 

"        Kellner's     experiments,     heavy 

rations 

Oil,  Sample  II 

Wheat  gluten,  Kellner's  experiments.. . 

Coarse  Fodders  : 

Meadow  hay 

Oat  straw 

Wheat  straw 

Extracted  .straw 


43.88 

52.44 

68.33 
83.46 
33.71 


31.80 
29.97 
12.92 
54.53 


THE  UTILIZATION  OF  ENERGY. 


49: 


PERCENTAGE    UTILIZATION    OF   METABOLIZABLE    ENERGY. 


Real 
Utilization 
as  Computed 
by  Kellner. 

Apparent 
Utilization. 

By  Ruminants. 

Concentrated  Feeding-stuffs : 

49.0 

58.9 

58.9 

59.4 

45.2* 

67.7* 

65.0* 

41.5 
37.6 
17.8 
63.1 

78.0 
70.9 
68.5 

50  0 

"       Kellner's  expts. 
Oil,  Sample  II,  Ox  G 

moderate  rations  . . 
heavy  rations 

58.4 
61.5 
69.4 

Wheat  gluten,  Kellner's 

experiments 

40.3 

Coarse  Fodders  : 

41.4 

36.1 

17.4 

63.0 

By  Swine. 

*  Of  protein. 

The  Expenditure  of  Energy  in  Digestion,  Assimilation,  and 
Tissue  Building. — As  was  shown  in  the  introductory  paragraphs  on 
p.  466.  the  recorded  data  do  not  permit  us  to  distinguish  between  the 
energy  expended  in  the  digestion,  resorption,  and  assimilation  of 
the  various  feeding-stuffs  experimented  upon  and  the  energy  which 
we  have  reason  to  believe  is  required  for  the  conversion  of  the  assim- 
ilated material  into  tissue.  Accordingly  these  two  factors  have 
been  grouped  together  in  the  foregoing  summaries  of  results.  Some 
interesting  facts  are  revealed,  however,  by  a  comparison  of  the 
total  expenditure  of  energy  for  these  two  purposes  in  the  several 
cases.  Kellner's  results,  as  the  latest  and  apparently  most  accurate 
and  representative,  have  been  made  the  chief  basis  of  the  compari- 
son, the  figures  being  those  computed  by  the  writer  and  therefore 
showing  the  aggregate  net  effect  upon  the  balance  of  energy,  that  is, 
the  "apparent"  utilization. 

Coarse  Fodders. — A  comparison  of  the  coarse  fodders  with 
each  other  brings  out  the  interesting  fact  that  while  the  percentage 
of  the  gross  energy  recovered  in  the  gain  varied  from  5.5  to  47.6. 


492 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


the  percentage  expended  in  digestion,  assimilation,  and  tissue  build- 
ing varied  only  from  22.85  to  27.98.  Expressing  the  same  thing  in 
absolute  figures,  we  have  the  following: 

ENERGY  PER  GRAM  OF  ORGANIC  MATTER. 


Gross, 
Cals. 

Expended  in  Diges- 
tion, Assimilation, 
and  Tissue  Building, 
Cals. 

Meadow  hay 

4.751 

4.816 
4.743 
4.251 

1.327 
1.100 
1.214 
1.190 

Wheat  straw 

Extracted  straw 

4.640 

1.208 

In  other  words,  the  combined  energy  required  to  separate  the 
digestible  from  the  indigestible  portion  of  one  gram  of  organic 
matter,  resorb  it,  and  convert  the  resorbed  portion  into  tissue  was 
not  greatly  different  for  these  four  materials.  They  differed  widely 
in  their  nutritive  effect,  not  because  of  a  greater  or  less  expendi- 
ture of  energy  for  these  purposes,  but  chiefly  because  the  same 
expenditure  of  energy  resulted  in  making  a  much  larger  amount  of 
material  digestible  in  some  cases  than  in  others. 

Concentrated  Feeding-stuffs. — A  still  more  striking  result  is 
reached  when  we  compare  the  results  on  coarse  fodders  with  those 
on  concentrated  feeding-stuffs.  Taking  the  figures  of  Kellner's 
experiments  for  the  latter,  and  omitting  his  results  on  heavy 
rations  of  starch,  we  have  the  following  data  for  starch,  oil,  and 
wheat  gluten: 

ENERGY  PER  GRAM  OF  ORGANIC  MATTER. 


Gross, 
Cals. 

Expended  in 

Digestion, 

Assimilation, 

and  Tissue 

Building, 

Cals. 

Starch  (Kellner) 

Oil 

Gluten  (Kellner)  .... 

4.168 
9.464 
5.742 

1.277 

1.728 
2.2S4 

We  thus  reach  the  seemingly  paradoxical  result  that  the  total 
expenditure  of  energy  in  the  production  of  new  tissue  is  decidedly 


THE  UTILIZATION  OF  ENERGY. 


493 


greater  in  the  case  of  these  three  materials,  and  notably  the  last 
two,  than  in  the  four  coarse  fodders  previously  tabulated. 

The  paradox  largely  disappears,  however,  when  we  remember 
that  while  the  larger  share  of  the  work  of  digestion  has  to  do  with 
the  total  dry  matter  of  the  food,  the  work  of  assimilation  and  tissue 
building  has  to  be  performed  only  upon  the  digested  matter,  and 
that  the  proportion  of  the  latter  is  much  larger  in  the  starch,  oil, 
and  gluten  than  in  the  coarse  fodders.  We  have  already  (pp.  375 
and  445)  seen  reason  to  suppose  that  the  processes  of  assimilation 
and  tissue  building  consume  a  considerable  share  of  the  metaboliz- 
able  energy  of  the  food,  although  we  are  still  ignorant  as  to  how 
much  and  as  to  how  the  proportion  differs  with  different  materials, 
and  the  above  results  serve  to  confirm  this  conclusion. 

If,  simply  as  an  illustration,  we  assume  that  the  uniform  pro- 
portion of  30  per  cent,  of  the  metabolizable  energy  of  the  several 
feeding-stuffs  is  thus  consumed,  then  if  we  deduct  this  amount  from 
the  totals  above  computed  we  shall  have  the  work  of  digestion  alone 
as  follows: 

ENERGY  PER  GRAM  OF  ORGANIC  MATTER. 


Metaboliz- 
able 
Energy 
(p.  297), 
Cals. 

Assumed 
Work  of 
Assimilation 
and  Tissue 
Building 
(30  Per  Cent, 
of  Metaboliz- 
able), Cals. 

Total  Ex- 
penditure 
as  Above. 
Cals. 

Work  of 

Digestion  Alone 

Cals. 

2.213 
1.724 
1.475 
3.213 
3.079 
5.298 
3.831 

0.664 
0.517 
0.443 
0.964 
0.923 
1.5S9 
1.149 

1.327 
1.100 
1.214 
1.190 
1.277 
1.728 
2.284 

0.663] 

0  583     n  „„„ 

0771  r0-672 

0  192  J 

Starch  (Kellner) 

Oil 

0.354 
0  139 

Wheat  gluten  (Kellner)  .  . 

1.1,5 

This  arbitrary  assumption  reduces  the  work  of  digestion  of  the 
starch  to  about  one  half  that  expended  upon  a  like  amount  of  mate- 
rial in  the  form  of  coarse  fodders  which  yield  chiefly  carbohydrates 
to  the  organism.  Moreover,  we  must  remember  that  in  the  case  of 
starch  there  is  a  considerably  greater  loss  of  energy  in  the  methane 
fermentation  than  with  the  same  amount  of  total  organic  matter 
in  coarse  fodders,  and  that  this  loss  is  included  in  the  work  of  diges- 
tion.    The  high  figure  found  for  the  wheat  gluten  we  might  be 


494  PRINCIPLES  OF  ANIMAL   NUTRITION. 

inclined  to  explain  by  its  well-known  effect  in  stimulating  the  met- 
abolism in  the  body — that  is,  by  supposing  that  for  this  substance 
our  assumption  of  30  per  cent,  for  the  work  of  assimilation  and 
tissue  building  is  too  low. 

The  computed  work  of  digestion  is  small  in  the  case  of  the  oil, 
as  the  results  obtained  in  other  experiments  would  lead  us  to  expect. 
At  the  same  time  it  should  be  remembered  that  the  figures  given 
are  derived  from  two  experiments  only,  while  a  third  gave  quite 
different  results,  showing  in  particular  a  decidedly  higher  figure  for 
the  combined  work  of  digestion,  assimilation,  and  tissue  building. 
It  is  obvious,  therefore,  that  further  investigation  is  necessary  to 
fix  the  value  of  oil  in  this  respect. 

Crude  Fiber. — Finally,  it  will  be  observed  that  our  arbitrary 
assumption  results  in  making  the  work  of  digestion  of  the  extracted 
straw  less  than  two  thirds  that  of  starch.  We  should  naturally 
suppose  that  the  mechanical  work  involved  in  digestion  would  be 
fully  as  great  in  the  case  of  the  former  as  in  that  of  the  latter,  while, 
as  the  figures  for  methane  show,  the  extracted  straw  underwent  a 
more  extensive  fermentation  than  the  starch.  Obviously,  the 
mechanical  and  chemical  treatment  to  which  the  straw  was  sub- 
jected so  modified  the  cellulose  and  removed  incrusting  matters  as 
to  produce  a  material  which  behaved  substantially  like  starch  in  the 
alimentary  canal,  both  as  regards  its  digestibility  and  its  relation  to 
ferments.*  Correspondingly,  the  total  work  of  digestion,  assimila- 
tion, and  tissue  building  is  not  widely  different  in  the  two  cases.  It 
is  only  when  we  arbitrarily  assume  a  high  percentage  for  the  work  of 
assimilation  and  tissue  building,  as  was  done  above  for  the  sake  of 
illustrating  the  general  question,  that  this  difference  and  that  in  the 
amount  of  metabolizable  energy  combine  to  give  the  relatively  low 
figure  for  digestive  work  noted  above. 

§  2.  Utilization  for  Muscular  Work. 

When  a  muscle  is  subjected  to  a  suitable  stimulus  (normally  a 
nerve  stimulus)  there  occurs,  as  we  have  seen,  a  sudden  and  rapid 
increase  in  its  metabolism.     This  increased  metabolism  appears  to 

*  Lehmann  (Landw.  Jahrb.,  24,  Supp.  I,  118)  had  previously  shown  that 
the  apparent  digestibility  of  the  crude  fiber  and  nitrogen-free  extract  of  straw 
and  chaff  thus  treated  was  increased  by  from  79  to  133  per  cent. 


THE  UTILIZATION  OF  ENERGY.  495 

consist  largely  in  a  breaking  down  or  cleavage  of  some  substance  or 
substances  contained  in  the  muscle,  resulting  in  a  rapid  increase  in 
the  excretion  of  carbon  dioxide  and  the  consumption  of  oxygen  by 
the  animal.  In  this  process  of  breaking  down  or  cleavage  there  is  a 
corresponding  transformation  of  energy,  a  portion  of  the  potential 
energy  of  the  metabolized  material  appearing  finally  as  heat,  while 
a  part  may  take  the  form  of  mechanical  energy.  The  inquiry 
naturally  arises  what  proportion  of  the  total  energy  liberated  during 
the  increased  metabolism  is  recovered  as  mechanical  work  and  what 
proportion  takes  the  form  of  the  (for  this  purpose)  waste  energy  of 
heat.  The  question  is  not  only  one  of  great  theoretical  interest  to 
the  physiologist,  but  the  efficiency  of  the  working  animal  regarded 
as  a  machine  for  the  conversion  of  the  potential  energy  of  feeding- 
stuffs  into  mechanical  work  is  also  of  the  highest  practical  im- 
portance. 

Efficiency  of  Single  Muscle. — A  large  amount  of  experi- 
mental work  has  been  devoted  to  the  study  of  the  single  muscle  as  a 
machine.  The  subject  is  a  complicated  one,  and  unanimity  of  views 
upon  it  has  by  no  means  been  attained,  especially  as  to  the  mechan- 
ism of  muscular  contraction.  As  regards  the  efficiency  of  the  muscle 
as  a  converter  of  energy,  however,  one  fact  is  perfectly  well  estab- 
lished, viz.,  that  it  varies  within  quite  wide  limits. 

If  the  two  ends  of  a  muscle  be  attached  to  fixed  points,  so  that 
it  cannot  shorten,  a  suitable  stimulus  will  still  cause  it  to  contract 
in  the  technical  sense  of  the  word;  that  is,  a  state  of  tension  will 
be  set  up  in  1he  muscle  tending  to  pull  the  two  supports  nearer 
together  (isometric  contraction).  In  such  a  contraction  there  is 
an  expenditure  of  potential  energy  and  a  corresponding  increase 
of  muscular  metabolism,  but  no  external  work  is  done.  In  other 
words,  all  the  potential  energy  finally  takes  the  form  of  heat  and 
the  mechanical  efficiency  is  zero.  This  is  the  case,  for  example,  in 
the  standing  animal.  A  not  inconsiderable  muscular  effort  is 
required  to  maintain  the  members  of  the  body  in  certain  fixed 
positions,  and  a  corresponding  generation  of  heat  takes  place,  but 
no  mechanical  work  is  done. 

But  even  when  the  muscle  is  free  to  shorten  and  thus  do  mechan- 
ical work,  its  efficiency  is  found  to  be  variable,  the  chief  determin- 
ing factors  being  the  load  and  the  degree  of  contraction.      The 


496  PRINCIPLES   OF  ANIMAL   NUTRITION. 

maximum  efficiency  of  the  muscle  is  reached  when  the  load  is  such 
that  the  muscle  can  just  raise  it,  while  this  maximum  load  dimin- 
ishes as  the  muscle  contracts  until  when  the  latter  reaches  the  limit 
of  shortening  it  of  course  becomes  zero.  Conversely,  if  the  muscle 
be  stretched  beyond  what  may  be  called  its  normal  length,  as  is 
the  case  in  the  living  body,  the  weight  which  it  can  lift,  and  conse- 
quently its  efficiency,  is  increased.  In  these  respects  the  muscle 
behaves  like  an  elastic  cord,  and  some  authorities,  notably  Chau- 
veau,*  regard  the  essence  of  muscular  contraction  as  consisting  of 
a  direct  conversion  of  the  potential  energy  of  the  "contractile 
material "  of  the  muscle  into  muscular  elasticity. 

Efficiency  of  the  Living  Animal. — According  to  the  above 
principles  the  maximum  efficiency  of  a  muscle  would  be  obtained 
when  it  was  loaded  to  its  maximum  at  each  point  in  the  contraction ; 
that  is,  when  the  load  diminished  uniformly  from  the  maximum 
corresponding  to  the  initial  length  of  the  muscle  to  zero  at  the  point 
of  greatest  contraction.  Such  conditions,  however,  rarely  if  ever 
obtain  in  the  animal.  Of  its  many  muscles  some  serve  largely 
or  wholly  to  maintain  the  relative  positions  of  the  different  parts  of 
the  body,  and  consequently  have  an  efficiency  approaching  zero. 
Others  contract  to  a  varying  extent  and  under  loads  less  than  the 
maximum.  Some  muscles,  owing  to  their  anatomical  relations, 
work  at  a  less  mechanical  advantage  than  others,  while  the  extent 
to  which  a  given  group  of  muscles  is  called  into  action  will  vary 
with  the  nature  of  the  work. 

If,  then,  the  efficiency  of  the  single  muscle  is  variable,  that  of 
the  body  as  a  whole  would  seem  likely  to  be  even  more  so,  thus 
rendering  it  difficult  to  draw  any  trustworthy  direct  conclusions 
as  to  the  efficiency  of  the  bodily  machine  from  studies  of  the  effi- 
ciency of  the  single  muscle.  Moreover,  the  performance  of  labor 
by  an  animal  sets  up  various  secondary  activities,  notably  of  the 
circulatory  and  respiratory  organs,  which  consume  their  share  of 
potential  energy  and  yet  do  not  contribute  directly  to  the  per- 
formance of  the  work,  and  the  extent  of  these  secondary  activities 
varies  with  the  nature  and  the  severity  of  the  work.  When,  there- 
fore, as  is  here  the  case,  we  consider  the  whole  animal  in  the  light 
of  a  machine  for  converting  the  potential  energy  of  the  food  into 
*  Le  Travail  Musculaire.     Paris,  1891. 


THE   UTILIZATION  OF  ENERGY.  497 

mechanical  work,  we  are  perforce,  by  the  very  complexity  of  the 
problem,  driven  to  the  statistical  method  of  comparing  the  total 
income  and  outgo  of  energy  in  the  various  forms  of  work. 


THE   UTILIZATION    OF   NET   AVAILABLE    ENERGY. 

Both  the  activity  of  the  skeletal  muscles  in  the  performance  of 
work  and  the  supplementary  activity  of  the  muscles  concerned  in 
circulation,  respiration,  etc.,  is  carried  on  at  the  expense  of  energy 
stored  in  the  muscles  themselves  or  perhaps  in  the  blood  which 
circulates  through  them.  The  body  thus  suffers  a  loss  of  energy 
which  is  replaced  from  the  energy  of  the  food.  If,  then,  we  supply 
a  working  animal,  in  addition  to  its  maintenance  ration,  with  an 
amount  of  food  exactly  sufficient  to  make  good  the  loss,  the  total 
energy  metabolized  in  the  performance  of  the  work  will  repre- 
sent the  net  available  energy  of  the  excess  food,  since  this  by 
definition  is  that  portion  of  the  gross  energy  which  contributes 
to  the  maintenance  of  the  store  of  potential  energy  in  the 
body. 

It  is  true  that  in  our  discussion  of  the  net  available  energy  of 
the  food  we  regarded  it  as  making  good  the  losses  that  occur  below 
the  maintenance  requirement,  and  the  question  may  arise  whether 
the  availability  as  thus  measured  is  the  same  as  the  availability  for 
the  production  of  muscular  work.  In  reality,  however,  the  two 
cases  are  not  radically  different.  Even  below  the  point  of  mainte- 
nance the  internal  work  of  the  body  consists  very  largely  of  muscu- 
lar work,  and  it  is  the  energy  metabolized  in  the  performance  of  this 
work  which  appears  to  constitute  the  chief  demand  for  available 
food  energy.  It  would  appear  highly  probable,  therefore,  that  the 
net  availability  of  the  metabolizable  energy  of  the  food  will  be  found 
to  be  substantially  the  same  whether  that  energy  be  employed  to 
prevent  a  loss  from  the  body  as  a  consequence  of  its  internal  work 
below  maintenance  or  on  account  of  the  performance  of  external 
work  above  maintenance. 

If,  then,  we  cause  an  animal  to  perform  a  known  amount  of 
external  work  and  measure  the  increase  in  the  amount  of  energy 
metabolized  in  the  body,  we  may  regard  the  latter  as  representing 
net  available  energy  derived  from  previous  food,  and  a  comparison 


498  PRINCIPLES  OF  ANIMAL  NUTRITION. 

of  this  quantity  with  the  work  done  will  give  the  coefficient 
of  utilization  for  the  particular  animal  and  kind  of  work  experi- 
mented on. 

The  Efficiency  of  the  Animal  as  a  Motor. 

The  relation  just  indicated  between  the  work  performed  and  the 
total  energy  metabolized  in  its  performance  is  not  infrequently  re- 
garded as  expressing  the  efficiency  of  the  animal  as  a  motor,  but  it 
should  be  clearly  understood  that  this  is  true  only  in  a  limited  sense. 
A  coefficient  computed  in  the  manner  outlined  above  takes  account 
only  of  the  loss  which  occurs  in  the  conversion  of  the  stored  energy 
of  the  body  into  external  mechanical  work.  It  neither  includes  the 
expenditure  of  energy  required  for  the  digestion  and  assimilation  of 
the  food,  nor  does  it  take  account  of  the  large  amount  of  energy  con- 
tinually consumed  in  the  internal  work  of  the  animal  machine.  It 
does  not,  therefore,  furnish  a  direct  measure  of  the  economy  with 
which  the  animal  machine  uses  the  energy  supplied  to  it,  but  is 
comparable  rather  to  the  theoretical  thermo-dynamic  efficiency  of 
a  steam-engine.  With  this  limitation,  however,  the  phrase  may  be 
used  as  a  matter  of  convenience. 

Quite  extensive  investigations  upon  this  point  are  already  on 
record.  They  have  generally  taken  the  form  of  what  may  be  called 
respiration  experiments.  The  respiratory  exchange  of  carbon  di- 
oxide and  oxygen  has  been  determined,  first,  in  a  state  of  rest,  andr 
second,  during  the  performance  of  a  measured  amount  of  work. 
From  the  difference  between  these  two  values  the  quantity  of  ma- 
terial metabolized  and  the  amount  of  energy  consequently  liberated 
have  been  computed  and  compared  with  the  energy  recovered  in 
the  form  of  mechanical  work. 

This  method  of  experimentation  has  been  largely  developed  and 
employed  by  Zuntz  and  his  associates  *  in  experiments  upon  man, 
the  dog,  and  especially  the  horse.  Since  the  present  work  relates 
especially  to  the  nutrition  of  domestic  animals,  the  results  upon  the 
latter  animal  are  of  peculiar  interest,  but  their  study  may  be  ad- 
vantageously preceded  by  a  somewhat  brief  consideration  of  the 
results  upon  the  dog  and  upon  man. 

*  Compare  Chapter  VIII,  pp.  251-2 


THE   UTILIZATION  OF  ENERGY. 


499 


Experiments  on  the  Dog. — The  following  experiments  by 
Zuntz,*  while  not  the  earliest  upon  record,  may  serve  to  illustrate 
the  general  methods  employed  and  as  introductory  to  the  more 
elaborate  experiments  upon  the  horse. 

The  following  table  shows  the  average  oxygen  consumption  and 
carbon  dioxide  excretion,  determined  by  the  Zuntz  apparatus,  of 
a  dog  when  lying,  standing,  and  performing  work  upon  a  tread- 
power,  and  also  the  amount  of  work  done,  all  computed  per  minute : 


Weight 

No  of 
Ex- 
peri- 
ments. 

Respiration  per 
Minute. 

Work  per  Minute. 

of  Ani- 
mal 
and 

Load 
Kgs. 

Oxy- 
gen 
c.c. 

CO-i. 
c.c. 

Respir- 
atory 
Quo- 
tient. 

Work 

of 
Ascent 
Kgm. 

Work 

of 
Draft. 
Kgm. 

Dis- 
tance 
travel- 
led, 
Meters. 

6 

2 
8 
5 
10 

L  vine 

174.3 
172. 
245.6 
725.3 

1285.3 
1028.8 

124.7 
123.8 
170.2 
525.2 
990.6 
798.9 

0.71 
0.72 
0.69 
0.73 
0.77 
0.77 

"        (Magnus-Levy)  .  . 

26.932 
26.674 
27.175 

Ascending  slight  incline. 

steeper     " 
Draft   nearly  horizontal  . 

13.23 
365 . 82 
22.83 

202 .91 

78.566 
79.497 
70.420 

The  work  per  minute  as  given  in  the  above  table  does  not  in- 
clude the  energy  expended  in  horizontal  locomotion.  The  work  of 
draft  is  the  product  of  the  distance  traversed  into  the  draft;  the 
work  of  ascent  equals  the  same  distance  multiplied  by  the  sine  of 
the  angle  of  ascent.  A  remarkable  increase  (41  per  cent.)  in  the 
metabolism  when  standing  over  that  when  lying  was  observed 
(compare  p.  343)  but  does  not  enter  into  the  subsequent  com- 
putations. 

The  two  experiments  on  ascending  a  grade  afford  data  for  com- 
puti  lg  the  increased  metabolism  corresponding,  on  the  one  hand, 
to  one  gram-meter  of  work  done  against  gravity,  and,  on  the 
other,  to  the  transportation  of  one  kilogram  through  one  meter 
horizontally.  The  latter,  of  course,  is  not  work  in  the  mechanical 
sense,  but  it  requires  the  consumption  of  a  certain  amount  of 
material,  the  liberated  energy  being  employed  in  successive  liftings 
of  the  body  and  in  overcoming  internal  resistances  and  ultimately 
appearing  as  heat.  It  includes,  of  course,  the  increased  metab- 
olism required  for  the  maintenance  of  the  erect  position. 

*  Arch.  ges.  Physiol.,  68,  191. 


500  PRINCIPLES   OF  ANIMAL   NUTRITION. 

If  from  the  totals  given  in  the  table  we  subtract  the  figures 
for  rest,  we  have  the  following  as  the  increments  of  the  respiration 
due  to  the  work,  including  the  work  of  standing: 


Oxygen, 

c.c. 

Carbon  Dioxide, 
c.c. 

Ascending  slight  incline  .  . 
"          steeper    " 

551.0 
1111.0 

00.5 
865.9 

The  weight  of  the  animal  and  the  distance  traversed  having 
differed  somewhat,  the  results  may  be  rendered  comparable  by  com- 
puting them  per  kilogram  of  weight  and  per  meter  of  distance  trav- 
ersed— that  is,  by  dividing  in  each  case  by  the  product  of  weight 
into  distance.  Expressing  the  results  in  gram-meters  and  cubic 
millimeters  for  convenience  we  have — 


Oxygen 
c.mm 

Carbon  Dioxide 
c  mm. 

Work  of  Ascent, 
gr.-m. 

Ascending  slight  incline .... 
"            steeper    "      .... 

260.40 
523.93 

189.27 
408.35 

6.252 
172.512 

If  we  let  x  equal  the  oxygen  consumption  required  for  the  trans- 
portation of  1  kg.  through  1  meter  and  y  that  required  per  gram- 
meter  of  work  of  ascent  we  have 


x+     6.252y-- 
*+ 172.512?/  = 


260. 40  c.mm. 
=  523.93  c.mm. 


whence  we  have 


250.49    c.mm. 
1.585  c.mm. 


A  similar  computation  for  the  carbon  dioxide  gives 

Locomotion,  per  kg.  and  meter 181 .  033  c.mm. 

Per  gram-meter  of  work  of  ascent ....       1.317  c.mm. 

and  the  corresponding  respiratory  quotient  is  0.723. 

With  these  data  in  hand  it  is  easy  to  compute  the  increased, 
respiratory  exchange  corresponding  to  one  gram-meter  of  work  of 
draft  as  follows: 


THE   UTILIZATION  OF  ENERGY. 


5°; 


Oxygen, 
c.c. 

Carbon 
c 

3ioxide, 
c. 

Total 

1028.80 
174.30 

479.36 
36.19 

124.70 

346.55 
30.07 

798   90 

Rest 

Transportation   of   27.175    kgs. 

through  70.42  meters 

Ascent — 22.83  kgm 

Total 

689.85 

501  32 

338.95 

297  58 

For  one  gram-meter  of  work  of  draft  we  have,  therefore, 

Oxygen 1 .  6704  c.mm 

Carbon  dioxide 1 .  467    c.mm 

Respiratory  quotient 0.878 

It  appears  from  the  above  that  the  work  of  draft  required 
somewhat  more  metabolism  than  the  same  amount  of  work  of 
ascent.  The  individual  experiments  of  this  and  other  series  like- 
wise show  that  variations  in  the  speed  and  in  the  angle  of  ascent 
affect  the  result.  For  the  present,  however,  we  may  confine  our- 
selves to  a  consideration  of  the  average  figures. 

It  remains  to  compute  from  the  results  for  oxygen  and  carbon 
dioxide  the  corresponding  amounts  of  energy  liberated.  The  data 
are  insufficient  for  an  exact  computation.  It  having  been  shown, 
however  (compare  Chapter  VI),  that  even  severe  work  causes  but  a 
slight  increase  in  the  proteid  metabolism,  the  author  assumes  that 
the  additional  metabolism  in  these  experiments  was  entirely  at  the 
expense  of  carbohydrates  and  fat  and  computes  the  proportion  of 
each  from  the  respiratory  quotient.  The  results  are  admittedly 
not  exact.  Besides  the  uncertainty  just  mentioned,  there  is  the 
possibility  that  irregularities  in  the  excretion  of  carbon  dioxide 
may  affect  the  respiratory  quotient  in  short  trials  and,  more- 
over, we  must  bear  in  mind  the  possibility  of  various  cleavages 
and  hydrations  as  affecting  the  evolution  of  energy  in  such  experi- 
ments (compare  Berthelot's  criticism  on  p.  254).  The  author  does 
not,  however,  regard  these  possible  errors  as  very  serious.  Com- 
puted on  this  basis  the  results  are  as  follows,  expressed  both  in 
terms  of  heat  (calories)  and  in  gram-meters  (1  cal.  equals  425 
gram-meters) : 


502  PRINCIPLES  OF  ANIMAL   NUTRITION. 

For  1  gram-meter,  ascent 0 .  0076681  cal.  =  3 .  259  gr.-m. 

"    1     "         "      draft 0.008180     "    =3.476     " 

"    locomotion  per  kg.  and  meter.  .    1 .  1787       cals.  =  500 .95  " 

According  to  the  above  figures  the  performance  of  one  gram- 
meter  of  work  required  the  metabolizing  of  material  whose  potential 
energy  was  equal  to  3.259  gr.-m.  in  the  one  case  and  3.476  gr.-m.  in 
the  other.  In  other  words,  these  amounts  of  net  available  energy- 
were  liberated  in  the  kinetic  form  in  the  body,  one  gram-meter  in 
each  case  being  recovered  as  external  work  while  the  remainder 
ultimately  took  the  form  of  heat. 

This  is  equivalent  to  a  utilization  of  30.7  per  cent,  of  the  net 
available  energy  in  ascent  and  of  28.77  per  cent,  in  draft.  It  is  to 
be  noted  that  these  figures  refer  only  to  that  portion  of  the  in- 
creased metabolism  which  is  applied  to  the  production  of  external 
work  and  do  not  include  that  necessary  for  the  transportation  of  the 
animal's  weight.  The  corresponding  ratio  for  this  portion  could 
only  be  obtained  on  the  basis  of  complicated  and  uncertain  compu- 
tations of  the  mechanical  work  of  locomotion.  If,  however,  instead 
of  this  we  assume  that  this  most  common  form  of  muscular  activity 
is  performed  with  the  same  economy  as  the  work  of  ascent,  we  can 
conversely  compute  the  mechanical  work  of  locomotion  for  1  kg. 
through  1  meter  as 

500 .  95  gr.-m.  X  .  307  =  153 . 8  gr.-m. 

Experiments  on  Man. — In  connection  with  his  experiments  on 
the  dog  already  described,  Zuntz  *  cites  the  results  of  a  number  of 
experiments  with  man  upon  the  work  of  locomotion  and  of  ascent, 
the  average  results  of  which  are  summarized  in  the  table  opposite, 
to  which  have  been  added  the  results  of  later  experiments  by 
Frentzel  &  Reach. f 

Experiments  on  the  Horse. — Very  extensive  investigations  on 
the  production  of  work  by  the  horse  have  been  made  by  Zuntz  in 
conjunction  with  Lehmann  and  Hagemann.J  Some  of  the  results 
of  these  investigations  have  already  been  discussed  in  their  bearing 
on  the  question  of  digestive  work  (pp.  385-393),  and  the  method 

*  hoc.  cit.,  p.  20S. 

t  Arch,  ges   Physiol..  83,  494. 

JLandw   Jahr.;  18,  1;   23,  125;   27,  Supp   III. 


THE   UTILIZATION  OF  ENERGY. 


503 


Weight 
(with 

Appara- 
tus), 
Kgs. 

Energy  Expended  in 

Horizontal 
Velocity 

per 
Minute, 
Meters. 

Experimenter. 

Loco- 
motion 
per  Kg. 
and  Meter, 
Kgm. 

Per  Kgm. 

Work  of 

Ascent, 

Kgm. 

Grade. 
Per  Cent. 

55.5 
72.9 
67.9 
80.0 
88.2 
72.6 
81.1 
80.0 

86.5 
86.5 

65.8 
55.8 

0.334 
0.217 
0.211 
0.288 
0.263 
0.284 
0.231 
0.244 

0.219 
0.233 

0.230 
0.251 

2.857 
3.190 
3.140 
3.563 
3.555 
2.913 
2.921 
2.729 

[2.746  | 
[ 2.846  | 

74.48 
71.32  \ 
71.46  \ 
51.23  / 
43.34  S 
62.04  ) 
60.90  [ 
56.54  ) 

66.94  1 
3, .92  1 

> 

63.95  1 
34.58  J 

9  6-13  3 

r 

Schumburg    &    Zuntz  -j 

I 

Loewy •] 

Frentzel : 

Normal  gait 

6.5 
30.7-62.0 

23.0-30.5 

Slow         "   

Reach : 

Normal  gait 

Slow          "    

23.3 

of  computing  the  total  metabolism  in  the  rest  experiments  has 
been  explained;  it  remains  to  consider  the  results  of  the  work 
experiments.  The  larger  proportion  of  the  experiments  were 
upon  the  same  horse  (No.  Ill),  and  the  summaries  and  averages 
on  subsequent  pages  represent  chiefly  the  results  with  this  animal. 

The  work  was  done  upon  a  special  tread-power  located  in  the 
open  air,  and  during  the  rest  experiments  the  animal  likewise  stood 
in  the  tread-power.  The  inclination  of  the  platform  of  the  power 
could  be  varied,  and  it  could  also  be  driven  by  a  steam-engine,  so 
that  by  setting  it  horizontal  the  work  performed  by  the  animal  was 
reduced  to  that  of  locomotion  alone.  The  distance  traversed  was 
measured  by  a  revolution-counter,  and  in  the  experiments  on  draft 
the  animal  pulled  against  a  dynamometer. 

The  large  number  of  experiments  (several  hundred)  are  grouped 
by  the  authors  into  fourteen  periods  according  to  the  season  (winter 
or  summer)  and  the  kind  and  amount  of  food  consumed,  each  of 
these  periods  including  a  considerable  number  of  experiments  both 
on  rest  and  on  different  forms  of  work.  On  each  day  from  two  to 
eight  experiments  were  usually  made,  some  on  rest  and  some  on 
work  of  various  sorts.  The  average  of  all  the  rest  experiments  in 
each  period  is  then  compared  with  similar  averages  for  the  various 


5°4 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


kinds  of  work  in  order  to  eliminate  so  far  as  possible  the  influence 
of  variations  in  external  temperature  and  in  the  feeding,  as  well  as 
to  reduce  the  probable  error  of  experiment. 

Work  at  a  Walk.— The  experiments  may  be  grouped  into 
those  in  which  the  work  was  performed  respectively  at  a  walk  and  a 
trot.  Those  of  the  former  category,  being  the  more  numerous,  may 
be  considered  first. 

Work  of  Locomotion. — The  following  detailed  comparison  of  the 
experiments  of  Period  a  upon  rest  and  upon  walking  without  load 
or  draft  will  serve  to  further  explain  the  method : 

REST    EXPERIMENTS.       PERIOD    a. 
Ration,  6  Kg.  Oats,  1  Kg.  Straw,  6-7  Kg   Hay. 


No.  of  Experiment. 

Per  Kg  Live  Weight 
and  Minute. 

Respira- 
tory 
Quotient. 

Air  Tem- 
perature 
Deg.  C. 

Relative 
Velocity 
of  Wind. 

Hours 
Since  Last 
Feeding. 

Oxygen 
c.c. 

Carbon 

Dioxide 

c.c. 

37a    

3.94 
3.92 

3  98 
4.06 
4.11 
3.89 
3  71 

3.81 
4.02 
3.42 
4.04 
3  86 
3.63 
3.44 

0.968 
1.025 
0.861 
0.997 
0.940 
0.933 
0.929 

-5.0 
-0.5 
2.0 
5.3 
4.7 
2.0 
9.0 

0 

1 

3 

1 
1 
3 

3.0 

386         • 

2.5 

38/ 

5.6 

39a   

2.0 

44a   

1.5 

45a 

3.5 

46a 

1.5 

Average 

Corrected  *  

3.94 
4.04 

3.75 
3.86 

0.950 

2.5 

1.4 

2.8 

In  the  same  period  eight  experiments  were  made  in  which  the 
.  -^ad-power  was  set  as  nearly  horizontal  as  possible  and  driven  by 
the  steam-engine,  the  animal  being  simply  required  to  maintain  his 
place  on  the  power.  The  results  for  oxygen  were  as  shown  in  the 
first  portion  of  the  following  table : 

*  A  comparison  of  Zuntz's  method  with  the  results  obtained  in  the  Pet- 
tenkofer  respiration  apparatus  showed  that  the  gaseous  exchange  through 
the  skin  and  intestines  amounted  to  about  2\  per  cent,  of  the  pulmonary 
respiration  in  case  of  the  oxygen  and  3  per  cent  in  case  of  the  carbon  di- 
oxide These  additions  are  accordingly  made  to  the  figures  of  the  respira- 
tion experiments  and  the  results  designated  as  "corrected  " 


THE   UTILIZATION   OF  ENERGY. 


5°5 


WALKING    WITHOUT   LOAD    OR   DRAFT.       PERIOD    a. 
Per  Kg.  Live  Weight. 

Live 

Weight 

Kgs. 

Observed. 

Oxygen 
to 

Equivalent 

No.  of 
Experiment. 

Per  Minute. 

Work  of 

Ascent , 

Per  Meter 

Traveled. 

Gr.-m. 

Work. 

Oxygen 
c.c. 

Distance 
Traveled 
Meters. 

Work  of 

Ascent, 

Kgm. 

Per 

Minute, 

c.c. 

Per  Meter 

Traveled. 

c.mm. 

40d 

429 
434 
428 
428 
430 
430 
434 
434 

9.0 
11.3 
12.2 
12.7 
10.8 
11.7 
12.3 
11.2 

57 
87 
94 
95 
92 
99 
98 
93 

0.57 

0.84 
0.89 
0.87 
0.70 
0.74 
0.79 
0.76 

10 
10 
9 
9 

8 
8 
8 
8 

5.1 
7.3 
8.2 
8.7 
6.9 
7.8 
8.4 
7.3 

89 

446 

456 

45d 

466 

46c 

84 
88 
92 
74 
79 

476 

86 

47c 

78 

Average  ..  . 
Corrected. . 

430.9 

11.405 

89.338 

0.764 

8.643 

7.463 

83.793 

85 . 888 

If  from  the  oxygen  consumption  in  each  of  the  above  experiments 
we  subtract  the  average  rest  value  for  the  same  period  (3.94  c.c.) 
the  remainder  will  represent  the  increase  due  to  the  work,  as  shown 
in  the  seventh  column,  and  this  divided  by  the  distance  traveled 
gives  the  figures  of  the  eighth  column. 

The  average  respiratory  quotient  of  that  part  of  the  respiration 
due  to  the  work  in  these  eight  experiments  was  0.894.  On  the 
very  probable  assumption  that  the  work  caused  no  material  change 
in  the  metabolism  of  either  proteids  *  or  crude  fiber,  or  in  other 
words,  that  the  energy  for  work  was  derived  substantially  from  solu- 
ble carbohydrates  and  fat,  the  calorific  equivalent  of  1  c.c.  of  oxygen 
is  computed  and  the  following  calculation  of  energy  made  for  the 
average  of  the  eight  experiments  (compare  pp.  76  and  251).  These 
results  are  not  corrected  for  cutaneous  and  intestinal  respiration. 
Per  Kg.  Live  Weight  per  Minute. 

Oxygen  combined  with  fat 3.4415  c.c. 

Oxygen  combined  with  starch 4.0215  " 


Total 7.4630  " 

Equivalent  energy 36.420  cals. 

*  The  authors  show  that  even  a  considerably  increased  proteid  meta- 
bolism would  not  materially  affect  the  computation  of  energy. 


506 


PRINCIPLES   Oh'  ANIMAL   NUTRITION. 


Energy  per  Meter  Traveled  (Including  Work  of  Ascent). 

Per  kg.  total  mass  * 0.3948  cal. 

(0.4077    " 
Per  kg.  live  weight j  0  1733  kgln 

Work  of  ascent 8.643  gr.-m. 

Determinations  of  the  work  of  locomotion  were  made  in  six 
different  periods,  or  thirty-five  experiments  in  all.  The  average 
for  each  period,  computed  in  terms  of  energy  as  in  the  above 
example,  is  given  in  Table  VIII  of  the  Appendix.  It  is  to  be  noted 
that  these  results  still  include  the  small  amount  of  work  expended 
in  ascending  the  slight  incline.  This  factor  is  determined  in  the 
manner  shown  in  the  following  paragraph. 

Work  of  Ascent. — In  four  periods  experiments  were  made  (thir- 
teen in  all)  upon  the  work  of  ascending  a  moderate  grade  at  a  walk. 
The  average  results,  computed  oh  the  same  basis  as  before,  are 
contained  in  Table  IX  of  the  Appendix. 

By  comparing  the  average  results  of  these  two  series  of  experi- 
ments in  the  manner  explained  on  p.  500,  letting  x  equal  the  oxygen 
or  energy  required  per  kilogram  live  weight  for  locomotion  through 
1  meter  horizontally  and  y  the  corresponding  quantities  for  the 
performance  of  1  gram-meter  of  work  of  ascent  we  have  the  follow- 
ing equations: 

For  Oxygen. 
x  +     4.395?y  =   83.480  c.mm. 
x+107.041i/  =  222.941  c.mm. 

For  Energy. 
x+     4. 395?/  =  0.4035  cal. 
x+ 107 .  041?y  =  1 .  0795  cals. 
Solving  these  we  obtain  the  following  values  respectively  for 
the  work  of  locomotion  per  meter  and  for  the  energy  expended  in 


Oxygen 
c  mm. 

Energy. 

cals. 

Kgm. 

Locomotion  per  meter: 

Per  kg   live  weight 

77.509 
75.048 
1359.00 

0.3746 
0.3618 
6.5858 

0.1592 
0  1538 

Ascent,  per  kilogram-meter 

2.7990 

*  Weight  of  animal  plus  weight  of  apparatus  carried. 


THE   UTILIZATION  OF  ENERGY. 


5°7 


doing  1  kgm.  of  work  of  ascent,  and  the  utilization  of  the  available 
energy  in  the  latter  case  is  35.73  per  cent. 

Work  of  Draft. — For  the  work  of  draft  at  a  walk,  up  a  slight 
incline,  the  results  tabulated  in  Table  X  of  the  Appendix  were 
obtained. 

Giving  x  and  y  the  same  significance  as  before,  and  letting  z 
represent  the  oxygen  or  energy  corresponding  to  one  gram- meter  of 
vork  of  draft,  we  have  the  following  equation,  based  on  the  results 
per  kilogram  live  weight  and  meter  traveled : 

x  +  5. 1152/ +153. 1272  =  306. 561  c.mm.  =  1.5021  cals. 
Substituting  in  this  the  average  values  of  x  and  y  obtained  as  in- 
dicated in  the  previous  paragraph,  but  from  a  larger  number  of 
experiments,  we  have 

2=  1 .4504  c.mm.=  .007143  cal.  per  gram-meter. 

The  above  details  of  a  few  of  the  experiments  may  serve  to  illus- 
trate the  methods  of  computation  employed.  Similar  determina- 
tions were  made  upon  various  forms  of  work  under  differing  condi- 
tions, the  results  of  which  will  be  given  later. 

Correction  for  Speed. — Before  final  data  could  be  obtained, 
however,  it  was  found  necessary  to  take  account  of  the  speed  of  the 
animal,  since  comparisons  of  the  various  periods  showed  that  the 
metabolism  due  to  the  work  of  locomotion  at  a  walk  increased 
materially  as  the  velocity  increased. 

To  compute  the  necessary  correction,  the  authors  divide  the 
thirty-five  experiments  of  Table  VIII  into  three  groups  according 
to  the  speed.  For  each  group  the  oxygen  and  energy  correspond- 
ing to  the  work  of  ascent  are  computed,  using  the  values  of  y  given 
on  the  previous  page  (1359  c.mm.;  6.5858  cals.),  and  subtracted 
from  the  total,  leaving  the  following  as  the  amounts  per  kilogram 
live  weight  due  to  horizontal  locomotion: 


No.  of 
Experi- 
ments. 

Velocity 

per  Minute, 

Meters. 

Oxygen 
Consumed 

per  Kg. 
and  Meter, 

c.mm. 

Respira- 
tory 
Quotient. 

Oxygen  Re- 
calculated to 
Respiratory 
Quotient  of 
0.S6,  c.mm 

Increase  of 

Oxygen  per 

Meter 

Velocity, 

c.mm. 

Heat  Value 
of  Oxygen 
per  Meter 

(Corrected), 
cals. 

6 

20 

9 

78.00 
90.16 
98.11 

66.69 
76.04 
80.97 

0.896 
0.848 
0.873 

67.32 
75.80 
81.23 

0.697 
0.683 

0.3363 
0.3787 
0.4058 

5°8 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


On  the  average,  an  increase  of  1  meter  per  minute  in  the  speed 
was  found  to  cause  an  increased  metabolism  corresponding  to — 

Oxygen 0 .  692  c.mm. 

Energy 0.00345  cal. 

A  similar  computation  for  the  experiments  on  ascending  a  con- 
siderable grade  without  load  or  draft  showed  a  similar  difference, 
which,  however,  seemed  to  be  chiefly  or  entirely  due  to  variations 
in  the  work  of  locomotion.  When  the  amount  of  the  latter  was 
computed  with  the  correction  for  speed  just  given,  the  metabolism 
due  to  the  actual  work  of  ascent  seemed  to  be  independent  of  the 
speed,  the  only  exception  being  two  experiments  at  a  rapid  walk  in 
which  over  exertion  of  the  animal  was  suspected. 

In  the  thirteen  experiments  on  the  work  of  ascending  a  moderate 
grade  contained  in  Table  IX,  the  average  speed  was  81.95  meters 
per  minute,  while  in  the  thirty-five  experiments  with  which  they 
are  compared  (Table  VIII)  the  average  speed  was  90.16  meters. 
From  the  table  on  p.  506  we  compute  that  the  consumption  of 
oxygen  (R.Q.  =  0.86)  and  the  corresponding  energy  values  per  kilo- 
gram and  meter  at  these  speeds  would  be — 


Oxygen 

Energy, 
cals. 

At  90.16  M.  velocity 

"    81.95  M.        "       

75.80 
70.05 

0.3746 
0.3462 

Substituting  this  corrected  value  of  x  in  the  equations  on 
p.  506,  we  have  as  the  corrected  value  of  y  per  kilogram-meter  for 
ascending  a  moderate  grade 

6.851  cals.  =  2.912kgm.  =  34.3  per  cent. 

In  brief,  a  correction  for  the  value  of  x  is  computed,  using  the 
first  value  of  y,  and  then  this  corrected  value  of  x  is  used  to  com- 
pute the  corrected  value  of  y.  In  other  words,  the  method  is  one 
of  approximation,  but  the  errors  of  the  corrected  values  are  pre- 
sumably less  than  the  unavoidable  errors  of  experiment. 

Effect  of  Load. — In  a  number  of  experiments  the  horse  carried 
on  the  saddle  a  load,  consisting  of  lead  plates,  corresponding  to  that 
of  a  rider.    The  mere  sustaining  of  such  a  weight  at  rest  was  found 


THE  UTILIZATION  OF  ENERGY.  509 

to  increase  the  gaseous  exchange,  the  total  metabolism  being  sub- 
stantially proportional  to  the  total  mass  (horse  +  load),  but  in  com- 
puting the  work  experiments  the  same  rest  values  are  used  as  for 
the  preceding  experiments;  that  is,  the  results  include  the  work 
required  to  simply  sustain  the  weight  as  well  as  that  required  to 
move  it.  Computing  the  results  in  the  same  manner  as  before  the 
authors  obtain  for  an  average  speed  of  90.18  meters  per  minute 
the  following  results: 
Locomotion  per  Meter. 

Per  kg.  live  weight 0.5004  cal.    =  0.2126  kgm. 

"     "    total  mass 0.3914     "     =0.1663     " 

Ascent. 

Per  kilogram-meter 6 . 502  cals.  =  2 . 7640     "       =36.19£ 

A  comparison  of  these  figures  with  those  on  p.  506  shows 
that  for  this  animal  a  load  of  127  kgs.  caused  about  8  per  cent, 
increase  in  the  energy  expended,  per  kg.  of  total  mass,  in  horizon- 
tal locomotion,  but  no  increase  in  that  expended  per  kilogram- 
meter  in  ascent. 

Work  of  Descent. — In  descending  a  grade  the  force  of  gravity 
acts  with  instead  of  against  the  animal  and  tends  therefore  to 
diminish  the  metabolism.  On  the  other  hand,  however,  as  the 
steepness  of  the  grade  increases  the  animal  is  obliged  to  put  forth 
muscular  exertions  to  prevent  too  rapid  a  descent,  and  this  tends 
to  increase  the  metabolism.  It  was  found  that  an  inclination  of 
2°  52'  caused  the  maximum  decrease  in  the  metabolism.  At  5°  45' 
the  metabolism  was  the  same  as  at  0°,  while  on  steeper  grades  it 
was  greater  than  on  a  level  surface. 

Work  at  a  Trot. — A  smaller  number  of  experiments  were  made 
upon  work  at  a  trot  under  varying  conditions.  In  trotting,  the  up 
and  down  motion  of  the  body  is  much  greater  than  in  walking,  while 
but  a  small  part  of  the  muscular  energy  thus  expended  is  available 
for  propulsion.  It  was  therefore  to  be  expected  that  the  energy 
required  for  horizontal  locomotion  would  be  greater  at  a  trot  than 
at  a  walk,  and  the  results  of  the  experiments  corresponded  fully 
with  this  expectation,  the  computed  energy  per  meter  being  found 
to  be 

Per  kg.  live  weight 0 .  5660  cal. 

(horse  +  load) 0.5478    " 


5i° 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


at  a  speed  of  195  meters  per  minute.  The  fact  of  such  an  increased 
expenditure  of  energy  in  trotting  as  compared  with  walking  has 
also  been  confirmed  by  the  results  of  Grandeau,  which  will  be  con- 
sidered in  another  connection.  It  was  also  found  that  in  trotting, 
unlike  walking,  the  work  of  locomotion  was  independent  of  the 
speed  within  the  limits  experimented  upon  (up  to  a  speed  of  206 
meters  per  minute,  or  about  7£  miles  per  hour).  A  load  of  127.2  kgs. 
increased  the  work  of  locomotion  per  kg.  of  mass  by  about  10  per 
cent,  as  compared  with  the  increase  of  8  per  cent,  at  a  walk.  One 
experiment  on  work  of  ascent  and  one  on  horizontal  draft,  both 
without  load,  showed  a  utilization  of,  respectively,  31.96  percent, 
and  31.70  per  cent.,  but  two  other  experiments  on  horizontal  draft, 
in  which  the  work  was  thought  to  have  been  excessive,  gave  an 
average  of  only  23.35  per  cent. 

Summary. — The  final  results  of  the  experiments  upon  the  horse 
may  be  summarized  as  follows: 


Work  at  a  Walk. 


Available  Energy 
Expended. 


Kgm. 


Utiliza 
tion 
Per 
Cent. 


Work  at  a  Slow  Trot 


Available  Energy 
Expended. 


Kgm 


Utiliza 
tion. 
Per 
Cent. 


For  1  kgm    work  of  ascent, 
without  load  : 

10.7?  grade 

18.1?  grade 

For  1  kgm.  work  of  ascent, 
with  load : 

15.8?  grade 

For  1  kgm  work  of  draft : 

0.5  %  grade 

8.5  %  grade 

Locomotion  per  kg  mass  per 
meter  without  load  : 
Speed  of  78  00  M.  per  min 
"      "  90.16  "      "     " 
"      "  98.11  '•      "     " 
The  same  with  load  : 

Speed  of  90.18  M  per  min 


6.8508 
6.9787 


i.502 


7.5190 
10.3360 


0.3256 
0.3666 
0.3929 

0.3914 


2.9116 
2.9660 


2.7634 


3.1960 


34.3 
33.7 


36.2 


31.3 


4.3930  22.7 


7.3647' 


7.4240* 
10.0780f 


0.5478J 
0.6007J 


3.1300* 


3.1550* 

4.2820f 


31.96* 


31.7* 
23. 4f 


*  Single  experiment 

1  Two  experiments.     Work  probably  excessive. 

X  Independent  of  speed. 


THE   UTILIZATION  OF  ENERGY.  511 

Conditions  Determining  Efficiency. 

From  the  results  recorded  in  the  preceding  paragraphs  it  appears 
that,  as  we  were  led  to  expect  from  a  consideration  of  the  efficiency 
of  the  single  muscle,  the  efficiency  of  the  animal  as  a  converter  of 
potential  energy  into  mechanical  work  varies  with  the  nature  of  the 
work  and  the  conditions  under  which  it  is  performed,  although  the 
variations  are  perhaps  hardly  as  great  as  might  have  been  expected. 
In  general,  we  may  say  that  in  the  neighborhood  of  one  third  of  the 
potential  energy  directly  consumed  in  muscular  exertion  is  recov- 
ered as  mechanical  work.  This  appears  to  be  a  high  degree  of  effi- 
ciency as  compared  with  that  of  any  artificial  transformer  of  poten- 
tial energy  yet  constructed.  The  steam-engine,  the  chief  example 
of  such  transformers,  even  in  its  most  highly  perfected  forms,  rarely 
utilizes  over  15  per  cent,  of  the  potential  energy  of  the  fuel,  while 
in  ordinary  practice  one  half  of  this  efficiency  is  considered  a  good 
result. 

The  comparison  is  misleading,  however,  for  three  reasons:  First, 
the  figures  given  in  the  preceding  pages  relate  to  the  utilization  of 
the  net  available  energy  of  the  food.  As  we  have  seen,  however, 
a  certain  expenditure  of  energy  in  digestion  and  assimilation  is 
required  to  render  the  food  energy  available,  while  still  another 
portion  of  the  latter  is  lost  in  the  potential  energy  of  the  excreta. 
In  the  case  of  herbivorous  animals,  these  two  sources  of  loss  very 
materially  reduce  the  percentage  utilization  when  computed  upon 
the  gross  energy  of  the  food.  Second,  the  comparison  takes  no 
account  of  the  large  amount  of  energy  consumed  continually 
throughout  the  twenty-four  hours  for  the  internal  work  of  the 
body  of  the  animal,  and  which  continues  irrespective  of  whether 
the  animal  is  used  as  a  motor  or  not.  Third,  the  expenditure 
of  energy  in  locomotion  is  not  considered  in  computing  the 
efficiency  of  one  third.  When  these  three  points  are  allowed  for 
but  little  remains  of  the  apparent  superiority  of  the  animal  as  a 
prime  motor,  even  omitting  from  consideration  the  greater  cost  of 
his  fuel  (food). 

It  remains  now  to  consider  somewhat  more  specifically  the  in- 
fluence upon  the  efficiency  of  the  animal  machine  of  some  of  the 
more  important  conditions. 


5i2 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


Kind  of  Work. — Of  the  forms  of  work  investigated,  that  of 
ascent,  that  is,  of  raising  the  weight  of  the  body  (with  or  without 
load),  appears  to  be  the  one  which  is  performed  most  economically. 
The  horse  in  ascending  a  moderate  grade  without  load  showed  an 
efficiency  of  34.3  per  cent.,  while  with  a  load  of  127  kgs.  a  slightly 
higher  efficiency  was  obtained,  viz.,  36.2  per  cent.  (The  latter 
figure,  however,  includes  some  estimated  corrections  for  speed.) 
For  the  dog  (p.  502)  the  average  result  was  30.7  per  cent.  For 
man  the  figures  of  the  table  on  p.  503  correspond  to  from  28.1  to 
36.6  per  cent. 

The  efficiency,  however,  was  found  to  decrease  with  the  steep- 
ness of  the  grade.  Thus  with  the  horse  it  fell  from  34.3  to  33.7 
per  cent.,  with  an  increase  of  the  grade  from  10.7  to  18.1  per  cent. 
The  experiments  of  Loewy  on  man,  averaged  on  p.  503,  show  the 
same  result  in  a  more  striking  manner.  Taking  separately  the 
experiments  on  each  subject  we  have  the  following: 


Grade 
Per  Cent. 

Efficiency. 

A.  L. 
Per  Cent. 

J   L 
Per  Cent. 

L  Z 
Per  Cent 

23 

30.5 

36.6 

34  3 
34  3 
29.0 

36.1 
32.6 
32  3 

36.6 
36   6 

32.2 

The  work  of  horizontal  locomotion  consists  largely  of  successive 
liftings  of  the  weight  of  the  body.  It  might  therefore  be  expected 
from  the  above  results  that  this  work  would  be  performed  even  more 
economically  than  that  of  ascent,  since  it  is  obviously  the  form  of 
muscular  activity  for  which  animals  like  the  horse  and  dog  arc 
specially  adapted.  In  the  case  of  the  walking  horse,  Kellner  *  has 
proposed  a  formula  based  on  mechanical  considerations,  for  com- 
puting the  work  of  locomotion.  Zuntz  f  has  applied  this  formula 
to  the  animal  used  in  his  experiments  and  computed  the  mechanical 
work  of  locomotion  at  the  three  speeds  for  which  the  total  metabo- 
lism was  also  determined  (p.  507). 

Landw.  Jahrb.,  9  658. 
t  Ibid  ,  27.  Supp  III.  p  314. 


THE   UTILIZATION  OF  ENERGY. 


5*3 


A  comparison  of  these  figures,  expressing  the  total  metabolism 
in  its  mechanical  equivalent,  is  as  follows: 


Speed 

Meters  per 

Minute. 

Per  Kg.  Mass  and  Meter. 

Total 
Metabolism 
Gram-meters. 

Computed 

Work 

Gram-meters. 

Percentage 
Efficiency. 

78.00 
90.16 
98.11 

138.4 
155.8 
167.0 

49.14 
54.54 
58.40 

35.5 

35.00 

34.97 

This  computation  gives  an  efficiency  slightly  greater  than  that 
obtained  for  the  ascent  of  a  grade  without  load,  and  in  so  far  tends 
to  confirm  our  conjecture,  but  the  basis  on  which  the  work  of  loco- 
motion is  computed  can  hardly  be  regarded  as  sufficiently  accurate 
to  give  this  result  the  force  of  a  demonstration. 

The  work  of  draft  appears  to  be  performed  considerably  less 
economically  than  that  of  ascent  or  locomotion.  Thus,  for  the 
horse,  the  efficiency  for  nearly  horizontal  draft  was  found  to  be  31.3 
per  cent,  at  a  walk,  and  in  one  experiment  at  a  trot  31.7  per  cent.,  as 
against  34-36  per  cent,  for  ascent.  In  two  other  experiments  at  a 
trot,  in  which  the  work  may  have  been  excessive,  a  much  lower 
efficiency  was  found,  viz.,  23.4  per  cent.  For  draft  up  a  grade  of 
8.5  per  cent,  at  a  walk  the  efficiency  was  greatly  reduced,  viz.,  to 
22.7  per  cent.  The  above  figures  refer  to  the  work  of  draft  only, 
after  deducting  the  energy  required  for  locomotion  and  ascent.  A 
similar  difference  was  likewise  observed  with  the  dog  (p.  502),  the 
efficiency  in  nearly  horizontal  draft  being  28.8  per  cent,  as  compared 
with  30.7  per  cent,  for  work  of  ascent. 

Experiments  on  man,  not  cited  in  the  above  pages,  in  which 
the  work  was  performed  by  turning  a  crank,  have  shown  decidedly 
lower  figures  for  the  percentage  utilization. 

Speed  and  Gait. — The  energy  expended  by  the  horse  in  loco- 
motion at  a  walk  was  found  to  increase  with  the  speed  at  the 
rate  of  0.00334  cal.  per  meter  and  kilogram  mass  for  each  in- 
crease of  1  meter  in  the  speed  per  minute.  Kellner's  mechanical 
analysis  of  the  work  of  locomotion  mentioned  above  divides  it 
into  two  parts,  viz.,  that  expended  in  lifting  the   body  of   the 


5  M  PRINCIPLES  OF  ANIMAL  NUTRITION. 

animal  and  that  expended  in  imparting  motion  to  the  legs.  The 
former  portion  is  regarded  as  constant,  while  the  latter  portion 
would  increase  with  the  speed.  The  very  close  proportionality 
between  the  work  thus  computed  and  the  total  metabolism,  as 
Bhown  by  the  table  on  the  preceding  page,  is  a  strong  confirma- 
tion of  the  correctness  of  both  methods  and  places  the  conclusion 
as  to  the  influence  of  speed  upon  metabolism  beyond  reasonable 
doubt.  It  is  to  be  remembered,  however,  that  it  is  the  total 
metabolism  per  kilogram  and  meter  which  increases  with  the  speed. 
The  percentage  utilization  of  the  energy,  so  far  as  the  data  at  our 
command  enable  us  to  determine,  apparently  remains  constant. 
Practically,  however,  it  is  the  former  fact  which  interests  us,  since 
the  expenditure  of  energy  in  locomotion  is  comparable  to  that  in 
internal  work  and  has  only  an  indirect  economic  value.  A  similar 
effect  of  speed  on  the  metabolism  in  horizontal  locomotion  was 
observed  by  Zuntz  *  in  experiments  on  man.  In  those  with  the  dog, 
on  the  other  hand,  the  variations  in  speed  were  between  64.2  and 
85.9  meters  per  minute,  but  no  material  difference  in  the  metabo- 
lism due  to  locomotion  was  observed. 

In  trotting,  a  horse  expends  much  more  energy  per  unit  of  hori- 
zontal distance  than  in  walking.  Thus,  trotting  at  an  average 
speed  of  195  meters  per  minute  (a  little  over  7  miles  per  hour),  as 
compared  with  walking  at  an  average  speed  of  90.16  meters  per 
minute,  gave  the  following  results  for  the  metabolism  per  kilo- 
gram mass  and  meter  distance. 

Trotting 0.5478  cal. 

Walking 0.3666    " 

On  the  other  hand,  speed  is,  so  to  speak,  obtained  more  econom- 
ically at  the  trot  than  at  the  walk.  In  the  averages  just  given  the 
speed  was  increased  by  116  per  cent.,  while  the  metabolism  was  in- 
creased by  only  49  per  cent.  The  same  result  is  reached  in  another 
way  by  computing,  by  means  of  the  factor  given  at  the  beginning  of 
this  paragraph  (0.00334  cal.),  the  theoretical  walking  speed  which 
would  give  a  metabolism  equal  to  the  average  metabolism  in  trot- 
ting. We  find  this  to  be  147  meters  per  second,  as  compared  with  195 
meters  at  a  trot.  Moreover,  it  was  found  that  at  the  trot  the  metab- 
olism did  not  increase  with  the  speed,  within  the  limits  of  the  ex- 
*  Arch.  ges.  Physiol.,  68,  198. 


THE  UTILIZATION  OF  ENERGY.  515 

periments.  These,  however,  did  not  include  speeds  above  206 
meters  per  minute  (about  1\  miles  per  hour),  and  the  work  was  done 
on  a  tread-power,  so  that  there  was  no  air  resistance.  At  this 
moderate  speed  it  is  not  probable  that  the  latter  factor  would  be  a 
large  one,  but  it  is  one  which  increases  as  the  square  of  the 
velocity,  so  that  at  high  speeds  it  constitutes  the  larger  portion  of 
the  resistance.  At  high  speeds,  too,  the  muscles  contract  to  a 
greater  degree,  thus  decreasing  their  efficiency,  and  additional  auxil- 
iary muscles  are  called  into  play,  both  directly  and  to  aid  the  in- 
creased respiration.  It  is  a  matter  of  common  experience  that  while 
a  horse  is  able  to  travel  for  a  number  of  miles  consecutively  at  6  to 
7  miles  per  hour,  drawing  a  considerable  load,  he  can  maintain  his 
highest  speed  for  only  a  short  time  even  without  load,  and  does  this 
only  at  the  cost  of  largely  increased  metabolism.  It  is  evident  then 
that  there  is  a  limit  beyond  which  an  increase  of  trotting  speed 
must  increase  the  metabolism  with  comparative  rapidity. 

Load.— Supporting  a  load  on  the  back  while  standing  was  found 
to  increase  the  metabolism  of  the  horse  No.  Ill  approximately  in 
proportion  to  the  load — that  is,  the  metabolism  computed  per  unit 
of  mass  (horse  +  load)  increased  but  very  slightly.  In  locomotion 
with  a  load  the  metabolism  is,  of  course,  increased,  since  the  load 
as  well  as  the  body  of  the  animal  must  be  lifted  at  each  step.  The 
increase  over  the  metabolism  at  rest  and  without  load,  both  walking 
and  trotting,  was  found  in  the  case  of  Horse  III  to  be  somewhat 
greater  (8^10  per  cent.)  than  the  increase  in  the  mass  moved 
(horse  +  load). 

After  making  allowance  for  this  increase  in  the  work  of  locomo- 
tion, the  efficiency  in  ascent  with  a  load  was  found  to  be  unaffected 
by  the  latter;  that  is,  the  energy  expended  in  lifting  a  unit  of  mass 
(horse  +  load)  through  a  unit  of  distance  remained  substantially 
the  same.  Indeed  the  figure  obtained  (36.2  per  cent.)  is  slightly 
higher  than  that  without  load  (34.3  per  cent).  Interesting  indi- 
vidual differences  in  the  above  particulars  were,  however,  observed 
between  Horse  No.  Ill  and  some  of  the  other  animals  experimented 
upon,  particularly  Nos.  II  and  XIII,  which  form  the  subject  of  a 
succeeding  paragraph. 

Species  and  Size  of  Animal. — In  ascending  a  moderate  grade, 
the  efficiency  seems  to  be  about  the  same  in  the  horse  and  in 


5i6 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


man,  while  in  the  dog  it  is  apparently  somewhat  less,  as  is  seen 
from  the  following  comparison: 


Grade, 
Per  Cent. 

Efficiency, 
Per  Cent. 

23 
10.7 
18.1 
17.2 

35.7 
34.3 
33.7 
30.7 

Dog 

The  energy  expended  in  horizontal  locomotion,  on  the  other 
hand,  showed  more  marked  differences,  viz.: 


Speed,  Meters 
per  Minute. 

Energy  Expended 

per  Kg.  Mass 
per  Meter,  Kgm. 

Dog 

78.57 

42.32-74.48 

78.00 

0.501 

0.211-0.334 

0.138 

The  relatively  high  figure  for  the  dog  is  perhaps  due  in  part  to  the 
considerable  muscular  effort  apparently  required  (p.  499)  to  main- 
tain the  erect  posture.  It  has  been  shown  by  v.  Hosslin,*  however, 
by  a  mechanical  analysis  of  the  work  of  locomotion,  that  the  latter 
does  not  increase  as  rapidly  as  the  weight  of  the  animal,  but  in 
proportion  to  its  two-thirds  power,  or,  in  other  words,  approximately 
in  proportion  to  the  surface.  If  we  compare  the  experiments  upon 
different  species  of  animals  on  this  basis — that  is,  if  we  divide  the 
total  energy  expended  by  the  animal  for  locomotion  by  the  product 
of  the  distance  traversed  into  the  two-thirds  power  of  the  weight 
— we  obtain  the  following  figures : 

Dog 1.501  kgm. 

Man 0 .  861-1 .  274  kgm. 

Horse 1 .058  kgm. 

(Computed  in  this  way,  the  figures  for  the  horse  and  those  for  man  at 
a  comparable  speed  (74.48  M.  per  min.)  do  not  differ  greatly,  and 
v.  Hosslin's  conclusions  are  to  this  extent  confirmed.  The  figures 
for  the  dog  still  remain  higher  than  the  others.  If,  in  the  case  of 
*  Arehiv  f.  (Anatomie  u.)  Physiol.,  1888,  p.  340. 


THE   UTILIZATION  OF  ENERGY.  517 

this  animal,  we  compare  the  total  metabolism  in  locomotion  with 
that  during  standing  instead  of  lying,  as  was  done  in  the  case  of 
the  horse,  the  figure  is  reduced  to  1.303  kgm.,  or  not  much  higher 
than  in  the  case  of  man.  It  must  be  remembered,  however,  that 
the  figures  above  given  for  man  include  the  metabolism  due  to 
standing. 

Individuality. — Zuntz  &  Hagemann's  investigations  show  that 
the  efficiency  of  the  horse  is  affected  to  a  considerable  degree  by 
the  individual  differences  in  animals.  The  experiments  whose 
results  are  summarized  on  p.  510  were  upon  a  single  animal 
(No.  III).  In  addition  to  these  a  small  number  of  experiments 
were  made  with  several  other  animals,  mostly  old  and  more  or  less 
worthless  ones,  besides  the  considerable  number  upon  Horse  No.  II 
previously  reported  by  Lehmann  &  Zuntz.*  The  results  are  com- 
puted by  the  authors  in  terms  of  energy  and  corrected  for  speed 
upon  the  basis  of  the  results  obtained  with  Horse  No.  III. 

In  a  single  case  the  work  of  ascent  required  slightly  less  expen- 
diture of  energy  than  with  Horse  No.  Ill,  and  in  another  case  the 
work  of  horizontal  locomotion,  computed  to  the  same  live  weight 
in  proportion  to  the  two-thirds  power  of  the  latter  (see  the  oppo- 
site page)  was  also  less  than  for  Horse  No.  Ill,  but  as  a  rule  these 
old,  defective  horses  gave  higher  results.  For  ascent,  omitting 
one  exceptional  case,  the  range  was  as  follows : 
Per  Kgm.  of  Work. 

Minimum 5 .  906  cals.  =  39 .  84  per  cent,  efficiency 

Maximum 9.027    "    =26.07    "      " 

Horse  No.  Ill  .. .  6.851    "    =34.30    "      " 
With  one  very  lame  horse  (string-halt)  the  figures  reached  the 
maximum  of  12.343  cals.,  or  an  efficiency  of  only  16.6  per  cent. 

A  similar  range  was  observed  in  the  results  on  horizontal  loco- 
motion. Reduced  to  a  speed  of  78  M.  per  minute  and  to  the  live 
weight  of  No.  Ill,  the  range  was  as  follows: 

Per  Meter  and  Kilogram  Live  Weight. 

Minimum 0 .  284  cal. 

Maximum 0 .  441    " 

Horse  No.  Ill 0.336    " 

*  Landw.  Jahrb.,  18,  1. 


5'S 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


The  very  lame  horse  mentioned  above  gave  a  still  higher  figure, 
viz.,  0.566  cal. 

A  somewhat  larger  number  of  experiments  with  Horse  No.  XIII 
brought  out  the  interesting  fact  that  the  increase  in  the  metabo- 
lism caused  by  carrying  a  load  on  the  back  was  markedly  less  than 
in  the  case  of  No.  III.  both  at  rest  and  in  motion. 


PER    KILOGRAM 

MASS    (HORSE  4- LOAD). 

Without  Load, 
cals.  per  Minute. 

With  Load, 
cals.  per  Minute. 

Standing  : 

Horse  XIII 

15.990 
18.311 

cals.  per  Meter. 
0.389 
0.367 

0.553 
0.548 

14.670 
18.389 

cals.  per  Meter. 
0.388 
0.391 

0.488 
0.601 

"      III 

Walking  horizontally  : 
Horse  XIII 

"      III 

Trotting  Horizontally  : 
Horse  XIII 

"     III 

While,  without  load,  Horse  No.  XIII  showed  a  greater  metabo- 
lism, both  while  walking  and  trotting  than  did  Horse  No.  Ill,  the 
additional  effort  required  for  carrying  a  load  was  relatively  less, 
so  that  in  every  case  the  metabolism  per  unit  of  mass,  instead  of 
increasing,  remained  unchanged  or  even  diminished.  The  percent- 
age efficiency  of  the  animal  in  ascending  a  grade  was  also  not 
materially  affected  by  the  load,  while  with  Horse  No.  Ill  it  ap- 
peared to  increase  slightly. 

The  experiments  with  Horse  No.  II  previously  reported,*  when 
recalculated  f  in  the  same  manner  as  the  later  ones,  likewise  show 
interesting  individual  differences.  For  horizontal  locomotion, 
after  correcting  for  varying  speeds,  we  have  per  kilogram  mass 
(horse  +  load)  the  following : 


Walking  without  load. 

"        with  load.... 
Trotting  without  load. 

"         with  load 


Horse  No.  II, 
cals.  per  Meter. 


0.415 
0.385 
0.499 
0.415 


Horse  No.  Ill, 
cals.  per  Meter. 


0.367 
0.391 
0.548 
0.601 


*  Landw.  Jahrb.,  18,  1. 


t  Ibid.,  27,  Supp.  Ill,  355 


THE   UTILIZATION   OF  ENERGY. 


5*9 


As  these  figures  show,  No.  II  was  decidedly  inferior  to  No.  Ill 
in  walking  without  load.  In  trotting,  on  the  other  hand,  he  was 
somewhat  the  superior  of  No.  Ill,  or  in  other  words  the  change 
from  walking  to  trotting  caused  much  less  increase  in  his  metabo- 
lism. Like  No.  XIII,  he  carried  a  load  with  decidedly  less  expendi- 
ture of  energy  than  did  No.  III.  For  the  forms  of  work  in  which 
the  percentage  efficiency  could  be  measured  the  results  wTere  as 
follows,  the  grades,  however  being  not  exactly  the  same  for  No.  II 
as  for  No.  Ill: 


Horse  No.  II, 
Per  Cent. 

Horse  No.  Ill, 
Per  Cent. 

Ascending,  moderate  grade  .  .  . 

"            heavier  grade 

Draft,  nearly  horizontal 

"      up  a  grade 

33.2 
31.7 
29.0 
22.4 

34.3 
33.7 
31.3 
22.7 

It  seems  a  fair  presumption  that  such  individual  differences 
as  those  above  instanced  are  caused,  in  large  part  at  least, 
by  differences  in  the  conformation  of  the  animals  resulting  from 
heredity  or  "spontaneous"  variation.  A  strain  of  horses  which 
has  been  bred  and  trained  especially  for  the  saddle  through  a 
number  of  generations  might  very  naturally  be  expected  to  be 
more  efficient  in  carrying  a  load  than  a  strain  which  has  been  bred 
for  speed  in  harness  or  strength  in  draft,  while  the  latter  might  as 
naturally  excel  the  former  in  efficiency  at  the  trot  or  in  draft. 
Similarly,  a  race  of  horses  developed  in  a  hilly  country  might  be 
expected  to  be  more  efficient  in  ascending  a  grade  than  one  in- 
habiting a  flat  region.  It  would  seem,  too,  that  these  differences 
may  be  not  inconsiderable.  The  results  cited  suggest  an  interest- 
ing fine  of  thought  and  investigation  for  the  student  of  breeding. 

Training  and  Fatigue. — It  is  a  familiar  experience  that  any 
unaccustomed  form  of  work  is  much  more  fatiguing  at  first  than  it 
is  later.  This  is  due  in  part  to  the  fact  that  in  making  unfamiliar 
motions  more  accessory  groups  of  muscles  are  called  into  activity 
than  are  necessary  later  when  more  skill  has  been  acquired.  The 
experience  of  a  learner  on  the  bicycle  is  an  excellent  example  of 
this.     In  the  second  place,  however,  simple  exercise  of  a  group  of 


52° 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


muscles  in  a  particular  way  seems  to  increase  their  average  mechan- 
ical efficiency. 

Gruber,*  in  two  series  of  experiments  upon  himself,  obtained 
the  following  figures  for  the  excretion  of  carbon  dioxide  during 
rest,  horizontal  locomotion,  and  hill  climbing,  all  the  trials  being 
made  about  the  same  length  of  time  (four  to  five  hours)  after  the 
last  meal: 


Work  of 
Ascent, 
Kgm. 

Carbon  Dioxide 
Excreted  in  20 
Minutes,  Grms. 

Series  I: 

Rest     .            

9.706* 

19.390* 

5892 
6076 

40 . 982 

"            "    "  after  12  days'  practice 

Series  II  (2  months  later) : 

Rest 

32.217 
12.833 

22.418 

7376 
7539 

38.832f 

"            "    "  after  14  days'  practice 

31.001 

*  Some  carbon  dioxide  may  have  escaped  absorption, 
t  Some  carbon  dioxide  lost. 

Schnyder  f  has  confirmed  and  extended  Gruber's  results.  In 
experiments  in  a  treadmill  upon  two  different  subjects  he  ob- 
tained the  following  figures  for  the  work  performed  per  gram  of 
carbon  dioxide  excreted  in  excess  of  that  given  off  during  rest : 

Kgm. 

(  Without  training 218. 13 

"  ° I  After  2  months'  training 253 .  18 

I  Without  training 243 . 93 

No.  2 ■<  After  6  days'  training 285 .  52 

(     "     55     "  "       349.40 

XT    n,  ,       .    .  I  Without  training 302.76 

No.  2  (second  series)  <   ...       ._  ,       7~°  .  .  ...  orw 

|  After  47  days'  training 404.39 

That  the  greater  efficiency  after  training  is  not  due  solely  to  a 
diminished  use  of  accessory  muscles  is  shown  by  Schnyder's  experi- 
ments on  convalescents.     His  results  were  as  follows: 

*  Zeit.  f.  Biol.,  28,  466 
t  Ibid.,  33,  289. 


THE  UTILIZATION  OF  ENERGY. 


bill 

( First  trial 

Work  per 
Gram,  Car- 
bon Dioxide 
Kgm. 
215   18 

No.  1 — Climbing  a 

306.18 

r  First  trial 

182.70 

248.34 

10     "        "        "       "    

253.74 

No.  2— Treadmill . 

12     "        "        "       "    

238.85 

14     "        "        "       "    

210.87 

15     "        "        "       "    

227.04 

21     "        "         "       "    

227.50 

^    2£  months  after  first  trial 

[  First  trial 

441.17 

231.24 

No.  3— Treadmill . 

.  . .  ]  2  days  after  first  trial 

231.24 

l4     "        "         "       "    

286.25 

In  walking  the  same  distance  (468  M.)  No.  1  excreted  the  following 
excess  of  carbon  dioxide  over  the  rest  value: 

First  trial 4 .  505  grams 

A  week  later 3.690      " 

A  month  later 2.780      " 

It  appears  from  these  results  that  the  gradual  strengthening  of  the 
muscles  during  convalescence  results  in  a  more  economical  per- 
formance of  their  work,  largely  independent  of  any  special  training 
for  a  particular  kind  of  work.  It  seems  a  justifiable  conclusion, 
therefore,  that  a  part  of  the  gain  due  to  training  arises  from  its 
direct  effect  in  strengthening  the  muscles,  as  well  as  from  the  in- 
creased skill  acquired  in  their  use.  Conversely,  the  effect  of  fatigue 
in  increasing  the  relative  metabolism,  as  shown  by  Loewy,*  would 
seem  to  be  in  part  a  direct  effect.  Schnyder  summarizes  the  matter 
in  the  statement  that  it  is  not  the  work  itself,  but  the  muscular 
effort  required,  which  determines  the  amount  of  metabolism. 

In  the  case  of  domestic  animals  kept  chiefly  for  work,  however, 
we  may  safely  assume  that  they  are  constantly  in  a  state  of  training, 
and  that  the  results  obtained  by  Zuntz  and  his  associates  on  the 
horse  are  applicable  to  work  done  by  normal  animals  within  the 
limits  of  the  experimental  conditions. 


Arch.  ges.  Physiol.,  49,  405. 


522  PRINCIPLES   OF  ANIMAL   NUTRITION. 

Relative  Values  of  Nutrients. — In  the  foregoing  discussion 
it  has  been  tacitly  assumed  that  the  stored-up  energy  of  the  pro- 
teids,  fats,  and  carbohydrates  of  the  body  is  all  net  available  energy, 
ready  to  be  utilized  directly  for  the  production  of  mechanical  work. 
As  we  have  seen,  however,  on  previous,  pages,  a  school  of  physiolo- 
gists, of  which  Chauveau  may  stand  as  the  representative,  denies 
this,  and  holds  that  the  fat  in  particular  must  be  converted  into 
a  carbohydrate  before  it  can  become  directly  available. 

In  discussing  the  source  of  muscular  energy  in  Chapter  VI  it 
was  shown  that  the  recorded  results  as  regards  the  nature  of  the 
material  metabolized  were  insufficient  to  decide  the  question,  since 
the  final  excretory  products  are  qualitatively  and  quantitatively 
the  same  whether  the  fat  is  directly  metabolized  in  the  muscle  or 
undergoes  a  preliminary  cleavage  in  the  liver  or  elsewhere  in  the 
body.  The  results  as  to  energy,  however,  would  be  materially 
different  in  the  two  cases.  The  dextrose  resulting  from  the  cleavage 
of  fat,  according  to  Chauveau's  schematic  equation  (p.  38),  would 
contain  but  about  64  per  cent,  of  the  potential  energy  of  the  fat,  the 
remainder  being  liberated  as  heat.  We  cannot,  however,  suppose 
that  the  energy  of  this  dextrose  can  be  utilized  by  the  muscle  any 
more  completely  than  that  of  dextrose  derived  directly  from  the 
food.  It  follows,  then,  that  the  percentage  utilization  of  the  total 
energy  metabolized  during  muscular  work  should  be  materially 
greater  when  the  metabolized  material  consists  largely  or  wholly  of 
carbohydrates  than  when  it  consists  chiefly  of  fat.  By  supplying 
food  consisting  largely  of  one  or  the  other  of  these  materials,  it  is 
possible  to  bring  about  these  conditions,  and  a  determination  of 
the  respiratory  exchange  and  the  nitrogen  excretion  will  then 
afford  a  check  upon  the  nature  of  the  material  metabolized  and  the 
means  of  computing  the  utilization  of  its  potential  energy. 

Investigations  of  this  sort  have  been  reported  from  Zuntz's 
laboratory.  The  earliest  of  these  were  by  Zuntz  &  Loeb  *  upon  a 
dog,  the  method  being  substantially  the  same  as  that  with  which 
the  preceding  pages  have  made  us  familiar.  Their  final  results  for 
the  energy  metabolized  per  kilogram  and  meter  traveled  (including 
the  work  of  ascent)  were: 

*  Arch.  f.  (Anat.  u.)  Physiol.,  1894,  p.  541. 


THE   UTILIZATION   OF  ENERGY.  523 


Diet. 


Respiratory- 
Quotient. 


Energy,  cals. 


Proteids  only 

Chiefly  fat 

"  "  (body  freed  from  carbohydrates  by 
phloridzin) 

Much  sugar  with  proteids 

"        "       and  httle  proteids 


0.78 
0.74 

0.71 
0.83 
0.88 


2.58 
2.43 

2.71 
2.58 
2.63 


The  differences  are  quite  small,  while,  as  Zuntz  points  out,  if 
2.6  cals.  represent  the  demand  for  energy  per  unit  of  work  when 
carbohydrates  are  the  source  it  should,  according  to  Chauveau's 
theory,  rise  to  about  3.68  cals.  when  the  energy  is  derived  exclu- 
sively from  fat. 

Altogether  similar  results  have  been  recently  reported  from 
Zuntz's  laboratory  by  Heineman,*  and  by  Frentzel  &  Reach,f  in 
experiments  on  man. 

In  Heineman's  experiments  the  work,  which  was  never  exces- 
sive, consisted  in  turning  an  ergostat,  the  respiratory  exchange 
being  determined  by  means  of  the  Zuntz  apparatus  and  the  total 
urinary  nitrogen  being  also  determined.  From  these  data,  reckon- 
ing 1  gram  of  urinary  nitrogen  equivalent  to  6.064  liters  of 
oxygen,|  the  average  amount  of  energy  metabolized  on  the  vari- 
ous diets,  and  the  proportion  derived  respectively  from  proteids, 
fats,  and  carbohydrates,  is  computed.  By  comparison  with  rest 
experiments  the  increments  of  oxygen  and  carbon  dioxide  due  to 
the  work  were  determined,  and  from  these  the  energy  consumed 
per  kilogram-meter  of  work  was  calculated  upon  three  different 
assumptions:  first,  that  the  proteid  metabolism  was  not  increased 
by  the  work;  second,  that  it  increased  proportionally  to  the  oxy- 
gen consumption;  third,  that  as  large  a  proportion  of  the  energy 
for  the  work  was  furnished  by  the  proteids  as  is  consistent  with 
the  observed  respiratory  exchange.  The  results  are  summarized 
in  the  following  table : 

*  Arch.  ges.  Physiol.,  83,  441. 

t  Ibid.,  83,  477. 

$  Zuntz,  Arch.  ges.  Physiol.,  68,  204. 


524 


PRINCIPLES   OF  ANIMAL    NUTRITION. 


Respira- 
tory 
Quo- 
tient. 

Total  Energy 
Supplied  by 

Energy  per  Kg 
of  Work. 

m. 

Predominant  Nutrient. 

Fat, 
Cals. 

Car- 
bohy- 
drates, 
Cals. 

Pro- 
teids, 
Cals. 

First 
As- 
sump- 
tion, 
cals. 

Second 

As- 
sump- 
tion, 
cals. 

Third 

As- 
sump- 
tion, 
cals. 

** it:: 

Carbohydrates.  ].'" 

As  much  proteids  as 
possible 

0.783 
0.724 
0.805 
0.901 

0.796 

3829 
4422 
3414 
1543 

3381 

1379 

246 

1823 

3374 

1620 

163 
163 
139 
139 

377 

10.98 
9.39 
11.15 
10.67 

11.40 

"9.35' 
•16!  63' 
11.27 

10.35 
9.27 
10.46 
10.37 

10.64 

The  subject  was  not  able  to  consume  even  approximately 
enough  proteids  to  supply  the  demands  for  energy,  so  that  the 
experiments  are  virtually  a  comparison  of  the  utilization  of  fat  and 
carbohydrates  in  different  proportions.  With  the  exception  of  the 
third  group,  the  results  seem  to  show  that  the  energy  of  the  fat 
metabolized  was  utilized,  if  anything,  rather  more  fully  than  that 
of  the  carbohydrates. 

Frentzel  &  Reach  experimented  upon  themselves,  the  work 
being  done  by  walking  in  a  tread-power;  otherwise  the  methods 
were  similar  to  those  of  Heineman.  In  computing  the  results  of 
the  experiments  on  a  carbohydrate  and  a  fat  diet  they  assume  that 
there  was  no  increase  in  the  proteid  metabolism  as  a  consequence 
of  the  work.  For  the  experiments  on  a  proteid  diet  they  com- 
pute the  results  both  on  this  assumption  and  also  on  the  assump- 
tion of  a  maximum  participation  of  the  proteids  in  work  produc- 
tion. Calculated  in  this  way  the  total  evolution  of  energy  per  kilo- 
gram weight  and  meter  traveled  was  as  given  in  the  table  on  p.  525. 
The  results  show  a  slight  advantage  on  the  side  of  the  carbo- 
hydrates, which  in  the  case  of  Frentzel  is  regarded  by  the  authors 
as  exceeding  the  errors  of  experiment.  They  compute,  however, 
that  it  is  far  too  small  to  afford  any  support  to  Chauveau's  theory. 

Zuntz  *  has  recalculated  Heineman's  results,  using  slightly 
different  data  but  reaching  substantially  the  same  result.  He 
shows,  however,  that  they  are  affected  by  the  influence  of  train- 
ing already  discussed  on  p.  519.  Arranging  the  experiments  in 
chronological  order,  it  becomes  evident  that  the  work  was  done 
*  Arch.  ges.  Physiol.,  83,  557. 


THE  UTILIZATION  OF  ENERGY. 


525 


FrentzeL — fat  diet: 

First  week 

Second  week. 


Average. 


Frentzel — carbohydrate  diet : 

First  week 

Second  week 


Average. 


Frentzel — proteid  diet: 

First  assumption  .  .  . 

Second  assumption. . 
Reach — fat  diet : 

First  week 

Second  week 


Average. 


Reach — carbohydrate  diet : 

First  week 

Second  week 


Average. 


Respiratory 
Quotient. 


0.766 
0.778 


0.773 


0.896 
0.880 


0.799 


0.805 
0.766 


0.781 


0.899 
0.901 


0.900 


Energy  per  Kg. 
and  Meter,  cals. 


2.088 
2.049 


2.066 


1.932 
2.031 


1.933 
1.824 


2.259 
2.034 


2.119 


2.202 
2.005 


2.086 


with  increasing  efficiency,  largely  independent  of  the  food,  and  the 
fact  that  most  of  the  experiments  with  fat  came  later  in  the  series 
than  those  with  carbohydrates  largely,  although  perhaps  not  en- 
tirely, accounts  for  the  observed  difference  in  efficiency,  while  the 
low  figure  for  proteids  is  accounted  for  by  the  fact  that  these  were 
among  the  earliest  experiments.  A  similar  effect  appears  in  the 
experiments  of  Frentzel  &  Reach,  although  it  is  less  marked,  since 
walking  is  a  more  accustomed  form  of  work  than  turning  a  crank. 
On  the  whole,  Zuntz  concludes  that  these  experiments  warrant  the 
conclusion  that  in  work  production  the  materials  metabolized  in 
the  body  replace  each  other  in  proportion  to  their  heats  of  combus- 
tion— that  is,  in  isodynamic  and  not  isoglycosic  proportions. 


THE   UTILIZATION    OF   METABOLIZABLE   ENERGY. 

ihe  investigations  just  discussed  give  us  fairly  full  data  as  to 
the  utilization  of  the  stored-up  energy  of  the  body  in  the  produc- 
tion of  external  work,  and  this,  as  we  have  seen  (p.  497),  is  sub- 
stantially equivalent  to  a  knowledge  of  the  utilization  of  the  net 


526  PRINCIPLES   OF  ANIMAL    NUTRITION. 

available  energy  of  the  food.  These  determinations  by  Zuntz  and 
his  co-workers,  however,  do  not  bring  the  energy  recovered  as 
mechanical  work  into  direct  relation  with  the  energy  of  the  food; 
that  is  to  say  (aside  from  such  computations  of  available  energy  as 
those  made  by  Zuntz  &  Hagemann*  for  the  food  of  the  horse), 
they  do  not  tell  us  how  much  of  the  energy  contained  in  a  given 
feeding-stuff  we  may  expect  to  recover  in  the  form  of  mechanical 
work,  but  only  what  proportion  of  the  stored-up  energy  resulting 
from  the  use  of  this  feeding-stuff  is  so  recoverable. 

It  is  the  former  question  rather  than  the  latter,  however,  which 
is  of  direct  and  immediate  interest  to  the  feeder  of  working  animals. 
The  feeding-stuffs  which  he  employs  are  comparable  to  the  fuel  of 
an  engine,  and  the  practical  question  is  how  much  of  the  energy 
which  he  pays  for  in  this  form  he  can  get  back  as  useful  work. 

Methods  of  Determination. — Two  general  methods  are  open 
for  the  determination  of  the  percentage  utilization  of  the  energy 
of  the  food. 

It  is  obvious  that  if  we  know  the  net  availability  of  the  energy 
(gross  or  metabolizable)  of  a  given  food  material  we  can  compute 
its  percentage  utilization  in  work  production  from  the  data  of  the 
foregoing  paragraphs  with  a  degree  of  accuracy  depending  upon 
that  of  the  factors  used.  For  example,  if  we  know  that  the  net 
available  energy  of  a  sample  of  oats  is  60  per  cent,  of  its  gross  energy, 
then  if  the  oats  are  fed  to  a  draft  horse  utilizing,  according  to  Zuntz 
&  Hagemann,  31.3  per  cent,  of  the  net  available  energy,  it  is  obvious 
that  the  utilization  of  the  gross  energy  of  the  oats  is  60x0.313  = 
18.78  per  cent.  An  entirely  similar  computation  could  of  course  be 
made  of  the  percentage  utilization  of  the  metabolizable  energy  of 
the  oats. 

Unfortunately,  however,  as  we  have  already  seen,  our  present 
knowledge  of  the  net  availability  of  the  energy  of  feeding-stuffs  and 
nutrients  for  different  classes  of  animals  is  extremely  defective,  and 
extensive  investigations  in  this  direction  are  an  essential  first  step 
in  the  determination  of  the  percentage  utilization  of  the  energy 
of  feeding-stuffs  in  work  production  by  this  method.  Until  trust- 
worthy data  of  this  sort  are  supplied,  results  like  those  of  Zuntz  & 
Hagemann  can  be  applied  to  practical  conditions  only  on  the  basis 
*  Landw.  Jahrb.,  27,  Supp.  Ill,  279  and  429. 


THE   UTILIZATION  OF  ENERGY.  527 

of  more  or  less  uncertain  estimates  and  assumptions  regarding  the 
expenditure  of  energy  in  digestion  and  assimilation  such  as  those 
discussed  in  Chapter  XI,  §  3. 

The  second  possible  general  method  for  the  determination  of 
the  percentage  utilization  of  the  energy  of  the  food  in  work  pro- 
duction is  that  employed  in  the  determination  of  the  utilization  in 
tissue  production.  Having  brought  the  animal  into  equilibrium  as 
regards  gain  or  loss  of  tissue  and  amount  of  work  done  with  a  suit- 
able basal  ration,  the  material  to  be  tested  is  added  and  the  work 
increased  until  equilibrium  is  again  reached.  The  increase  in  the 
work  performed  compared  with  the  energy  of  the  material  added 
would  then  give  the  percentage  utilization  of  the  latter. 

The  accurate  execution  of  this  method  would  require  the  em- 
ployment of  a  respiration  apparatus  or  a  respiration-calorimeter 
for  the  exact  determination  of  the  equilibrium  between  food  and 
work,  while  the  skill  of  the  experimenter  would  doubtless  be  taxed 
in  the  endeavor  to  so  adjust  food  and  work  as  to  secure  either  no 
gain  or  loss  of  tissue  or  equality  of  gain  or  loss  in  the  two  periods 
to  be  compared.  Indeed,  it  may  safely  be  said  that  exact  equality 
would,  as  a  matter  of  fact,  be  reached  rarely  and  by  accident,  and 
that  as  a  rule  it  would  be  necessary  to  correct  the  observed  results 
for  small  differences  in  this  respect.  To  make  such  corrections 
accurately,  however,  requires,  as  we  have  seen  in  §  1  of  this 
chapter,  a  knowledge  of  the  net  availability  and  percentage  utili- 
zation of  the  food,  and  we  are  thus  brought  back  to  the  necessity 
for  more  accurate  knowledge  upon  fundamental  points. 

The  extensive  investigations  of  Atwater  &  Benedict  *  upon  man 
appear  to  be  the  only  ones  yet  upon  record  in  which  the  actual 
balance  of  matter  and  energy  during  rest  has  been  quantitatively 
compared  with  that  during  the  performance  of  a  measured  amount  of 
work.  Unfortunately,  however,  the  gains  and  losses  of  energy  by 
the  bodies  of  the  subjects  in  these  experiments  were  relatively 
considerable,  while  the  experiments  thus  far  reported  seem  to 
afford  no  sufficient  data  for  computing  the  net  availability  of  the 
food  for  maintenance  or  its  percentage  utilization  for  the  production 
of  gain.     Moreover,  the  authors  appear  to  regard  the  measurements 

*  U.  S.  Department  Agr.,  Office  of  Experiment  Stations,  Bull.  109;  Mem- 
oirs Nat.  Acad.  Sci.,  8,  231. 


52  8  PRINCIPLES  OF  ANIMAL   NUTRITION. 

of  the  work  done  as  not  altogether  satisfactory.  In  a  preliminary 
paper*  Atwater  &  Rosa  compute  a  utilization  of  21  per  cent. 
Inasmuch  as  they  have  not  further  discussed  the  question  of  the 
utilization  of  the  food  energy  for  work  production  it  would  seem 
premature  to  attempt  to  do  so  here.  It  may  be  remarked,  however, 
that  the  figures  given  seem  to  indicate  a  rather  low  degree  of  effi- 
ciency for  the  particular  form  of  work  investigated  (riding  a  station- 
ary bicycle). 

Wolff's  Investigations. 

The  horse,  being  par  excellence  the  working  animal,  has  natu- 
rally been  the  subject  of  experiments  upon  the  relation  of  food  to 
work.  While  as  yet  the  respiration  apparatus  or  calorimeter  has 
not  been  applied  to  the  study  of  this  phase  of  the  subject,  two  ex- 
tensive and  important  series  of  investigations  have  been  made 
upon  the  work  horse,  viz.,  by  Wolff  and  his  associates  in  Hohen- 
heim  and  by  Grandeau,  LeClerc,  and  others  f  in  Paris,  in  which  the 
attempt  has  been  made  to  judge  approximately  of  the  equilibrium 
between  food  and  work  from  the  live  weight  and  the  urinary  nitro- 
gen. 

Grandeau 's  experiments  were  made  for  the  Compagnie  generate 
des  Voitures  in  Paris,  and  were  directed  specifically  toward  a  scientific 
investigation  of  the  rations  already  in  use  by  the  company  and  to 
a  study  of  the  most  suitable  rations  for  the  different  kinds  of  ser- 
vice required  of  the  horses.  They  were,  therefore,  while  executed 
with  the  greatest  care  and  exactness,  largely  "practical"  in  their 
aim. 

Wolff's  experiments  were  made  at  the  Experiment  Station  at 
Hohenheim  and  were  broader  in  their  scope,  being  directed  largely 
to  a  determination  of  the  ratio  of  (digested)  food  to  work.  The 
following  paragraphs  are  devoted  chiefly  to  an  outline  of  Wolff's 
experiments,  but  with  more  or  less  reference  also  to  Grandeau's 
results. 

Methods. — In  discussing  the  effects  of  muscular  exertion  on 
metabolism  in  Chapter  VI,  mention  was  made  of  the  interesting 

*  Phys.  Rev.,  9,  248;  TJ.  S.  Dept.  Agr.,  Office  of  Experiment  Station,  Bull. 
98,  p.  17. 

t  L'alimentation  du  Cheval  de  Trait,  Vols.  I,  II,  III,  and  IV,  and  Annales 
de  la  Science  Ajjronomique,  1892,  I,  p.  1;   1893,  I,  p.  1;  and  1896,  II,  p.  113 


THE   UTILIZATION   OF  ENERGY. 


529 


results  obtained  by  Kellner  regarding  the  influence  of  excessive 
work  upon  the  proteid  metabolism  of  the  horse.  It  was  there  shown 
that  when  the  work  was  increased  beyond  a  certain  amount  there 
resulted  a  prompt  increase  of  the  urinary  nitrogen  and  at  the  same 
time  a  steady  falling  off  in  the  live  weight.  The  method  employed 
in  Wolff's  experiments,  and  which  originated  with  Kellner,  is  based 
upon  this  fact.  It  may  perhaps  be  best  illustrated  by  one  of 
Kellner's  earliest  experiments,*  in  which  starch  was  added  to  a 
basal  ration,  the  results  of  which  have  already  been  referred  to  in 
Chapter  VI  (p.  199). 

In  the  first  period  the  daily  ration  consisted  of  6  kgs.  of  oats 
and  6  kgs.  of  hay,  while  in  the  second  period  1  kg.  of  rice  starch  was 
added.  Digestion  trials  showed  that  there  was  digested  from  these 
rations  the  following: 


Period  I, 
Grms. 

Period  II, 
Grms. 

Increase. 
Grms. 

757.07 

636.10 

3874.36 

279.45 

750.53 
713.40 

4488.15 
275.43 

-     6.54 

+   77.30 

+613.79 

-     4.02 

5546.98 

6227.51 

+  680.53 

The  work  was  performed  in  a  special  sweep-power  which  was 
so  constructed  as  to  act  as  a  dynamometer.  With  a  uniform  draft 
of  76  kgs.,  the  daily  work  in  the  four  subdivisions  of  the  first 
period  consisted  of  300,  600,  500,  and  400  revolutions  respectively, 
while  in  the  two  subdivisions  of  the  second  period  it  was  800  and 
600  respectively.  From  the  daily  results  for  live  weight  and  urinary 
nitrogen  and  from  a  comparison  with  another  period  in  which  1.5 
kgs.  of  starch  was  fed,  Kellner  concludes  that  the  maximum  amounts 
of  work  which  the  animal  could  perform  vdthout  causing  an  increase 
in  its  proteid  metabolism  and  a  decrease  in  its  live  weight  were  for 
the  first  period  500  revolutions  and  for  the  second  period  700  revo- 
lutions. The  difference  of  200  revolutions,  then,  represents  the 
additional  work  derived  from  the  added  starch.  Two  hundred 
revolutions  with  a  draft  of  76  kgs.  equaled  438,712  kgm.,  to  which 
is  to  be  added  the  work  of  locomotion,  estimated  by  Kellner  (com- 
*  Landw.  Jahrb.,  9,  670. 


53°  PRINCIPLES   OF  ANIMAL   NUTRITION. 

pare  p.  539)  at  100,000  kgm.,  making  the  total  additional  work 
538,712  kgm.  Kellner  compares  this  difference  with  the  increased 
amount  of  nitrogen-free  extract  digested,  613.79  grams,  neglecting 
the  small  differences  in  the  other  nutrients.  As  corrected  in  a  later 
publication,*  the  results  are  as  follows: 

613.79  grms.  starch =2527. 601  Cals.  =  1,071,698  kgm. 
538,712-*- 1,071,698  =  50.27  per  cent. 

If  we  base  the  calculation  upon  the  difference  in  total  organic 
matter  digested,  the  percentage  will  of  course  be  somewhat  smaller. 

It  was  discovered  later  that  the  indications  of  the  dynamometer 
used  in  these  experiments  and  many  subsequent  ones  were  untrust- 
worthy, so  that  no  value  attaches  to  the  percentage  computed  above, 
but  it  serves  just  as  well  to  illustrate  the  method  employed,  and 
which  was  followed  in  the  whole  series  of  experiments.  In  brief, 
the  attempt  is  to  find  in  the  indications  of  live  weight  and  urinary 
nitrogen  a  partial  substitute  for  the  determination  of  the  respira- 
tory products.  As  Kellner  and  Wolff  do  not  fail  to  point  out,  the 
results  are  but  approximations,  and  in  any  single  experiment  may 
vary  considerably  from  the  truth,  but  on  the  average  of  a  large 
number  of  experiments  it  was  hoped  that  satisfactory  results  might 
be  reached.  In  later  experiments  rather  more  importance  seems 
to  be  attached  to  the  effects  upon  live  weight  than  to  those  upon 
urinary  nitrogen,  but  it  should  be  noted  that  the  live  weight  showed 
remarkably  small  variations  from  day  to  day,  under  the  carefully 
regulated  conditions  of  the  experiments,  and  was  quite  sensitive 
to  changes  in  the  amount  of  work  done. 

The  experiments  may  be  conveniently  divided  into  three  groups. 
The  first  of  these  f  includes  the  years  1877  to  1886,  inclusive,  in  which 
the  work  done  was  compared  with  the  total  digested  food.  The 
second  %  covers  the  experiments  of  1886-1891,  in  which  the  digested 
crude  fiber  was  omitted  in  computing  the  work-equivalent  of  the 
food,  while  the  third  group  §  includes  the  experiments  of  1891-1894 
with  a  new  and  more  accurate  form  of  dynamometer. 

*  Wolff,  Grundlagen,  etc.,  p.  89. 

t  Grundlagen  fur  die  rationelle  Fattening  desPferdes,  1886,  66-155;  Neue 
Beitrage,  Landw.  Jahrb.,  16,  Supp.  Ill,  1-48. 

%  Landw.  Jahrb.,  16,  Supp.  Ill,  49-131,  and  24,  125-192. 
§  Ibid.,  24,  193-271. 


THE   UTILIZATION   OF  ENERGY. 


531 


Experiments  of  1877-1886. — During  the  years  named,  in  addi- 
tion to  the  preliminary  investigations  necessary  in  working  out  the 
method,  a  large  number  of  experiments  were  made  on  three  different 
animals.  The  rations  consisted  largely  of  hay  and  oats  in  some- 
what varied  proportions,  together  with  smaller  amounts  of  other 
feeding-stuffs.  In  three  experiments  on  starch  and  four  on  oats  a 
comparison  of  the  increase  in  digested  nutrients  *  with  the  in- 
creased work  which  could  be  done  gave  the  following  results :  f 


Increase  in  Digested] 

Nutrients, 

Grms. 


Increase  in  Work  Done   Nutrients  Equivalent 
76  Kg.  Draft,  to  100  Revolutions, 

Revolutions.  Grms. 


Starch. 
Oats  .. 


677.3 
577.0 


217 
175 


Average. 


312 
318 


315 


The  Maintenance  Requirement. — As  already  stated,  it  was 
discovered  later  that  the  dynamometer  used  was  unreliable  and 
gave  too  high  readings,  so  that  the  above  result  cannot  be  em- 
ployed to  compute  the  utilization  of  the  energy  of  the  added  food. 
It  does,  however,  in  its  present  form,  enable  us  to  compute  the 
maintenance  requirements  of  the  horse  by  subtracting  from  the 
total  digested  food  the  nutrients  equivalent  to  the  work  performed 
(i.e.,  3.15  gramsXthe  number  of  revolutions).  The  results  of  such 
a  computation  made  by  Wolff  J  are  given  on  p.  532. 

The  actual  live  weights  in  these  experiments  were  somewhat 
below  the  normal  weights,  which  were  regarded  as  being  about 
533  kgs.  for  No.  I,  500  kgs.  for  No.  II,  and  475  kgs.  for  No.  III. 
Wolff  considers  the  maintenance  requirements  to  be  independent 
of  minor  changes  in  weight,  and  on  the  basis  of  the  above  "normal " 
weights  computes  the  maintenance  requirements  per  500  kgs.  live 
weight  as  follows: 

Horse  1 4143  grams 

"      II 4260      " 

"      III 4167      " 


Average 4190      " 

*  The  algebraic  sum  of  the  differences  in  the  single  nutrients  is  used,  and 
in  this  and  the  succeeding  comparisons  the  digested  fat  is  multiplied  by  2.44. 
f  hoc.  cit.,  pp.  125-129. 
J  hoc  cit.,  pp.  99  and  132. 


532 


PRINCIPLES    OF  ANIMAL   NUTRITION. 


No.  of 
Experi- 
ments. 

Total 

Nutrients, 

Grms. 

Nutritive 
Ratio. 

Live 

Weight, 

Kgs. 

No.  of 
Revolu- 
tions. 

Equiva- 
lent 
Nutrients, 
Grms. 

For 
Mainte- 
nance, 
Grms. 

Horse  I 

Horse  II: 

1881-82 

1882-83 

1883-84.... 

4 

7 
4 
6 

6305.6 

5831 . 1 
6748.3 
5920.2 

1:5.79 

1:6.64 
1:6.37 
1:7.26 

521 

477 
486 
457 

600 

546 

662 
567 

1890 

1720 

2085 
1786 

4416 

4111 
4663 
U34 

Average.. . 

Horse  III: 
1881-82.... 

1882-83 

1883-84.... 
1885 

17 

6 
6 
5 
4 

6078.4 

5313.8 
6061.3 
5*734.8 
5761.2 

1:6.80 

1:7.16 
1:6.88 
1:7.55 
1:7.57 

473 

454 
469 
473 
473 

577 

404 
683 
580 
575 

1818 

1273 
2152 
1827 
1811 

4260 

4041 
3909 
3908 
3.50 

Average... 

21 

5717.8 

1:7.29 

467 

5L1 

1766 

3952 

By  means  of  a  comparison  of  the  results  by  groups*  Wolff 
shows  that  the  maintenance  requirement  as  thus  computed  is  appar- 
ently independent  of  the  amount  of  work  done  and  of  the  nutritive 
ratio,  and  from  this  uniformity  concludes  that  the  relative  efficiency 
of  the  food  for  work  production  is  unaffected  by  these  factors, 
within  the  range  of  his  experiments. 

A  series  of  similar  experiments  on  Horse  No.  Ill  in  1885-86,t 
computed  in  substantially  the  same  way,  gave  results  for  the  main- 
tenance ration  agreeing  well  with  those  of  earlier  years,  viz., 

Period  1 3934  grams  total  nutrients 

"      II 3984      " 

"      III  and  V 4001      " 

"      VII6 4094      " 

"      VIII 4094      " 

Average 4021      " 

with  an  average  live  weight  of  475  kgs.,  equivalent  to  4230  grams 
per  500  kgs.  In  a  succeeding  period  (IX),  however,  in  which  hay 
alone  was  fed,  a  decidedly  higher  result  was  obtained,  viz.,  4357 
grams  per  head,  or  4586  grams  per  500  kgs. 

*  Loc.  cit.,  pp.  135  and  137. 

t  Landw.  Jahrb.,  16,  Supp.  Ill,  32. 


THE   UTILIZATION  OF  ENERGY. 


533 


Experiments  of  1886-91. — In  the  experiments  thus  far  de- 
scribed, with  the  exception  of  the  last,  the  proportions  of  grain  and 
coarse  fodder  in  the  rations  were  not  widely  different,  the  latter 
furnishing  on  the  average  fully  one  half  of  the  dry  matter  fed. 
Consequently  the  experiments  were  not  calculated  to  bring  out  any 
difference  in  the  nutritive  value  of  the  two  such  as  is  indicated  by 
the  results  of  the  one  trial  with  hay  alone. 

Grain  vs.  Coarse  Fodder  for  Maintenance. — The  results 

obtained  by  Grandeau  &  LeClerc  upon  the  maintenance  ration  of 

the  horse  when  fed  a  mixture  containing  about  75  per  cent,  of 

grain   fully  confirm  the   indications  of   Wolff's   trial    with   hay. 

Their  experiments   have  been  very  fully  discussed,  and  in   part 

recalculated,  by  Wolff  *  in  their  bearing  on  this  question.     The 

three  horses  experimented  on  were  fed  two  different  amounts  of 

the  same  mixture  in  several  different  thirty-day  periods,  eighteen 

such  periods  in  all  being  available  for  comparison.     In  all  of  them 

the  animals  were  led  daily,  at  a  walk,  over  a  distance  of  about  four 

kilometers.     Wolff  estimates  the  amount  of  work  of  locomotion  by 

1  (W\ 
means  of  the  formula  ^-( — )i>2  and  by  subtracting  the  equivalent 

amount  of  nutrients  from  the  total  digested  obtains  the  amount  re- 
quired for  maintenance.     The  results  are  as  follows: 


No.  of 
Experi- 
ments. 

Live 

Weight. 
Kgs. 

Digested 

Nutrients, 

Grms. 

Nutrients 
Equiva- 
lent to 
Work, 
Grms. 

For  Maintenance. 

Per 
Head, 
Grms. 

Per 

500  Kgs., 

Grms. 

Heavier  Ration : 

Horse  I 

"       II 

"       III 

3 
5 
4 

416.6 
405.9 
439.0 

3553 
3432 
3625 

110 
108 
119 

3443 
3324 
3506 

4132 
4078 
3994 

420.5 

411.0 
441.2 

3537 

3060 
3310 

112 

108 
119 

3425 

2952 
3191 

4068 

Lighter  Ration  : 

Horse  II 

"       III 

2 

4 

3636 
3617 

426.1 

3185 

114 

3071 

3626 

The  results,  and  particularly  those  on  the  lighter  ration,  which 
appeared  ample  for  maintenance,  are  much  lower  than  those  com- 
*  Landw.  Jahrb.,  16,  Supp.  Ill,  73-81. 


534 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


puted  in  the  previous  paragraph.  The  difference  is  too  great  to  be 
ascribed  to  experimental  errors  in  estimating  the  small  amount  of 
work  done,  and  can  most  reasonably  be  ascribed  to  the  difference 
in  the  character  of  the  ration.  Apparently  the  horse,  like  cattle 
(p.  433),  requires  less  digestible  food  for  maintenance  when  the 
latter  consists  largely  of  grain  than  when  it  is  chiefly  or  wholly  coarse 

fodder. 

Direct  experiments  by  Wolff  *  likewise  show  that  the  digestible 
nutrients  of  concentrated  feed  (oats)  are  more  valuable  for  work 
production  than  those  of  coarse  feed  (hay).  The  experiments 
were  made  in  the  manner  already  described,  the  draft  being  uni- 
formly 60  kgs.  Although  the  measurements  of  the  work  actually 
done  are  probably  incorrect,  it  may  be  assumed  to  have  been 
substantially  proportional  to  the  number  of  revolutions  of  the 
dynamometer.  A  ration  of  3  kgs.  of  hay  and  5.5  kgs.  of  oats  served 
as  the  basal  ration,  to  which  was  added  in  one  case  4  kgs.  of  hay 
and  in  another  1£  kgs.  of  oats.  The  nutrients  digested  in  each 
case  and  the  equivalent  amount  of  work  secured  were : 


Ration. 

Digested. 

1  « 

i 

Protein, 
Grms. 

Crude 
Fiber, 
Grms. 

Nitrogen- 
free 
Extract, 
Grms. 

Ether 
Ex- 
tract, 
Grms. 

Total 
(Fat   X 
2.4), 
Grms. 

>  ^ 

i-m 

v.... 

7  kgs.  hay,  5.5  kgs.  oats 
3    "       "      5.5    "        '• 

822 .  58 
626.46 

816.68 
422 . 74 

3889.64 
3068.46 

186.72 
184.78 

5973.62 
4561.13 

750 
350 

196.12 

393.94 

821.18 

1.94 

1412.49 
353.12 

5434.21 
4561.13 

400 

VI... 

v.... 

3  kgs.  hay,  7  kgs.  oats.. . 
3    "       "     5.5  " 

1.5  kgs.  oats  . 

754.52 
626.46 

355.24 
393.94 

3719.24 
3068.46 

252.17 

184. 78 

700 
350 

128.06 

-67.50 

650.78 

67.39 

873.08 
249.45 

350 

The  relative  value  of  the  digested  matter  of  hay  and  of  oats  for  work 
production  in  these  trials  was  thus  approximately  as  5  :  7. 

In  the  earlier  experiments  (p.  531)  it  was  found  that  when  oats 
or  starch  were  added  to  a  basal  ration,  approximately  315  grams 
of  digested  nutrients  were  required  to  produce  the  amount  of 
work  represented  by  100  revolutions  at  76  kgs.  draft.  Converting 
this  result  and  the  one  just  given  for  oats  into  kilogram-meters, 
Wolff  computes  that  100  grams  of  digested  nutrients  was  equivalent, 
*  Loc.  cit.,  pp.  84-95. 


THE   UTILIZATION  OF  ENERGY. 


535 


in  round  numbers,  to  85,400  kilogram-meters  in  the  earlier  experi- 
ments and  to  90,480  in  the  one  just  cited.  While  these  figures  are 
not  correct  absolutely,  they  are  probably  comparable,  being  ob- 
tained with  the  same  apparatus.  In  the  later  experiment  the 
work  of  locomotion  is  computed  by  Wolff's  formula,  which  gives 
higher  results  than  Kellner's.  Taking  this  into  account  we  may 
regard  the  agreement  of  the  two  equivalents  as  satisfactory. 

Value  of  Crude  Fiber. — In  all  the  experiments  with  con- 
centrated feeds  the  additional  nutrients  digested  from  the  added 
food  contained  no  crude  fiber,  the  apparent  difference,  indeed, 
being  in  most  cases,  as  in  the  above  experiment,  negative.  When 
hay  was  added,  on  the  other  hand,  over  one  fourth  of  the  addi- 
tional nutrients  digested  consisted  of  crude  fiber.  If,  now,  we 
neglect  this  crude  fiber  and  compare  the  work  and  the  fiber-free 
nutrients  we  have  1018.55-^4.00  —  254.64  grams  of  fiber-free  nutri- 
ents per  100  revolutions,  or  a  figure  corresponding  almost  exactly 
with  that  obtained  for  the  fiber-free  nutrients  added  in  oats  or 
starch.  In  other  words,  it  would  appear  from  this  result  that  the 
digested  crude  fiber  of  hay  is  as  valueless  for  work  production  as  it 
appears  to  be  for  maintenance. 

If,  however,  the  crude  fiber  is  valueless  both  for  maintenance 
and  work,  then  by  omitting  it  altogether  from  our  computations  we 
ought  to  get  results  for  the  maintenance  ration  and  for  the  ratio  of 
nutrients  to  work  which  are  independent  of  the  proportion  of  grain 
to  coarse  fodder  in  the  ration.  Confirmatory  evidence  of  this  sort 
is  abundantly  furnished  by  Wolff's  experiments  and  likewise  by 
the  results  of  Grandeau  on  maintenance.  Taking  first  the  averages 
of  the  experiments  of  1877-1886  (p.  532)  we  have — 


Nutrients  Digested. 

No.  of 
Revolu- 
tions atr 
76  Kgs. 

Equiva- 
lent 
Nutrients, 
Grms. 

Fiber-free  Nutrients 
for  Maintenance. 

Total, 
Grms. 

Crude 
Fiber, 
Grms. 

Without 
Crude 
Fiber, 
Grms. 

Per  Head, 
Grms. 

Per  500 
Kgs.. 
Grms. 

Horse  I 

"       II  ...  . 
"       III  .  .  . 

6306 
6078 
5718 

815 
978 
809 

5491 
5100 
4909 

600 
577 
561 

1890 
1818 
1766 

3601 

3282 

143 

3378 
3282 
3306 

Average  . .  . 

3322 

536 


PRINCIPLES  OF  AN  I M  ML   NUTRITION. 


The  results  of  the  series  made  in  1885-86  on  Horse  No.  Ill 
(p.  532),  computed  in  the  same  way,  give  the  following  as  the 
amounts  of  fiber-free  nutrients  required  for  maintenance : 


Per  Head. 
Grms. 

Per  500  Kgs. 

Live  Weight, 

Grms. 

3270 
3186 
3242 
3342 
3316 
3170 

3442 
3353 
3413 
3549 
3490 
3335 

"     II 

"      III  and  V 

"      VII 

"      VIII 

"      IX 

3254 

3430 

From  Grandeau's  experiments  (p.  533),  by  the   same  method,  we 
have  for  the  lighter  ration  the  following: 


Per  Head, 
Grms. 

Per  500  Kgs. 

Live  Weight, 

Grms. 

2732 
2935 

3324 
3328 

"    III 

3326 

Finally,  for  the  series  of  experiments  by  Wolff,  just  discussed, 
upon  the  relative  value  of  the  digested  matter  of  oats  and  of  hay, 
and  from  which  the  conclusion  as  to  the  lack  of  value  of  the  crude 
fiber  was  drawn,  by  computing  backwards,  we  get  figures  for  the 
fiber-free  nutrients  required  for  maintenance  which  not  only  agree 
with  each  other,  as  they  necessarily  must,  but  also  with  those  of 
the  earlier  experiments.    The  results  are: 


Per  Head,  Grms. 

Per  500  Kgs   Live 
Weight    Grms. 

Period  I-III 

3175 
3275 
3180 
3196 

3342 
3429 
3329 
3364 

rv 

V 

"        VI... 

3366 

THE   UTILIZATION  OF  ENERGY.  537 

Wolff's  conclusions  from  these  results  *  are — 

1.  The  digested  crude  fiber  is  apparently  valueless,  both  for 
maintenance  and  for  work  production. 

2.  The  remaining  nutrients  may  be  regarded  as  of  equal  value 
whether  derived  from  grain  or  coarse  fodder. 

3.  The  maintenance  of  a  500-kg.  horse  requires  approximately 
3350  grams  per  day  of  fiber-free  nutrients. 

Wolff's  subsequent  experiments  up  to  1891  f  gave  results  con- 
firmatory in  general  of  the  above  conclusions.  Particularly  was 
this  the  case  when  the  work  of  locomotion  was  computed  by  Kell- 

1  /W\ 

ner's  formula  and  not  by  the  formula  —  ( —  )  vi.     The  work  done 

2  \  flr  / 

(expressed  in  number  of  revolutions  of  the  dynamometer)  per  100 
grams  of  fiber-free  nutrients  was  reasonably  uniform  and  agreed 
well  with  the  results  previously  obtained,  while  the  fiber-free 
nutrients  required  for  maintenance  likewise  agreed  with  the  results 
given  above.  On  the  other  hand,  the  inclusion  of  the  digested 
crude  fiber  in  the  computations  gave  in  many  cases  strikingly 
discordant  results.  In  view  of  the  unreliability  of  the  measurement 
of  the  work  no  conclusions  can  be  drawn  as  to  the  percentage 
utilization  of  the  energy  of  the  food,  and  it  seems  unnecessary  to 
describe  the  individual  experiments. 

A  discussion  by  Wolff  %  of  the  results  of  some  of  the  experi- 
ments by  Grandeau  in  which  work  was  done,  although  rendered 
uncertain  by  the  difficulty  in  estimating  the  work  of  locomotion  at 
varying  velocities,  and  by  the  changes  in  live  weight  of  the  animals, 
seems  to  indicate  that  they  also  confirm  Wolff's  conclusions. 

Significance  of  the  Results. — In  drawing  his  conclusions 
Wolff  is  careful  to  say  that  the  digested  crude  fiber  is  apparently 
valueless,  and  while  calling  attention  to  Tappeiner's  then  recent 
results  on  the  fermentation  of  cellulose  in  the  digestive  tract  as 
probably  explaining  its  low  nutritive  value  he  points  out  that 
other  ingredients  of  the  food  may  also  undergo  fermentation.  He 
therefore  holds  fast  to  the  fact  actually  observed,  viz.,  the  lower 
nutritive  value  of  the  digested  matter  of  coarse  fodder  compared 

*  Loc.  cit.,  p.  95. 

t  Landw,  Jahrb.,  24,  125-192. 

t  Ibid.,  16,  Supp.  Ill,  110-126. 


538  PRINCIPLES   OF  ANIMAL   NUTRITION. 

with  that  of  grain,  and  virtually  regards  the  amount  of  crude 
fiber  as  furnishing  a  convenient  empirical  measure  of  the  difference. 

In  the  light  of  our  present  knowledge  this  reserve  seems  amply 
justified.  The  difference  in  the  value  of  coarse  fodder  and  grain 
we  should  now  regard  as  arising  largely  from  the  difference  in  the 
amounts  of  energy  consumed  in  digestion  and  assimilation.  Kell- 
ner's  experiments  on  extracted  straw  discussed  in  the  previous 
section  have  shown,  however,  that  with  cattle  this  difference 
is  by  no  means  determined  by  the  simple  presence  of  more  or  less 
crude  fiber,  but  is  related  rather  to  the  physical  properties  of  the 
feeding-stuff,  while  Zuntz  (see  p.  392)  has  shown  that  the  same 
factor  largely  affects  the  work  of  mastication  in  the  horse.  That 
the  nutritive  value  of  the  rations  in  Wolff's  experiments  was  pro- 
portional to  the  amount  of  fiber-free  nutrients  which  they  contained, 
or,  in  other  words,  that  the  energy  expended  in  digestion,  etc.,  was 
proportional  to  the  digested  crude  fiber,  is  explained  by  the  limited 
variety  of  feeding-stuffs  employed.  The  coarse  fodder  was  meadow 
hay  with,  in  some  cases,  an  addition  (usually  relatively  small)  of 
straw,  while  the  grain  was  commonly  oats,  part  of  which  was  in  some 
instances  replaced  by  maize,  beans,  barley,  flaxseed,  or  oil-meal, 
while  starch  was  added  to  the  ration  in  a  number  of  trials.  The 
larger  part  of  the  work  of  digestion,  under  these  circumstances,  was 
probably  caused  by  the  coarse  fodders,  viz.,  hay  and  straw,  while  the 
digested  crude  fiber  was  likewise  derived  chiefly  or  entirely  from 
these  substances.  Such  being  the  case,  it  follows  that  the  loss 
of  energy  through  digestive  work  would  be  in  general  proportional 
to  the  amount  of  crude  fiber  in  the  ration.  The  essential  point  in 
Wolff's  experiments  is  that  the  omission  of  crude  fiber  renders  the 
results  concordant,  and  this  is  as  well  explained  in  the  manner  just 
indicated  as  by  the  estimate  of  Zuntz  &  Hagemann  that  the  work 
of  digesting  and  assimilating  crude  fiber  consumes  the  equivalent 
of  its  metabolizable  energy. 

Experiments  of  1891-94. — In  the  dynamometer  employed  by 
Wolff  the  resistance  was  produced  by  the  friction  of  metallic  sur- 
faces. A  copy  of  his  dynamometer  was  employed  by  Grandeau  & 
LeClorc  in  their  investigations  at  Paris,  and  these  experimenters 
found*  that  the  measurement  of  the  work  was  subject  to  large  errors, 
*  Fourth  Memoir,  p.  49. 


THE  UTILIZATION  OF  ENERGY.  539 

particularly  in  experiments  at  a  trot,  owing  to  the  continual  changes 
in  the  friction.  Wolff  believes  that  in  his  experiments,  all  made 
at  a  rather  slow  walk,  the  errors  are  less,  but  admits  that  they  are 
sufficient  to  deprive  his  computations  of  utilization  of  all  val  e. 

Grandeau  &  LeClerc,  however,  were  successful  in  improving 
the  dynamometer,  by  the  addition  of  an  integrating  apparatus,*  so 
that  its  measurements  of  the  total  work  were  satisfactory,  and  this 
apparatus  was  added  to  Wolff's  dynamometer  in  1891.  Before  that 
date,  therefore,  Wolff's  experiments,  while  of  great  value  in  many 
other  respects,  afford  no  trustworthy  direct  data  as  to  the  utili- 
zation of  the  energy  of  the  food  for  work  production,  although,  as 
we  have  just  seen,  they  afford  some  information  on  subsidiary 
points.  From  1891,  however,  we  may  regard  the  measurements  of 
the  work  done  on  the  dynamometer  as  reasonably  accurate. 

Corrections.  —  Unfortunately,  in  the  light  of  subsequent 
investigation,  the  same  is  not  true  of  some  of  the  other  factors 
entering  into  the  comparison,  particularly  the  work  of  locomotion 
and  the  metabolizable  energy  of  the  food. 

In  all  his  later  experiments  Wolff  computes  the  work  of  hori- 

1  /W\ 
zontal  locomotion  per  second  by  means  of  the  formula  -  I  —  jv2, 

in  which  W  equals  the  weight  of  the  animal,  g  the  force  of  gravity, 
and  v  the  velocity  per  second.  Zuntz's  experiments,  however, 
appear  to  show  that  this  formula  gives  too  high  results,  the  error 
increasing  with  the  velocity,  and  Wolff  t  himself  recognizes  the  truth 
of  this  for  higher  speeds.  According  to  Zuntz's  determinations 
(p.  512),  Kellner's  method  of  computation  gives  results  agree- 
ing quite  closely  with  those  computed  from  his  respiration  experi- 
ments. Under  the  conditions  of  Wolff's  experiments  this  corre- 
sponds quite  closely  to  50,000  kgm.  per  100  revolutions  of  the 
dynamometer,  and  in  the  comparisons  which  follow  this  amount 
has  been  substituted  for  that  computed  by  Wolff,  thus  reducing 
materially  the  figures  for  the  total  work  performed. 

Wolff  estimates  the  metabolizable  energy  of  the  food,  on  the 
basis  of  Rubner's  results,  by  multiplying  the  digested  fat  by  2.4, 
adding  the  remaining  digested  nutrients,  and  reckoning  the  total 

*  Ann.  Sci.  Agron.,  1881,  I,  464. 

t  Landw.  Jahrb.,  16,  Supp.  Ill,  119. 


54° 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


at  4.1  Cals.  per  gram.  As  we  have  seen,  however  (Chapter  X), 
this  figure  is  probably  too  high  for  herbivora,  although  exact  figures 
for  the  horse  are  not  yet  fully  available.  Approximately,  however, 
we  may  estimate  the  metabolizable  energy  of  the  several  digested 
nutrients  as  follows  (p.  332): 

Protein 3.228  Cals  per  gram 

Crude  fiber 3.523     "     "       " 

Nitrogen-free  extract 4.185     "     "       " 

Etherextract 8.572     "     "       " 

Zuntz  *  estimates  the  metabolizable  energy  of  the  total  nutri- 
ents (including  fat  X  2.4)  at  3.96  Cals.  per  gram.  This  figure  is 
probably  somewhat  high,  especially  for  rations  containing  much 
crude  fiber  or  ether  extract,  but  may  serve  the  purpose  of  approxi- 
mate calculations. 

Experiments  on  Single  Feeding-stuffs. — Comparatively  few 
of  the  experiments  admit  of  a  direct  computation  of  the  utiliza- 
tion for  a  single  feeding-stuff,  since  in  most  cases  the  amounts  of 
two  or  more  feeding-stuffs  were  varied  simultaneously.  As  an 
example  of  the  former  class  we  may  take  Periods  I  and  II  of  the 
experiments  of  1892-93.  In  Period  I  the  ration  consisted  of  7.5  kgs. 
of  hay  and  4  kgs.  of  oats  per  day,  while  in  Period  II  the  oats  were 
increased  to  5.5  kgs.  The  quantities  of  nutrients  digested  and  the 
metabolizable  energy  of  the  difference  between  the  two  rations 
(computed  by  the  use  of  the  factors  just  given)  were — 


Protein, 
Grms. 

Crude 
Fiber. 
Grms. 

Nitrogen- 
free 
Extract, 
Grms. 

Ether 
Extract, 
Grms. 

Total 

Nutrients, 

Grms. 

Period  II 

"      I 

1022.4 

847.8 

849.6 
819.9 

4152.8 
3598.4 

175.8 
137.1 

6446.6 
5595.3 

Difference  .... 
Equiv.  energy .  .  . 

174.6 
Cals. 
564 

29.7 
Cals. 
105 

554.4 

Cals. 

2320 

38.7 

Cals. 
332 

851.3 

Cals. 

3321 

In  Period  I  (20  days)  the  daily  work  consisted  of  300  revolutions 
of  the  dynamometer.     With  this  amount  of  work  the  live  weight 
of  the  horse  underwent  very  little  change,  but  there  was  a  material 
*  Landw.  Jahrb.,  27,  Supp.  Ill,  418. 


THE   UTILIZATION  OF  ENERGY. 


541 


gain  of  nitrogen,  so  that  Wolff  estimates  that  the  work  might  have 
been  increased  to  350  revolutions.  In  Period  II  (23  days)  the 
daily  work  was  increased  to  450  revolutions  and  the  same  behavior 
was  observed,  while  a  further  increase  to  500  revolutions  during 
the  last  ten  days  checked  the  gain  of  nitrogen  without  causing  a 
decrease  in  live  weight.  Taking  350  and  500  revolutions  respec- 
tively as  representing  the  maximum  amount  of  work  that  could  be 
done  on  the  two  rations,  the  equivalent  of  the  oats  added  may  be 
computed  as  follows: 


Revolutions. 

Equivalent 
Work, 
Kgm. 

Period  II 

500 
350 

1,030,687 

"      I...                      

722,678 

Difference 

150 

308,009 
75,000 

383,009 

903  Cals. 

The  percentage  utilization  was  therefore  903^-3321  =  27.2  per  cent. 

The  above  figures  serve  to  exemplify  the  general  method  of 
computation  and  likewise  to  illustrate  the  weak  points  in  Wolff's 
experiments,  viz.,  the  uncertainty  in  the  determination  of  the  work 
of  locomotion  and  the  impossibility  of  demonstrating  the  equilib- 
rium of  food  and  work  without  the  use  of  the  respiration  apparatus 
or  calorimeter. 

Out  of  the  whole  number  of  experiments  between  1891  and  1894, 
seven  admit  of  a  comparison  of  this  sort,  viz.,  four  on  oats,  two  on 
straw,  and  one  on  beans.  Upon  making  the  computations,  how- 
ever, the  results  are  found  to  be  so  exceedingly  variable  (the  range 
for  oats,  e.g.,  being  from  16.89  to  63.96  per  cent.)  as  to  demonstrate 
that  the  data  of  Wolff's  experiments  are  not  sufficiently  exact  to  be 
used  in  this  way,  and  that  the  apparently  reasonable  result  just 
computed  is  purely  accidental. 

Utilization  of  Fiber-free  Nutrients. — But  although  Wolff's 
results  do  not  enable  us  to  compute  the  percentage  utilization  of 
single  feeding-stuffs,  if  we  accept  provisionally  his  conclusions  re- 
garding the  non-availability  of  the  crude  fiber  they  afford  data  for 
numerous  computations  of  the  utilization  of  the  fiber-free  nutrients, 


542 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


and  these  computations  in  turn  supply  a  check  upon  the  hypoth- 
esis of  the  non-availability  of  crude  fiber. 

Wolff  makes  the  comparison  by  deducting  from  the  total  fiber- 
free  nutrients  3300  grams  per  500  kgs.  live  weight  for  maintenance 
and  comparing  the  energy  of  the  remainder  with  the  amount  of 
work  done.  In  the  following  tabulation  of  his  results  this  method 
has  been  pursued.  For  the  energy  of  the  fiber-free  nutrients, 
Zuntz's  figure  (3.96  Cals.  per  gram)  has  been  used  and  the  work  of 
locomotion  has  been  estimated  at  50,000  kgm.  per  100  revolutions 
of  the  dynamometer  (compare  p.  539). 


la-d. 

lib.. 
in.. 

IV.. 

v.... 


i... 
ii.. 
in . 

IVb. 

V... 
Vic. 


1891-92. 

Hay,  7.0  kgs.;   oats.  4.5  kgs. 

"       7.0     "  "     5.5     "  . 

"       4.5     "  "     7.0     "  . 

Average 


1892. 
Hay,  7.5  kgs.;   oats,  4.0  kgs. ; 


4.5 
"  4.5 
"  4.5 
"  7.5 
Average 


5.5     "    straw,  1  kg — 

grain,  5.0  kgs. ;     "        1.5  kgs. 

"      5.0     "         "       1.5    "  . 

oats,  4.5    "    ....'. 


Hay. 


7.5 
6.0 
"  6.0 
"  4.0 
"  4.0 
Average 


1892-93. 
.  5  kgs. ;  oats,  4 . 0  kgs 


5.5 
5.5 
5.5 
7.5 
7.5 


straw,  1  kg.  . 
"  1  "  .  . 
"      2  kgs. 


1893-94. 
Hay,  6.5  kgs.;  oats,  4.0  kgs.;  straw,  1.0  kg. 
"  3  0"  "  7.0  "  "  2.5kgs. 
"  3.0  "  grain,  7.0  "  "  2.5  "  . 
"  3.0  "  "  6.5  "  "  2.5  "  . 
Average 


Fiber-free 

Nutrients 

Minus  3300 

Grms. 


1.424 
1.990 
2,259 


1 ,775 
1.873 

1.521 
1.S00 
1  91)3 


1,475 

2,297 
1,670 
2,036 
2,577 
2,092 


1 ,607 

2.5SO 
2,500 

2, ,ssi  I 


5.639 

7,881 
8,945 


7,026 
7.416 

0,023, 
7,305 
7,537 


5.841 
9,095 
6.613 
8.063 

10,210 

lo.ooo 


Kgm. 


931,676  2,197 
1,129,568  2,663 
1,094,328  2,581 


1,074,802  2.53.5 
1,153,813  2.720 
912,454  2.152 
1,186,577  2.799 
1,188,388  2,803 


897,678 
1,280,687 

905,568 
1,167,127 
1,421,285 
1.549,620 


6,362  900,267 
10,220  1,549.202 
10.140  1,545,702 
11.420  1 ,673.780 


38.95 
33.79 


36.07 
36.68 
35.73 
38.00 
37.18 
36.77 


2,116  36.24 
3.024  33.20 
2,135  32.28 
2,752  34.14 
3,352132.85 
3,655  34.28 
33 . 74 


2.12233.36 
3.653  35.76 
3,645  35.95 
3,948  34.61 
35.05 


In  every  instance  but  one  the  utilization  as  thus  computed 
exceeds  31.3  per  cent.  In  other  words,  the  energy  of  the  body 
material  which,  according  to  Zuntz  &  Hagemann's  results,  must 
have  been  metabolized  to  produce  the  amount  of  work  done  exceeds 
considerably  the  amount  computed  to  be  available  from  the  food. 
There  being  no  reason  to  question  the  substantial  accuracy  of  Zuntz 
&  Hagemann's  factor,  this  means,  of  course,  that  if  the  food  and 
work  were  in  equilibrium  our  estimates  of  the  energy  available  from 


THE   UTILIZATION  OF  ENERGY. 


543 


the  food  are  too  low.  Either  3300  grams  of  fiber-free  nutrients 
(13,068  Cals.)  is  too  large  an  allowance  for  maintenance,  or  the 
assumption  that  the  energy  of  the  digested  crude  fiber  is  substan- 
tially equivalent  to  the  work  of  digestion  and  assimilation  is  erro- 
neous, or,  finally,  the  figure  of  3.96  Cals.  per  gram  of  digested  nutri- 
ents is  too  small.  As  regards  the  latter  possibility,  while  it  may 
be  conceded  that  the  energy  per  gram  of  digested  matter  will  vary 
somewhat  in  different  experiments,  the  difference  will  be  too  small 
to  materially  affect  the  result.  The  uncertainty  regarding  the 
maintenance  requirement  may  be  readily  eliminated  by  a  computa- 
tion based  on  the  differences  between  the  several  periods,  thus  afford- 
ing, to  a  degree  at  least,  a  test  of  the  correctness  of  Wolff's  hypothe- 
sis regarding  the  crude  fiber.  The  following  table  contains  the 
results  of  such  comparisons.  In  each  series  the  period  with  the 
least  amount  of  digested  food  (fiber-free)  has  been  compared  with 
the  other  periods  of  the  same  series. 


Metabolizable 

Energy  of 

Fiber-free 

Nutrients. 

Cals. 

Work, 
Cals. 

Utilization, 
Per  Cent. 

1891-92. 
Period  III 

20,949 
18,707 

2663 
2197 

"       He 

Period  IV 

2,242 

22,013 

18,707 

466 

2581 
2197 

20.79 

"       He 

1892, 

3,306 

20,094 
19,091 

384 

2535 
2152 

11.62 

"       HI 

Period  116 

"       III   

1,003 

20,484 
19,091 

383 

2720 
2152 

38.19 

Period  IV 

1,393 

20433 
19,091 

568 

2799 
2152 

40.77 

"      III 

1,342 

647 

48.21 

544 


PRINCIPLES  OF  ANIMAL   NUTRITION. 


Metabolizable 

Energy  of 

Fiber -free 

Nutrients, 

Cals. 

Work, 
Cals. 

Utilization, 
Per  Cent. 

1892. 
Period  V 

20,605 
19,091 

2803 
2152 

"      III 

1892-93. 
Period  II           

1,514 

22,163 
19,295 

651 

3024 
2126 

43.00 

"       I  and  III 

Period  rV6  

2,868 

21,131 
19,295 

898 

2752 
2126 

31.31 

"       I  and  III 

Period  V 

1,836 

23,278 
19,295 

626 

3352 
2126 

34.10 

"       I  and  III 

Period  Vic 

3,983 

23,728 
19,295 

1226 

3655 
2126 

30.78 

"       I  and  III 

1893-94. 
Period  III 

4,433 

23,288 
19,430 

1529 

3653 
2122 

34.48 

"       I 

Period  V 

3,858 

23,208 
19,430 

1531 

3645 
2122 

39.68 

"       I 

Period  VI 

3,778 

24,488 
19,430 

1523 

3948 
2122 

-:o.3i 

I 

Totals  and  averages,  ex- 
cluding 1891-92 

5,058 
31,066 

1826 
11,408 

36.10 
36.73 

With  the  exception  of  the  experiments  of  1891-92,  which  were 
the  first  with  the  new  form  of  dynamometer  and  which  Wolff  con- 
siders unsatisfactory,  we  have  but  two  cases  in  which  the  apparent 
utilization  does  not  exceed  31.3  per  cent.  Having  eliminated  the 
uncertainty  as  to  the  maintenance  ration,  and  the  figures  for  the 
energy  of  the  food  being  regarded  as  substantially  correct,  this  can 


THE   UIILIZATION  OF  ENERGY.  545 

mean  only  one  of  two  things,  viz.,  that  the  figures  for  the  work  done 
are  too  high  or  that  the  deduction  on  account  of  the  crude  fiber  is 
too  great. 

That  a  determination  of  the  equivalence  of  food  and  work  by 
Wolff's  method  is  subject  to  considerable  uncertainty  in  an  indi- 
vidual case  is  obvious,  but  there  seems  to  be  no  apparent  reason 
why  it  should  be  uniformly  overestimated.  The  measurement  of 
the  work  was  made  with  great  care,  and  while  the  work  of  locomo- 
tion is  an  estimate,  its  close  agreement  with  the  results  of  Zuntz  & 
Hagemann  (p.  539)  renders  it  unlikely  that  it  is  seriously  in  error. 

It  would  appear,  then,  that  with  the  rations  used  in  these  ex- 
periments the  energy  required  for  digestion  and  assimilation  was 
less  than  the  energy  of  the  digested  crude  fiber.  How  much  less  it 
was,  however,  unfortunately  does  not  appear,  and  we  are  obliged 
to  content  ourselves  for  the  present  with  this  negative  conclusion. 

Zuntz  &  Hagemann's  Computations.— These  investigators  * 
have  recalculated  Wolff's  results  in  a  still  different  manner.  In- 
stead of  taking  for  the  amount  of  work  equivalent  to  the  ration  the 
figures  given  by  Wolff,  which,  as  already  explained,  are  to  a  certain 
extent  estimates,  they  take  the  amount  of  work  actually  performed 
in  each  case  and  correct  for  the  observed  gain  or  loss  of  five  weight. 
This  method  is  in  conception  more  scientific  than  Wolff's,  pro- 
vided the  requisite  correction  can  be  accurately  estimated.  As  the 
basis  for  such  an  estimate,  Zuntz  &  Hagemann  take  an  early  experi- 
ment by  Wolff,f  from  which  they  compute  that  one  gram  loss  of 
live  weight  is  equivalent  to  one  half  revolution  of  the  dynamometer 
(at  76  kgs.  draft).  From  the  same  experiment  they  compute  the 
mechanical  equivalent  of  one  revolution  as  2694  kgm.  This,  how- 
ever, aside  from  the  fact  that  it  is  the  result  of  a  single  series  of 
experiments,  was  obtained  with  the  old  form  of  dynamometer, 
whose  indications,  as  we  have  seen,  were  too  high,  but  the  later 
experiments  unfortunately  are  not  reported  in  a  way  to  permit  of 
an  estimate  of  the  difference. 

Taking  the  correction,  then,  as  estimated,  Zuntz  &  Hagemann 
divide  Wolff's  experiments  into  two  groups,  viz.,  those  in  which  the 
work  was  400  or  less  revolutions  and  those  in  which  it  was  more 

*  hoc.  cit.,  pp.  412-422. 
t  Grundlagen,  etc.,  v>.  80. 


546 


PRINCIPLES   OF  ANIMAL   NUTRITION. 


than  400  revolutions.     Comparing  the  averages  of  these  two  groups, 
they  obtain  the  following: 


Total 
Digested 
Nutrients, 

Grms. 

Loss  of 

Work,Kgm.        ^>g*t 

Grms. 

6236 

5851 

1,415,755          179.5 

Lighter      "      (13          "            ) 

995,225 

385 

420,530 
231,922 

172.2 

188,608 

According  to  this  computation,  the  385  grams  of  added  nutrients 
enabled  188,608  kgm.  of  work  to  be  performed.  At  3.96  Cals.  per 
gram  the  metabolizable  energy  of  the  added  nutrients  equals  1524 
Cals.  From  this,  according  to  Zuntz  &  Hagemann,  is  to  be  de- 
ducted 9  per  cent,  for  the  work  of  digestion  and  also  2.65  Cals. 
for  each  gram  of  total  crude  fiber  in  the  added  food.  On  this, 
basis  we  have  the  following: 


Weight, 
Grms. 

Energy. 
Cals. 

385 

2338 
2356 

-18 

1524 

Average  crude  fiber  fed : 

-48 

Work  of  digestion  (1524  X  0.9) .  . 

137 

89 
1435 

Work  done  (188,608  -*-  424) 

445 



The  work  done  is  31  per  cent,  of  the  computed  available  energy  of 
the  food,  a  figure  corresponding  very  closely  with  the  31.3  per 
cent,  found  by  Zuntz  &  Hagemann. 

The  difference  in  the  average  amount  of  crude  fiber  fed  in  the 
two  groups  of  experiments  is  so  small  that  the  estimate  for  the 


THE  UTILIZATION  OF  ENERGY.  547 

energy  required  by  its  digestion  hardly  affects  the  computation. 
What  the  result  appears  to  show  is  that  the  estimate  of  9  per 
cent,  for  the  digestion  and  assimilation  of  the  fiber-free  nutrients 
is  approximately  correct. 

The  difference  in  the  amount  of  digested  crude  fiber  was  some- 
what greater  than  that  in  the  total  amoimt.  If  we  make  the  com- 
parison of  the  two  averages  on  the  basis  of  the  fiber-free  nutrients 
in  the  same  manner  as  in  previous  cases  we  have — 

Fiber-free  nutrients : 

Heavier  work 5524  grams 

Lighter  work 5086      " 

Difference 438      " 

Equivalent  energy 1735  Cals. 

Energy  of  work 445     " 

Utilization 25 .  65  per  cent. 

Apparently  a  considerable  amount  of  energy  was  required  for 
the  work  of  digestion  and  assimilation  in  addition  to  that  equiva- 
lent to  the  digested  crude  fiber,  a  result  which  seems  to  conflict 
with  the  conclusions  drawn  from  a  discussion  of  the  same  experi- 
ments in  the  preceding  paragraph.  The  apparent  discrepancy  lies 
in  the  determination  of  the  amount  of  external  work  equivalent 
to  the  added  nutrients.  Wolff,  as  we  have  seen,  after  securing 
an  approximate  constancy  of  live  weight,  corrects  the  measured 
amount  of  work  in  accordance  with  his  judgment  of  the  amount 
which  would  have  been  equivalent  to  the  ration  given  and  relies  on 
the  "might  of  averages"  to  overcome  the  inherent  uncertainties  of 
his  method.  Zuntz  &  Hagemann,  on  the  other  hand,  reckon  with 
the  measured  amount  of  work,  but  are  then  compelled  to  correct 
their  final  result  for  the  loss  of  live  weight,  and  unfortunately  this 
correction  is  relatively  a  very  large  one  (over  50  per  cent.)  and  rests 
upon  a  rather  uncertain  basis.  While  it  would  perhaps  be  pre- 
sumptuous to  attempt  to  decide  the  relative  value  of  the  two  methods 
and  the  probability  of  the  divergent  conclusions  based  on  them,  one 
can  hardly  avoid  feeling  that  the  trained  judgment  of  the  actual 
experimenter  is  a  safer  reliance  than  such  a  relatively  large  cor- 
rection computed  by  a  critic. 


54«  PRINCIPLES  OF  ANIMAL  NUTRITION. 

In  any  case  it  is  obvious  that  while  the  extensive  researches 
of  Zuntz  and  his  associates  afford  very  reliable  data  as  to  the 
ratio  between  the  energy  liberated  in  muscular  work  and  the 
amount  of  external  work  accomplished,  or,  in  other  words,  as  to 
the  utilization  of  the  net  available  energy  of  the  food,  we  have  as 
yet,  notwithstanding  the  vast  amount  of  work  done  by  Wolff  and 
his  co-laborers  and  others,  but  very  fragmentary  and  uncertain 
data  as  to  the  utilization  of  the  metabolizable  energy  of  the  food  for 
work  production. 


APPENDIX. 


TABLE  I.    METABOLIZABLE   ENERGY   OF  COARSE   FODDERS. 


"3 
S 
f 

< 

■6 

Organic 

Matter 
Eaten. 

Energy  of 

Metabolizable 
Energy. 

Feed  Added. 

a 
o 

1 

I 

a 

Food, 
Cals. 

Feces, 
Cals. 

Urine 
(Cor- 
rected), 
Cals. 

Methane, 
Cals. 

Total, 
Cals. 

Per 
Grm. 
Or- 
ganic 
Mat- 
ter, 
Cals. 

Meadow  hay  V  -J 

F 
F 

G 

G 

H 
H 

H 
H 

1 

3 

2 
3 

2 

4 

7 
4 

9475 
6630 

6024 
3175 

44821.2 
31327.8 

16323.7 
9599.2 

2113.3 
1530.0 

3250.6 
2560.7 

23133.6 
17637.9 

Difference. . .  . 
Correction 

2845 

9405 
6651 

2849 

5950 
3206 

13493.4 
+  19.0 

6724.5 
+  5.9 

583.3 
+0.9 

689.9 
+  1.5 

5495 . 7 
+  10.7 

Percentage  . .  . 
Meadow  hay  V  -j 

13512.4 
100.00 

43811.3 
30750.7 

6730.4 
49.81 

15336.3 
9491.5 

584.2 
4.32 

1916.1 
1359.6 

691.4 
5.12 

3432.1 
2524.7 

5506.4 
40.75 

23126.8 
17374.9 

1.933 

Difference .... 
Correction. . . . 

2754 

9527 
6402 

2744 

6323 
3198 

13060.6 
-45.4 

5844.8 
-14.0 

556.5 
-2.0 

907.4 
-3.7 

5751.9 
-25.7 

Percentage  . .  . 
Meadow  hay  VI  ■! 

13015.2 
100.00 

45255.8 
30338 . 1 

5830.8 
44.80 

14103.7 
8574.9 

554.5 
4.26 

2576.3 
1795.0 

903.7 
6.94 

3306.6 
2579.4 

5726.2 
44.00 

25269.2 
17388.8 

2.087 

Difference .... 
Correction 

3125 

9743 
6402 

3125 

6495 
3198 

14917.7 
-8.9 

5528.8 
-2.5 

781.3 
-0.5 

727.2 
-0.8 

7880.4 
-5.1 

Percentage  . .  . 
Meadow  hayVI  \ 

14908.8 
100.00 

46275.0 
30338.1 

5526.3 
37.07 

14104.8 
8574.9 

780.8 
5.24 

2593.0 
1795.0 

726.4 
4.87 

3564.2 
2579.4 

7875.3 
52.82 

26013.0 
17388.8 

2.520 

Difference .... 
Correction.. . . 

3341 

3297 

15936.9 
-208.3 

5529.9 
-58.9 

798.0  1     984.8 
-12.3       -17.7 

8624.2 
119.4 

Percentage  . . . 

15728.6 
100.00 

5471.0 
34.78 

785.7        967.1 
5.001         6.15 

8504.8 
54.07 

2.580 

549 


55° 


APPENDIX. 


TABLE  I   {Continued). 


- 
j 

< 

a 

Organic 
Matter 
Eaten. 

Energy  of 

Metabolizable 
Energy. 

Feed  Added. 

i 

6 

o 

■a 

1 
II 

a 

Food, 
Cals. 

Feces, 
Cals. 

Urine 
(Cor- 
rected), 
Cals. 

Methane, 
Cals. 

Total, 
Cals. 

25646 . 2 
17830.5 

Per 
Grm. 
Or- 
ganic 
Mat- 
ter, 
Cals. 

Meadow  hay  VI J 

J 
J 

F 
F 

G 
G 

a 

n 

j 
j 

ii 

Ji 

j 

j 

•2 
4 

2 
3 

1 
3 

1 
4 

1 
4 

5 

4 

5 
4 

9539 
0458 

6340  45239.6 
3239  30548.5 

13218.1 
8171.2 

2755.1 
1824.6 

3620.2 

2722.2 

Difference .... 
Correction. .  .  . 

3081 

9819 
6630 

3101 

3170 
0 

14691 . 1 
101.8 

5046.9 
-j-27.2 

930.5 
6.1 

898.0 
9.1 

7815.7 
59.4 

Percentage  . .  . 
Oat  straw  II..  -j 

14792.9 
100.00 

46690.1 
31327.8 

5074.1 
34.30 

18296.3 
9599.2 

936.6 
6.33 

1884.2 
1529.8 

907.1 
6.13 

3239.9 
2560.7 

7875.1 
53.24 

23269.7 
17638.1 

2.540 

Difference  . . . 
Correction. . .  . 

3189 

9740 
6651 

3170 

3115 
0 

15362.3 
-94.3 

8697.1 
-28.9 

354.4 
-4.6 

679.2 

-7.7 

5631.6 
-53.1 

Percentage  . . . 
Oat  straw  II . .  j 

15268.0 
100.00 

45626.1 
30750.7 

8668.2 
56.77 

17983.1 
9491.5 

349.8 
2.29 

1633.6 
1359.6 

671.5 
4.40 

3448.1 
2524.7 

5578.5 
36.54 

22561.3 
17374.9 

1.760 

Difference .... 
Correction. . .  . 

3089 

9611 

C402 

3115 

3195 
0 

14875.4 
+  126.5 

8491.6 
+  39.0 

274.0 
+  5.6 

923.4 

+  10.4 

5186.4 
+  71.5 

Percentage.  .  . 
Wheat  straw  I  -j 

15001.9 
100.00 

45570.1 
30338.1 

8530.6 
56.86 

17751.7 
8574.9 

279.6 
1.86 

2084.7 
1795.0 

933.8 
6.23 

3792.4 
2579.4 

5257.9 
35.05 

21941.3 
17388.8 

1.688 

Difference. . . . 
Correction. . . . 

3209 

9583 
6458 

3195 

3188 
0 

15232.0 
-76.6 

9176.8 
-21.7 

289.7 
-4.5 

1213.0 
-6.5 

4552.5 
-43.9 

Percentage  . .  . 
Wheat  straw  I  -j 

15155.4 
100.00 

45365.9 
30548.5 

9155.1 
60.41 

16562.1 
8171.2 

285.2 
1.88 

2237.8 
1824.6 

1206.5 
7.96 

4003.2 
2722 . 2 

4508.6 
29.75 

22562.8 
17830.5 

1  411 

Difference .... 
Correction .... 

3125 

9114 

6402 

3188 

2665 
0 

14817.4 
+  302.4 

8390.9 
+  80.9 

413.2 
+  18.1 

1281.0 
+  26.9 

4732.3 
+  176.5 

Percentage  . .  . 

Extracted  rye  j 
straw | 

15119.8 
100.00 

41900.7 
30338.1 

8471.8 
56.03 

9926.4 
8574.9 

431.3 
2.85 

1756.5 
1795.0 

1307.9 
8.65 

4004.5 
2579.4 

4908.8 
32.47 

26213.3 
17388.8 

1.540 

Difference  .  .  . 
Correction  . .  . 

2712 

9142 

cj.-.s 

2665 

2659 
0 

11562.6 
-232.7 

1351.5 
-65.8 

-38.5 
-13.8 

1425.1 
-19.8 

8824.5 
-133.3 

Percentage  . . . 

Extracted   rye  1 
straw i 

11329.9 
100.00 

41962.6 
30548.5 

1285.7 
11.35 

9799.0 
8171.2 

-52.3 
-0.46 

1705.8 
1824.6 

1405.3 
12.40 

4147.4 
2722.2 

8691.2 
76.71 

26310.4 

17830.5 

3.261 

Difference.  . .  . 

Correction. .  .  . 

2684 

2659 

11414.1 
-113.3 

1627.8 
-30.3 

-118.8 
-6.8 

1425.2 
-  10.1 

8479.9 
-66.1 

Percentage  . .  . 

11300.8 
100  00 

1597.5 
14.14 

-125.6 
-1.11 

1415.1 
12.52 

8413.8 
74.45 

3.164 

APPENDIX. 


551 


TABLE 

II. 

METABOLIZABLE    ENERGY 

OF    BEET   MOLASSES. 

1 
a 
< 

o 

Organic 
Matter 
Eaten. 

Energy  of 

Apparent 

Metabolizable 

Energy. 

Feed  Added. 

i 

o 

1 

I 
% 

a 

Food, 
Cals. 

Feces, 
Cals. 

Urine 
(Cor- 
rected), 
Cals. 

Methane, 
Cals. 

Total, 
Cals. 

Per 

Gram 
Or- 
ganic 
Mat- 
ter, 
Cals. 

Beet  mol'ses  I  -j 

F 

F 

H 
H 

J 
J 

6 
3 

6 
4 

6 
4 

8262 
6630 

1702  37946.2 
0  31327.8 

11365.8 
9599.2 

1786.1 
1530.0 

2397.9 
2560.7 

22396.4 
17637.9 

Difference .... 
Correction .... 

1632 

8110 
6402 

1702 

1611 
0 

6618.4 
+  330.8 

1766.6 
+  101.3 

256.1 
+  16.2 

-162.8 
+  27.0 

4758.5 
+  186.3 

Percentage  . .  . 
Beet  mol'ses  II  - 

6949.2 
100.00 

37544.4 
30338.1 

1867.9 
26.87 

9070.0 
8574.9 

272.3 
39.2 

2035.2 
1795.0 

-135.8 
-1.95 

3458.8 
2579.4 

4944.8 
71.16 

22980.4 
17388.8 

5591.6 
-263.3 

2.905 

Difference .... 
Correction..  .  . 

1708 

8104 

6458 

1611 

1595 
0 

7206.3 
-459.4 

495.1 
-129.8 

240.2 
-27.2 

879.4 

Percentage  . . . 
Beet  mol'ses  II  j 

6746.9 
100.00 

37461 . 1 
30548  .-5 

365.3 
5.40 

9198.7 
8171.2 

213.0 
3.16 

2017.2 
1824.6 

840.3 
12.44 

3422.7 
2722 . 2 

5328.3 
79.00 

22822.5 
17830.5 

3.308 

Difference .... 
Correction. . .  . 

1646 

1595 

6912.6 
-234.3 

1027.5 
-62.7 

192.6 
-14.0 

700.5 
-20.9 

4992.0 
—136.7 

Percentage . . . 

6678.3 
100.00 

964.8 
14.45 

178.6 
2.67 

679.6 
10.18 

4855.3 
72.70 

3.044 

552 


APPENDIX. 


TABLE   III.   METABOLIZABLE    ENERGY    OF   STARCH. 
EXPERIMENTS. 


I 

q 

< 

III 
III 

IV 
IV 

V 
V 

V 
V 

VI 
VI 

V] 

VI 

1 

Organic 
Matter 
Eaten. 

Energy  of 

Apparent 

Metabolizable 

Energy. 

Feed  Added. 

i 

a 
•i 

o 
H 

s 

—  9 

£  1 

+  '_" 
o 

a 

Food. 
Cals. 

Feces 
Cals. 

Urine* 
(Cor- 
rected), 
Cals. 

Methane, 
Cals. 

Total, 
Cals. 

Per 
Grm. 
Or- 
ganic 
Mat- 
ter, 
Cals. 

Starch  I... .  | 

2 

1 

2 

2<z 

1 

26 

1 

2b 

1 

3 

1 

8839 

;::js 

1651 
0 

40964.5 
34603.2 

16615.5 
15505.1 

1430.3 
1549.6 

3225.3 
2670.1 

19593.4 
14878.4 

Difference .  . 
Correction. . 

1511 

8787 
7074 

L651 

1  fillS 

0 

6361.3 
+  658.0 

1110.4 
+  294.8 

-119.3 
+  29.5 

655.2 

+  50.8 

4715.0 
282.9 

Percentage . 
Starch  I  .  . .  -j 

7019.3 
100.00 

40725.6 
33405.1 

1405.2 
20.02 

17202.1 
15250.6 

-89.8 
-1.29 

1434.9 
1481.5 

706.0 
10.06 

3348.0 
2491.3 

4997.9 
71.21 

18740.6 
14181.7 

3.029 

Difference  . 
Correction  . 

1713 

8767 
7199 

1608 

1621 
0 

7320.5 
-492.0 

1951.5 
-224.6 

-46.6 
-21.8 

856.7 
-36.7 

4558.9 
-208.9 

Percentage  . 
Starch  II.  .  .  -j 

6828.5 
100.00 

40827.5 
34211.5 

1726.9 
25.29 

15804.1 
15312.2 

-68.4 
-1.01 

1618.3 
1559.3 

820.0 
12.01 

3021 . 1 
2268.5 

4350.0 
63.71 

20384.0 
15071.5 

2.705 

Difference.  . 
Correction. . 

1568 

8792 
7199 

1621 

166:- 
0 

6616.0 
+  255.0 

491.9 
+  114.2 

59.0 
+  11.6 

752.6 
+  16.9 

5312.5 
+  112.3 

Percentage  . 
Starch  II...  -J 

6871.0 
100.00 

40917.4 
34211.5 

606.1 
8.82 

16270.0 
15312.2 

70.6 
1.03 

1524.8 
1559.3 

769.5 
11.20 

2944.0 
2268.5 

5424.8 
78.95 

20181.6 
15071.5 

3.347 

Difference .  . 
Correction. . 

1593 

8861 
7125 

1663 

166! 
0 

6705.9 
+  334.1 

957.8 
+  149.5 

-34.5 
+  15.2 

672.5 
+  22.2 

5110.1 
+  147.2 

Percentage  . 
Starch  1 1...  ] 

7040.0 
100.00 

41245.9 
33855.4 

1107.3 
15.73 

15485.9 
13765.2 

-19.3 
-0.27 

1569.6 
1737.9 

694.7 
9.86 

3130.5 
2480.6 

5257 . 3 
74.68 

21059.9 
15871.7 

3.161 

Difference . . 
Correction. . 

1736 

9953 

7125 

1669 

2788 

C 

7390.5 
-320.9 

1720.7 
-130.5 

-168.3 
-16.5 

649.9 
-23.5 

5188.2 
-150.4 

Percentage  . 
Starch  II...  -j 

7269.6 
100.00 

45859.6 
33855.4 

1590.2 
22.49 

16091.4 
13765.2 

-184.8 
-2.61 

1643.9 
1737.9 

626.4 
8.86 

3897.8 
2480.6 

5037.8 
71.26 

24226.5 
15871.7 

3.018 

Difference. . 
Correction . . 

2828 

2788 

12004.2 
-193.4 

2326.2 
-78.6 

-94.0 
-9.9 

1417.2 
-14.2 

8354.8 
-90.7 

Percentage 

11810.8 
100.00 

2247 . 6 
19.03 

-103.9 
-0.88 

1403.0 
11.87 

8264.1 
69.98 

2.964 

Computed  from  carbon  content. 


APPENDIX. 


553 


METABOLIZABLE    ENERGY   OF   STARCH. 
EXPERIMENTS. 


KELLNER  S 


■a 

| 

< 

— ' 
z 

I 

Organic 
Matter 
Eaten. 

Energy  of 

Apparent 

Metabolizable 

Energy. 

Feed  Added. 

a 

o 

I 

-of 

■So 

■v 

■1 

Food, 
Cals. 

Feces, 
Cals. 

Urine 
(Cor- 
rected), 
Cals. 

Methane 
Cals. 

Total, 
Cals. 

Per 

Grm. 
Or- 
ganic 
Mat- 
ter, 
Cals. 

Starch   I   and) 
II 1 

B 
B 

C 

C 

D 
D 

F 

F 

G 
G 

II 
H 

J 
J 

2 

4 

2 
1 

2 
1 

4 

3 

4 
3 

3 
4 

3 
4 

11695 
10067 

3231 
1607 

52928.6 
46129.1 

15915.8 
11874.4 

1740.1 
1958.5 

3382.7 
3716.3 

31890.0 
28579.9 

Difference  .  . 
Correction  .  . 

1631 

11980 
10407 

1624 

3193 
1602 

6799.5 
-30.9 

4041.4 
-8.0 

-218.4 
-1.3 

-333.6 
-2.5 

3310.1 
-19.1 

Percentage  . . 

Starch  I  and     ( 
II   1 

6768.6 
100. OC 

54016.5 
47458.0 

4033.4 
59.60 

19185.6 
15746.8 

-219.7 
-3.25 

1723.7 
1785.7 

-336.1 
-4.97 

3250.6 
3255.9 

3291.0 
48.62 

29856.6 
26669.6 

2.027 

Difference   .  . 
Correction  .  . 

1573 

11636 
9974 

1591 

1583 
0 

6558.5 
+  70.8 

3438.8 
+  23.5 

-62.0 
+  2.7 

-5.3 
+  4.9 

3187.0 
+  39.7 

Percentage  . . 
Starch  III  . .  | 

6629.3 
100.00 

53902.2 
46945.4 

3462.3 
52.22 

17817.9 
15718.3 

-59.3 
-0.89 

2211.2 
2407.0 

-0.4 
-0.01 

3381.4 
2957.0 

3226.7 
48.68 

30491.7 
25863.1 

2.028 

Difference   .  . 
Correction  .  . 

1662 

8374 
6630 

1583 

1687 
0 

1687 

1676 
0 

6956.8 
-379.5 

2099.6 
-127.1 

-195.8 
-19.5 

424.4 
-23.9 

4628.6 
-209.0 

Percentage  . . 
Starch  III  .  .  -j 

6577.3 
100.00 

38608.3 
31327.8 

1972.5 
29.99 

10833.9 
9599.2 

-215.3 

1594.3 
1530.0 

400.5 
6.08 

3382.7 
2560.7 

4419.6 
67.20 

22797.4 
17637.9 

2.792 

Difference  .  . 
Correction  .  . 

1744 

8380 
6651 

7280 . 5 
-268.3 

1234.7 
-82.5 

64.3 
-13.2 

822.0 
-22.0 

5159.5 
-150.6 

Percentage  . . 
Starch  III  .  .  -j 

7012.2 
100.00 

37963.6 
30750.7 

1152.2 
16.42 

10497.1 
9491.5 

51.1 

0.73 

1394.7 
1359.6 

800.0 
11.41 

3170.5 
2524 . 7 

5008.9 
71.44 

22901.3 
17374.9 

2.969 

Difference  .  . 
Correction . .  . 

1729 

8373 
6402 

1676 

2013 
0 

7212.9 
-246.2 

1005.6 
-76.0 

35.1 
-10.9 

645.8 
-20.2 

5526.4 
-139.1 

Percentage  . . 
Starch  IV. .  .  -j 

6966.7 
100.00 

38562.4 
30338.1 

929.6 
13.35 

9843.8 
8574.9 

24.2 
0.35 

1588.4 
1795.0 

625.6 
8.98 

3183.9 
2579.4 

5387.3 
77.32 

23946.3 
17388.8 

3.214 

Difference   .  . 
Correction  .  . 

1971 

8004 
6458 

2013 

1600 
0 

8224 . 3 
+  193.2 

1268.9 
+  54.6 

-206.6 
+  11.4 

604.5 
+  16.4 

6557.5 
+  110.8 

Percentage  . . 
Starch  IV.  .  .  -j 

8417.5 
100.00 

36982.6 
30548.5 

1323.5 
15.72 

9096.8 
8171.2 

-195.2 
-2.32 

1885.4 
1824.6 

620.9 
7.38 

3492.1 
2722 . 2 

6668.3 
79.22 

22508.3 
17830.5 

3.313 

Difference   .  . 
Correction  . . 

1546J1600 

6434.1 
+  254.1 

925.6 
+  68.0 

60.8 
+  15.2 

769.9 
+  22.6 

4677.8 
+  148.3 

Percentage  . . 

6688.2 
100.00 

993.6 
14.85 

76.0 
1.14 

792.5 
11.85 

4826.1    ! 
72.16 

5.017 

554 


APPENDIX. 


TABLE   V.    METABOLIZABLE   ENERGY   OF  WHEAT   GLUTEN. 


Feed  Added. 


Wheat  gluten  -, 


Difference . 
Correction. 


Percentage . 
Wheat  gluten 


Difference . 
Correction. 


Percentage 

Wheat  gluten  J 

Difference.  . 
Correction . 

Percentage 
Wheat  gluten 

Difference   . 
Correction  . 

Percentage  . . 

Wheat  gluten  J 
I I 

Difference   . . 
Correction  .  . 

Percentage  . . 

Wheat  gluten  I 
II / 

Difference   .  . 
Correction  .  . 


Percentage  . . 
Wheat  gluten  j 

Difference  .  , 
Correction  . 


Percentage  .. 


Organic 
Matter 
Eaten. 


H 
9311 


11630 
1(1007 


11533 

111007 


Energy  of 


Food, 
Cals. 


576  44025 . 3 
0  40964.5 


576    3060.8 
+  502.9 


3563.7 
100.00 


48293.6 
40964 . 5 


7329.1 
-171.2 


7157.9 
100.00 


4134.7 
-534.4 


3600.3 
100.00 


1746  54939 . 3 
261  46129.1 


1485  8810.2 
80.5 


1742  54469.0 
261  46129.1 


1481  8339.9 
62.3 


11578  1407 
9974|       0 


Urine 
Feces,        (Cor-      Methane 
Cals.       rected) 

Cals. 


16041.5 
16615.5 


574.0 
-204.0 


•370.0 
-10.3$ 


16593.3 
10015.5 


-91 

-1.28 


16845.6 
17202.1 


356.5 
225.7 


-582.2 
-16.17 


14514.7 
11874.4 


2040 . 3 
-97.9 


2542.4 
30.16 


13753.4 
11874.4 


1879.0 
+  16.0 


8402.2 
100.00 


1895.0 
22.55 


17643.2 
15746.8 


Cals. 


2048.8*    3669.6 
1430.3*     3325.3 


618.5 
+  17.6 


636.1 
17.85 


2990.4* 
1430.3* 


1554.1 
21.71 


2036.3* 
1434.9* 


601.4 
-18.8 


582.6 
16.18 


3372.3 
1958.5 


1413.8 
-16.2 


1397.6 
16.58 


3092 . 1 
1958.5 


1133.6 

+  2.6 


344.3 

+  40.8 


3703.0 
3325 . 3 


377.7 
-13.9 


Apparent 

Metabolizable 

Energy. 


Total, 
Cals. 


22265.4 
19593.4 


2672.0 
f240.5 


Per 
Grm. 

Or- 
ganic 
Mat- 


2912.5  5.057 
81.72 


25006.9 
19593.4 


5413.5 
-81.9 


5331 
5.08   74.49 


3346.7  22631.7 
3348.0  18740.6 


•45.2 
-1.26 


J891.1 
246.0 


3645 . 1 
101.25 


37.4 
-30.7 


6.7 
0.08 


3574.9 
3716.3 


-141.4 
+  5.0 


1136.2 
13.52 


2744.1 
1785.7 


1890.4 
-136.5 


8424.2 
100.00 


46945.4  J15718.3 


9108.2  I  1604.6 
-934.1  -312.7 


8174.1 
100.  ool 


1291.9 
15.80 


3468.0 
2407.0 


2973.0 
3255.9 


282 . 9 
-28.2 


311.1 
-3.6! 


3171.9 
2957.0 


1061.0  |  214.9 
-47.9  I  -58.8 


1013.1 
12.39! 


156.1 
1.91 


4718.7 
-235.7 


4483.0 
53.18 


34048.6 
28579.9 


5507.4 
65.55 


32933.3 
26669.6 


6263.7 
-231.2 


6032.5 
71.61 


32090.8 
25863.1 


6227.7 
-514.7 


5713.0 
69.90 


Estimated  from  carbon  content. 


APPENDIX. 


555 


TABLE   VI.    METABOLIZABLE    ENERGY    OF    PEANUT   OIL. 


3 

1 

D 
D 

F 
F 

G 

G 

■6 
a 

I 

Organic 
Matter 
Eaten. 

Energy  of 

Apparent 

Metabolizable 

Energy. 

Feed  Added. 

i 

c 

"3 
o 

•o 
1 

<d  a 

< 

Food, 
Cals. 

Feces. 
Cals. 

Urine 
(Cor- 
rected), 
Cals. 

Methane, 
Cals. 

Total, 
Cals. 

Per 
Gram 
Or- 
ganic 
Mat- 
ter, 
Cals. 

Peanut  oil  I.,  -j 

3 
1 

5 
3 

5 
3 

10752 
9974 

709 
0 

54007.3 
46945.4 

1749.2 
15718.3 

2351.2 
2407.0 

2909.0 
2957.0 

31279.6 
25863.1 

Difference.  .  . 
Correction. .  . 

778 

7491 
6630 

709 

798 
0 

7061.9 
-331.1 

1749.2 
-110.9 

-55.8 
-17.0 

-48.0 
-20.8 

5416.5 
-182.4 

Percentage  . . 
Peanut  oil  II .  \ 

6730.8 
100.00 

39185.9 
31327.8 

1638.3 
24.34 

14585.7 
9599.2 

-72.8 
-1.08 

1455.0 
1530.0 

-68.8 
-1.02 

1369.1 
2560 . 7 

5234.1 

-77.76 

21776.1 
17637.9 

4138.2 
-170.0 

7.382 

Difference . . . 
Correction. .  . 

861 

7396 
6651 

798 

798 
0 

7858.1 
-302.0 

4986.5 
-92.5 

-75.0 
-14.8 

-1191.6 
-24.7 

Percentage  . . 
Peanut  oil  II.  j 

7556.1 
100.00 

38057.3 
30750.7 

4894.0 
64.77 

12512.9 
9491.5 

-89.8 
-1.19 

1452.1 
1359.6 

1216.3 
-16.10 

2371.2 
2524.7 

3968.2 
52.52 

21721.1 
17374.9 

4.973 

Difference.  .  . 
Correction. .  . 

745 

798 

7306.6 
+  249.5 

3021.4 
+  77.0 

92.5 
+  11.0 

-153.5 

+20.5 

4346.2 
+  141.0 

Percentage  . . 

7556.1 
100.00 

3098.4 
41.00 

103.5 
1.37 

-133.0 
-1.76 

4487.2 
59.39 

5.623 

556 


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INDEX. 


PAGE 

Acid,  acetic,  effect  of,  on  proteid  metabolism 123 

total  metabolism 160 

replacement  value  of 160 

aspartic,  formed  from  proteids 39 

oxidized  in  body 52 

benzoic,  formation  of  hippuric  acid  from 44 

butyric,  effect  of,  on  total  metabolism 158 

replacement  value  of 158 

glutaminic,  formed  from  proteids 39 

hippuric,  effect  of  proteids  on  formation  of 463 

formation  of,  from  benzoic  acid 44 

glycocol 44 

in  urine „ 44 

loss  of  energy  in 313,  322 

non-nitrogenous  nutrients  as  source  of 45 

origin  of 44 

pentose  carbohydrates  as  source  of 46 

source  of  benzoyl  radicle  of 44,  45 

lactic,  effect  of,  on  proteid  metabolism 123 

total  metabolism 158 

production  of,  in  metabolism  of  carbohydrates 23 

replacement  value  of 158 

Acids,  organic,  absent  from  excreta 27 

effect  of,  on  proteid  metabolism 123 

total  metabolism 1 57 

net  available  energy  of 425 

metabolism  of 26 

oxidized  in  body 27 

produced  by  fermentation  of  carbohydrates 13,  26 

replacement  values  of 157 

Acid,  uric,  in  perspiration 48 

in  urine 43 

origin  of 43 

573 


574  INDEX. 

PAGE 

Activity,  muscular,  general  features  of 185 

Adipose  tissue 29 

Alanine  oxidized  in  body 53 

Albuminoids 7 

compound 7 

composition  of 62 

derived 7 

modified 7 

simple 7 

simple,  composition  of 62 

Albumins 7 

Albumoses  formed  from  proteids 38,  39 

Amido-acids 7 

Amides : 7 

formed  from  proteids 7,  39,  52 

influence  of,  on  digestibility 54 

of  carbohydrates 57 

crude  fiber 57,  58 

nitrogen-free  extract 57 

fermentation  of  carbohydrates 55 

in  digestive  tract 54 

metabolism  of 52 

not  synthesized  to  proteids 53 

oxidized  in  body 52 

replacement  of  proteids  by 53 

urea  formed  from 39 

Ammonium  acetate,  influence  of,  on  digestibility  of  carbohydrates.  ...   57,  58 

crude  fiber 57 

nitrogen-free  extract.     57 

carbonate  as  antecedent  of  urea 43 

lactate  as  antecedent  of  urea 43 

salts  influence  of,  on  digestibility  of  carbohydrates 57 

fermentation  of  carbohydrates 56 

in  digestive  tract 56 

in  perspiration 48 

Amount  of  food,  critical 408 

influence  of,  on  effects  of  muscular  exertion 197 

net  availability  of  energy 430 

utilization  of  energy 466 

Anabolism 16,17 

absorption  of  energy  in 17 

of  proteids 38.  41 

Animal,  efficiency  of 496,  498,  51 1 

as  motor 498 

conditions  determining 511 


INDEX.  575 

PAGE 

Animal,  efficiency  of,  influence  of  fatigue  on 519 

gait  on 513 

grade  on 512 

individuality  on 517 

kind  of  work  on 512 

load  on 515 

size  on 515 

species  on 515 

speed  on 513 

training  on    519 

method  of  determining 498 

Antecedents  of  urea 42 

Aromatic  compounds  in  urine 46 

Ascent,  work  of,  in  dog,  consumption  of  oxygen  in 500 

utilization  of  energy  in 510 

by  dog 502 

horse 506 

man 503 

effect  of  grade  on 512 

load  on 509,  510,515 

Ash  ingredients,  balance  of 79 

Asparagin,  influence  of,  on  digestibility 54 

of  carbohydrates 57 

crude  fiber 57,  58 

nitrogen-free  extract 57 

fermentation 54 

of  carbohydrates 55 

nutritive  value  of 54 

oxidized  in  body 52 

replacement  of  proteids  by 54 

typical  of  non-proteids 8 

Aspartic  acid  formed  from  proteids 39 

oxidized  in  body 52 

Assimilation,  expenditure  of  energy  in  digestion  and.     372.  375 

tissue  building  and 491 

of  fat,  loss  of  energy  in 35 

work  of 337, 372,375 

digestion  and 80,  93,  406 

above  critical  point 407 

of  bone 381 

carbohydrates 379,  382,  384 

fat 378,  382,  384,  385 

mixed  diet 382,  384 

proteids 381,  382,  384 

indirect  utilization  of  heat  from 406 


576  INDEX 

PAOB 

Assimilation,  work  of,  digestion  and,  in  dog 378 

horse 385 

man 382 

methods  of  determining 377 

relation  of,  to  surface 408 

Availability  of  energy,  gross 270,  395 

for  maintenance 396,  406,  410,  413,  427,  497 

work 497 

net    394,412 

net,  of  energy,  determination  of 413, 427, 428 

in  carnivora 413,  427,  428 

herbivora 418,  427, 428 

distinction  between  utilization  and 395 

of  carbohydrates 417,  419,  427, 428 

crude  fiber 422,  428 

fat 416,  419, 427,  428 

organic  acids 423 

pentoses 420, 428 

proteids 414,  427,  428 

timothy  hay 424,  428 

influence  of  amount  of  food  on 430 

character  of  food  on 431 

relation  of  maintenance  ration  to 432 

Barley,  utilization  of  energy  of 483.  491 

Beet  molasses,  metabolizable  energy  of 293,  297,  301 

digestible  protein  of 318,  332 

utilization  of  energy  of 483.  490.  491 

Benzoic  acid,  formation  of  hippuric  acid  from 44 

Benzoyl  radicle  of  hippuric  acid,  source  of.  ... 44,  45 

Blood,  consumption  of  dextrose  of.  in  muscles 22 

parotid  gland 22 

dextrose  of 17, 18 

fat  production  from 23 

percentage  of 18 

variations  in 18 

fate  of  dextrose  of 22 

la;vulose  in 17 

peptones  absent  from 40 

regulation  of  supply  of  dextrose  to 18,  20 

Body,  animal,  components  of 1 

composition  of 60.  64,  66 

conservation  of  energy  in 228,  258 

liberation  of  energy  in 1 

store  of  energy  in 1 

transformations  of  energy  in 2 


INDEX.  577 

PAGE 

Body,  schematic 60, 66 

Bone,  work  of  digestion  and  assimilation  of 381 

Butyric  acid,  effect  of,  on  total  metabolism 158 

replacement  value  of 158 

Calorie 232 

Calorimeters,  animal 246 

Carbohydrate  radicle  in  proteids 50 

Carbohydrates 8 

apparent  digestibility  of,  influence  of  amides  on 57 

ammonium  salts  on.     57 

asparagin  on 57 

non-proteids  on 57 

as  source  of  energy  to  body 91 

consumption  of,  in  muscular  contraction 220 

digestible,  gross  energy  of 308 

metabolizable  energy  of 324.  327,  332 

utilization  of  energjr  of 475, 477, 490, 491 

disappearance  of,  in  fasting 85 

effects  of,  on  minimum  of  proteids 135 

proteid  metabolism 115 

compared  with  fat 127 

total  metabolism 146 

fermentation  of 12, 13 

influence  of  amides  on 55 

ammonium  salts  on 56 

asparagin  on 55 

non-proteids  on 55 

on  nutritive  value   13 

organic  acids  from 13,  26 

products  of 13 

formation  of  dextrose  from,  in  liver 19,  20,  21 

fat  from 24,  30.  165 

equation  for 24 

respiratory  quotient  in 179 

glycogen  from 20,  21 

milk  fat  from.  .    174 

hexose 8,  9 

formation  of  glycogen  from 20,  21 

metabolism  of.     See  Metabolism 

resorption  of        12.  17 

rate  of 18 

liver  as  reservoir  of 20 

metabolism  of      See  Metabolism 

mutual  replacement  of  fat  and 151 

net  available  energy  of 41 7.  419.  427,  428 


578  INDEX. 


Carbohydrates,  of  crude  fiber 9 

food,  replacement  of  proteids  by 149 

nitrogen-free  extract 9 

oxidized,  computation  of,  from  respiratory  quotient  76 

pentose 8,  9 

assimilability  of 25 

as  source  of  hippuric  acid 46 

determination  of 9 

digestibility  of 24 

effects  of,  on  proteid  metabolism 124 

total  metabolism 156 

formation  of  fat  from . .  . 183 

glycogen  from i 25,  26 

metabolism  of.     See  Metabolism 

of  crude  fiber 9 

nitrogen-free  extract 9 

oxidized  in  body 25 

replacement  value  of 156 

replacement  value  of 152 

resorption  of 12 

respiratory  quotient  of 74 

subdivisions  of 8,  9 

substitution  of,  for  body  fat 146 

utilization  of  energy  of 461,  462,  473,  490,  491 

value  of,  for  maintenance 400,402 

work  of  digestion  and  assimilation  of 379,  382,  384 

Carbon  balance  computation  of  fat  from 77 

heat  production  from  nitrogen  and 255 

dioxide,  determination  of,  in  respiration 69,  73 

produced  by  fermentation  of  carbohydrates 13 

production  of,  in  fasting 84 

metabolism 14, 15 

of  carbohydrates 23,  27 

fat 36 

proteids 42 

equilibrium,  amount  of  proteids  required  to  produce 105 

factor  for  computation  of  fat  from 62,  78 

income  and  outgo  of 69 

determination  of 69-73 

of  excreta,  determination  of 69 

metabolism,  effect  of  muscular  exertion  on 209 

(  nrnivora,  determination  of  net  availability  of  energy  in 413,  427.  428 

hippuric  acid  in  urine  of 44 

metabolizable  energy  of  food  of 272 

utilization  of  energy  by 448,  466 


INDEX.  579 

PAGE 

Cattle,  excretion  of  methane  by 243 

Cellulose,  effects  of,  on  proteid  metabolism 117 

total  metabolism 162 

fermentation  of 13 

formation  of  fat  from 181 

of  crude  fiber 9 

replacement  value  of 162 

Changes,  chemical,  during  muscular  contraction 186,  189 

thermal,  during  muscular  contraction 189 

Chymosin 40 

Circulation,  effects  of  muscular  exertion  on , 191 

work  of 341 

Cleavage  digestive  of  proteids 38 

purpose  of 38 

nitrogen  of  proteids 98 

cause  of 100,  101,  103 

effects  of  non-nitrogenous  nutrients  on 131 

independent  of  total  metabolism 99 

of  fat  in  digestion 12 

proteids  in  digestion 12, 38 

purpose  of 38 

products  rebuilding  of  proteids  from 40 

Cleavages,  influence  of,  on  computation  of  heat  production 253 

by  an  enzym 40 

Coarse  fodders,  expenditure  of  energy  of,  in  digestion,  assimilation,  and 

tissue  building 491 

metabolizable  energy  of. .  .285,  286,  287,  290,  297,  298,  300,  301 

digestible  protein  of 320,  332 

carbohydrates  of.  .327,332 

non-nitrogenous  matter  of  urine  derived  from 28 

relative  value  of  grain  and,  for  maintenance 433,  533,  537 

work  production 534,  537 

utilization  of  energy  of 484,  490,  491 

Coefficient  of  utilization  of  energy 444,  498 

Collagens 7 

composition  of 62 

Combustion,  heats  of 229 

Concentrated  feeding-stuffs,  metabolizable  energy  of 289,  297,  299 

digestible   protein 

of 315,332 

utilization  of  energy  of 472,  490,  491 

Conservation  of  energy 228 

in  animal  body 229,  258 

Contractile  substance  of  muscle 17 

Contraction,  muscular 185 


580  INDEX. 

PAGE 

Contraction,  muscular,  chemical  changes  during 186,  189 

consumption  of  carbohydrates  in 220 

dextrose  in 220,  221 

isometric 495 

isotonic 495 

oxidations  in,  incomplete 186 

oxygen  not  essential  to 188 

respiratory  quotient  of  muscle  in 187 

thermal  changes  during 189 

transformation  of  energy  in 495 

Creatin 46 

Creatinin 46 

in  perspiration 48 

Crude  fat 8 

fiber 9 

apparent  digestibility  of 12 

influence  of  amides  on 57,  58 

ammonium  acetate  on  .      57 

asparagin  on 57,  58 

non-pro teids  on 57,  58 

carbohydrates  of 9 

cellulose  of 9 

digestible,  gross  energy  of 303 

metabolizable  energy  of 329,  332 

digestive  work  for 389 

effect  of,  on  total  metabolism 161 

expenditure  of  energy  of,  in  digestion,  assimilation,  and  tissue 

building 494 

formation  of  fat  from 181 

f urf uroids  of 9 

ligneous  material  of 9 

modified  in  digestive  tract 12 

net  available  energy  of 422,  42S 

pentose  carbohydrates  of 9 

replacement  value  of 161 

value  of,  for  maintenance 435,  535,  537 

work  production 535,  537 

Descent,  work  of 509 

influence  of  grade  on 509 

Dextrose,  amount  of,  produced  by  liver 19 

consumption  of,  in  muscles 221 

muscular  contraction 220,  221 

formation  of  fat  from 23 

from  carbohydrates  in  liver 19,  20,  21 

fat 36,385 


INDEX.  581 

Dextrose,  formation  of,  from  fat  during  muscular  exertion 223 

equation  for 38  51 

in  liver 36>  37 

proteids 19,  21,  49,  50 

glycogen  from 20,  21,  22 

in  muscles 23 

importance  of  constant  supply  of 18  21 

liver  as  source  of 18  19  21  49 

on  carbohydrate  diet 19 

on  proteid  diet 19 

method  of  formation  of,  in  liver 20 

of  blood 17  i8 

consumption  of,  in  muscles 22 

parotid  gland 22 

fate  of 22 

fat  production  from 23 

percentage  of 18 

variations  in 18 

reconversion  of  glycogen  into 20  22  37 

regulation  of  supply  of,  to  blood 18  20 

resorption  of,  rate  of 18 

storage  of,  in  resting  muscle 222 

Digestibility 9 

apparent l0j  n 

determination  of 10, 11 

influence    of    metabolic    prod- 
ucts on 10 

of  carbohydrates,  influence  of  amides  on 57,  58 

ammonium  salts  on  57 
asparagin  on.  . .  .  57,  58 
non-proteids  on.    57,  58 

crude  fiber 12 

influence  of  amides  on 57,  58 

ammonium  acetate  on.     57 

asparagin  on 57,  58 

non-proteids  on 57  58 

nitrogen-free  extract 12 

influence  of  amides  on. .  .     57 
ammonium 

acetate  on  57 
asparagin  on.  57 
non-proteids 

on 57 

significance  of  results  on 11 

determination  of ,  q 


582  INDEX. 


Digestibility,  determination  of,  influence  of  products  of  metabolism  on .  .     10 

of  pentose  carbohydrates 24 

real 10,  H 

determination  of 10 

of  fat 10 

protein 10 

Digestion,  changes  in  proteids  during 12 

cleavage  of  fat  in 12 

proteids  in 12,  38 

purpose  of 38 

•    expenditure  of  energy  in  assimilation  and 337, 372 

tissue  building  and.   491 
influence  of  amides  on 54 


asparagin  on. 


54 


non-proteids  on 54 

peptones  produced  during 12 

proteoses  produced  during 12 

saponification  of  fat  in 12 

work  of 337,  372,  493 

assimilation  and 80,  93,  337,  372,  376,  406 

above  critical  point 407 

below  critical  point 406 

indirect  utilization  of  heat  from.  .  .  .  406 

in  the  dog 378 

the  horse 385 

man 382 

methods  of  determining 377 

of  bone 384 

carbohydrates 379,  382,  384 

fat 378,  382,  384,  385 

mixed  diet 382,  384 

proteids 381,  382,  384 

relation  of,  to  surface 408 

factors  of 374 

for  crude  fiber 389 

Digestive  tract,  functions  of,  in  excretion 10 

Dog,  consumption  of  oxygen  by,  in  locomotion 500 

work  of  ascent 500 

draft 501 

expenditure  of  energy  by,  in  locomotion 502 

utilization  of  energy  by,  in  muscular  work 499 

work  of  ascent 502 

draft 502 

work  of  ascent  by,  consumption  of  oxygen  in 500 

utilization  of  energy  in 502 


INDEX.  583 

PAGE 

Dog,  work  of  digestion  and  assimilation  in 378 

draft  by,  consumption  of  oxygen  in 501 

utilization  of  energy  in 502 

Draft,  work  of,  consumption  of  oxygen  in,  by  dog 501 

horse 507 

utilization  of  energy  in 502,  507,  510,  513 

by  dog 502 

horse 507,  510 

Dynamometer  for  experiments  on  horses 538,  539 

Dyne 231 

Efficiency  of  animal.     See  Animal. 

single  muscle 495 

Emission  of  heat,  rate  of,  influence  of  temperature  on 350 

regulation  of 349 

Energy , 226 

absorption  of,  in  anabolism 17 

available 269,  394 

gross 270,  395 

net 270, 395 

determination  of,  in  carnivora 413,  427,  428 

herbivora 418,  427, 428 

availability  of,  distinction  between  utilization  and 395 

for  maintenance 396,  406,  410,  413,  427,  497 

influence  of  amount  of  food  on 430 

character  of  food  on 431 

relation  of  maintenance  ration  to 432 

carbohydrates  as  source  of,  to  body 91 

coefficient  of  utilization  of 440 

conservation  of 228 

in  animal  body .  228,  258 

Atwater's  and  Benedict's  inves- 
tigations    265 

early  experiments 261 

Laulanie's  experiments  on.  .  .  .   265 

nature  of  evidence 259 

Rubner's  experiments  on 263 

expenditure  of,  by  the  body 2,  226,  336,  339 

in  digestion  and  assimilation 372,  375,  376 

and  tissue  building.  491 
method    of    deter- 
mining   377 

Energy,  expenditure  of,  in  locomotion ' 510 

by  dog 502 

horse,  at  a  trot 509,  510,  514 


584  INDEX. 

PAGET 

Energy,  expenditure  of,  in  locomotion,  by  horse,  at  a  walk  504,  506,  508,  510 

533,  539 

man 503 

influence  of  gait  on 513 

individuality  on 517 

load  on 509,510,515 

size  of  animal  on.  . . .   516 

species  on 516 

speed  on 513 

standing 499 

sustaining  load 508,  515 

influence  of  individuality  on .  .    518 

food  as  source  of 2,  269 

gross,  of  digestible  crude  fiber 303 

ether  extract 304 

nutrients 302,  306 

organic  matter 309 

income  and  expenditure  of 3,  226 

kinetic 226 

determination  of 245 

liberation  of,  in  animal  body 1 

loss  of,  in  assimilation  of  fat 35 

fermentations 374 

hippuric  acid 313,  322 

methane 310,  325,  330,  335 

tissue  building 444, 447 

warming  ingesta 374 

metabolizable 269,  270 

apparent 291 

factors  for 279,  281,  333 

Atwater's 281 

Rubner's 279,  333 

of  coarse  fodders. 285,  286,287,  290,  297,  298,  300,  301 

concentrated  feeding-stuffs 289,    297,  299 

digestible  carbohydrates 324,  332 

crude  fiber 329,  332 

ether  extract 323,  332 

nutrients 310,  332,  333 

organic  matter 297,  307 

protein. 310,  315,  317,  318,  320,  327,  332 
fiber-free  nutrients,  utilization  of,  in  work  pro- 
duction    541,  543,  547 

food  of  carnivora 272 

herbivora 281 

man 277,  280,  282 


INDEX.  585 


Energy,  metabolizable,  of  nutrients,  utilization  of,  in  work  production . . .  545 

proteids 272,  276,  277 

total  organic  matter 284,  285 

real 291 

utilization  of,  in  work  production 525,  540 

methods  of  de- 
termina- 
tion. . . .   526,  528 
Wolff's  investi- 
gations    528 

muscular,  fat  as  source  of 200,  223 

proteids  as  source  of 201,  207 

source  of 196 

starch  as  source  of '. 199 

nature  of  demands  for 340 

net  available 394 

determination  of 413,  427,  428 

for  maintenance 396, 406, 410, 413,  427,  497 

work 497 

of  carbohydrates 417, 419,  427,  428 

crude  fiber 422,  428 

fat 416,  419,  427,  428 

organic  acids 423 

pentoses 420.  428 

proteids 414, 427,  428 

timothy  hay 424,  428 

utilization  of,  in  work 497 

of  food 2 

protein,  losses  of,  in  methane 310 

urine 312 

potential 226 

determination  of 235 

of  combustible  gases 243 

excreta,  computation  of 241 

determination  of 240 

feces,  computation  of 242 

food,  determination  of 235 

gain  of  fat 244 

protein 244 

tissue 244 

perspiration 244 

urine 272,  275.  278,  312 

computation  of 241.  277,  312 

store  of,  in  animal  body 1 

transformation  of,  in  animal  body 2 


586  INDEX. 

PAGE 

Energy,  transformation  of,  in  muscular  contraction 495 

units  of  measurement  of 231,  233 

utilization  of,  in  tissue  building 444,  447,  448,  461 

by  carnivora 448, 466 

man 451 

ruminants 455,  461,  467 

swine 452,  466 

earlier  experiments  on 460 

effect  of  amount  of  food  on 466 

character  of  food  on  . . .  472 
differences    in    live 

weight  on 457 

thermal  environment  on  471 

work 444, 447, 494 

by  dog 494 

horse     502 

at  a  trot 509 

walk 504 

man 502 

influence  of  fatigue  on 519 

individuality  on 517 

kind  of  work  on 512 

load  on 508 

size  of  animal  on 515 

species  on 515 

speed  on 507,  513,  514 

training  on 519 

of  ascent 502,  503,  506,  510 

by  dog 502 

horse 506 

man 503 

effect  of  grade  on 51 2 

load  on 509,510,  515 

draft 502,  510,  .513 

by  dog 502 

horse 507 

locomotion,  computed.         513 

of  barley 483.  491 

beet  molasses 483,  490.  491 

carbohydrates 461,462,473,490,491 

coarse  fodders 484,  490,  491 

concentrated  feeding-stuffs 472,  490,  491 

digestible  carbohydrates 475,  477,  490,  491 

protein 481,491 

extracted  straw 488,  490,  491 


INDEX.  Shl 

PAGE 

Energy,  utilization  of,  of  meadow  hay 484,  490,  491 

mixed  grains 483,  491 

oat  straw 485,  490,  49  L 

oil 478,  490,  491 

proteids 463,  482,  491 

rice 483, 491 

starch 473,  490,  491 

wheat  gluten 480,  490,  491 

straw 487,  490.  491 

Environment,  thermal,  critical 358 

influence  of,  on  heat  production  in  fasting 347 

maintenance  ration 435 

utilization  of  energy 471 

Enzym,  rebuilding  of  proteids  from  cleavage  products  by 40 

Epidermis,  composition  of 63  • 

Ether  extract 8 

digestible,  gross  energy  of 304 

metabolizable  energy  of 323,  332 

Exchange,  gaseous,  computation  of  heat  production  from 249 

effect  of  load  on 509 

muscular  exertion  on 209 

respiratory,  determination  of 73 

in  fasting 84,  85 

intermediary  metabolism 405 

Excreta,  determination  of  carbon  of 69 

dioxide  in 69 

hydrocarbons  in 69,  72 

methane  in 69,  72 

hydrogen  in 72 

organic  acids  absent  from 27 

percentage  of  oxygen  in 15 

potential  energy  of,  computation  of 241 

determination  of 240 

total,  computation  of  heat  production  from 252 

Excretion,  functions  of  digestive  tract  in 10 

nitrogen,  ptoteid  metabolism  and 97 

rate  of 98 

effect  of  non-nitrogenous  nutrients  on 130 

of  free  nitrogen 42 

methane  by  cattle 243 

Exertion,  muscular  (see  also  Work) 185 

effects  of,  influence  of  amount  of  food  on 197 

on  carbon  metabolism 209 

circulation 191 

gaseous  exchange 209 


5  88  INDEX. 

PAQB 

Exertion,  muscular,  effects  of,  on  metabolism 185,  193 

proteid  metabolism 194,  206 

respiration 192 

respiratory  quotient 211,  212,  216 

work  of  heart 192 

formation  of  dextrose  from  fat  during 223 

functions  of  proteids  in 207 

gain  of  proteids  caused  by 204 

general  features  of 185 

intermediary  metabolism  during 219 

nature  of  non-nitrogenous  material  metabolized  in.  .   218 

respiratory  quotient  during,  conclusions  from 218 

secondary  effects  of 191 

Expenditure  of  energy.     See  Energy. 

Extracted  straw,  metabolizable  energy  of 290,  297,  300,  301 

digestible,  carbohydrates  of .  327,  332 

utilization  of  energy  of 488,  490,  491 

Extractives 7 

of  muscle  8 

Factor  for  computation  of  fat  from  carbon 62,  78 

non-proteids  from  nitrogen 8 

protein  from  nitrogen 67,  68,  77 

Factors  for  metabolizable  energy  of  digestible  nutrients 302,  332,  333 

human  food 279,  281 

protein  in  human  foods 6 

of  proteid  metabolism  in  fasting 81,  90 

work  of  digestion 374 

Fasting,  constant  loss  of  tissue  in 83 

disappearance  of  carbohydrates  in.    85 

glycogen  from  liver  in 21 

heat  production  in 344 

constancy  of 345 

influence  of  size  of  animal  on 359 

thermal  environment  on 347 

is  a  minimum 347,  356 

measure  of  internal  work 344 

metabolism  in 80,  90,  340 

effect  of  body  fat  on 88,  90 

loss  of  protein  on 90 

ratio  of  proteid  to  total 86, 88,  89,  90, 93 

total 83,  90 

proportional  to  active  tissue 86,  93 

of  fat  in 85,  88,  90 

proteids  in 81,  90 

minimum  of  proteids  less  than 136 


INDEX.  589 


PAGE 


Fasting,  metabolism  of  proteids  in,  tends  to  become  constant 81,  90 

two  factors  of 81,  90 

minimum  of  proteids  in 82,  83,  90,  94 

oxygen  consumption  in 84 

production  of  carbon  dioxide  in 84 

ratio  of  fat  to  protein  in  body  in 88,  89,  90 

respiratory  exchange  in 84,  85 

Fat '    8 

assimilation  of,  loss  of  energy  in 35 

as  source  of  muscular  energy 200,  223 

body,  effect  of,  on  fasting  metabolism 88,  90 

formation  of,  from  food  fat 164 

proteids  substituted  for 104 

replacement  of  proteids  by 149 

substitution  of  non-nitrogenous  nutrients  for 144 

carbohydrates  for 146 

fat  for 144 

cleavage  of,  in  digestion 12 

composition  of,  constancy  of 35 

from  different  animals 61 

parts  of  animal 33 

influence  of  feeding  on 32 

computation  of,  from  carbon  balance 77 

crude 8 

deposition  of  iodine  addition  products  of 31 

digestibility  of,  real . .  . 10 

effects  of,  on  proteid  metabolism 114 

compared  with  carbohydrates.  . .  .   127 

factor  for  computation  of,  from  carbon 62  78 

food,  formation  of  body  fat  from 164 

quantitative  relation  of,  to  fat  production 34 

replacement  of  proteids  by 149 

foreign,  deposition  of 30 

formation  of  dextrose  from 23,  385 

during  muscular  exertion 223 

equation  for 38,  51 

in  liver 36,  37 

from  carbohydrates 24,  30, 165 

equation  for 24 

respiratory  quotient  in 179 

cellulose 181 

crude  fiber 181 

dextrose  of  blood 23 

food  fat 164 

non-nitrogenous  nutrients  of  feeding-stuffs 180 


590  INDEX. 


Fat,  formation  of,  from  other  ingredients  of  food 163 

pentose  carbohydrates 183 

proteids 30,  50,  98,  107 

difficulty  of  proof 113 

equations  for .51 

later  experiments Ill 

Pettenkofer  and  Voit's  experiments.  .  .  .  108 

Pfliiger's  recalculations 109 

functions  of  food 30 

gain  or  loss  of,  determination  of 69,  77 

influence  of  glycogen  on  computation  of ,66,  78 

potential  energy  of 244 

influence  of,  on  minimum  of  proteids 135 

in  muscular  tissue 63,  64 

katabolism  of 35 

loss  of  energy  in  assimilation  of 35 

manufactured  in  body 29,  30,  163 

metabolism  of.     See  Metabolism. 

mutual  replacement  of  carbohydrates  and 151 

net  availability  of  energy  of 416,419,427,  428 

of  plant,  nature  of 8 

oxidized,  computation  of,  from  respiratory  quotient 76 

production,  quantitative  relation  of  food  fat  to 34 

ratio  of,  to  protein  in  body  in  fasting 88,  89,  90 

resorption  of 12,  30 

respiratory  quotient  of 74 

saponification  of,  in  digestion 12 

sources  of  animal 29,  30,  163 

substitution  of,  for  body  fat 144 

value  of,  for  maintenance 400,  402 

work  production 522 

work  of  digestion  and  assimilation  of 378,  382,  384,  385 

Fatigue,  influence  of,  on  utilization  of  energy  in  work 519 

Fattening,  influence  of,  on  maintenance  ration 441,  458 

Feces,  computation  of  potential  energy  of 242 

metabolic  nitrogen  in 47 

products  in 10,  47 

determination  of 10 

influence  of,  on  determination  of  digesti- 
bility      10 

nature  of 47 

nitrogenous 42 

Feeding,  influence  of,  on  composition  of  fat 32 

Feeding-stufffl,  concentrated,  expenditure  of  energy  of,  in  digestion,  assim- 
ilation, and  tissue  building 492 


INDEX.  59 1 


I'M  IK 


Feeding-stuffs,  concentrated,  metabolizable  energy  of 289,  297,  299 

digestible    protein 

of 315,332 

utilization  of  energy  of 472,  490,  491 

metabolizable  energy  of,  utilization  of,  in  work  production  540 
non-nitrogenous  nutrients  of,  effects  of,  on  total  metab- 
olism     154 

formation  of  fat  from.  ...    ISO 
mutual  replacement  of .  .  .    154 

non-proteids  in G 

protein  in 5 

Fermentation  of  carbohydrates 12, 13 

influence  of  amides  on 55 

ammonium  salts  on 5G 

asparagin  on 55 

non-proteids  on 55 

organic  acids  from 13,  26 

products  of 13 

cellulose 13 

Fermentations  in  digestive  tract 12, 13 

influence  of  amides  on 55 

ammonium  salts  on 56 

asparagin  on 54,  55 

non-proteids  on 55 

Fermentations,  influence  of,  on  nutritive  value  of  carbohydrates 13 

loss  of  energ}-  in 374 

Fiber,  crude.     See  Crude  Fiber. 

Flesh  bases 8 

Flesh,  proteid  metabolism  expressed  in  terms  of 68 

Food 5 

amount  of,  critical 408 

influence  of,  on  effects  of  muscular  exertion 197 

net  availability  of  energy 430 

utilization  of  energy 466 

as  source  of  energy 269 

character  of,  influence  of,  on  net  availability  of  energy 431 

utilization  of  energy 472 

composition  of 5 

digested 12 

consumption,  influence  of,  on  heat  production. 338,  372,  387 

metabolism 387 

energy  of 2 

functions  of 3 

fat,  functions  of 30 


592  INDEX. 

PAGE 

Food,  increases  metabolism 372 

ingredients,  heats  of  combustion  of 236 

metabolizable  energy  of.     See  Energy. 

nature  of 2 

potential  energy  of,  determination  of 235 

purposes  to  which  applied 80 

Foods,  heats  of  combustion  of 236,  237 

Food-supply,  relation  of  metabolism  to 93 

Foot-pound 231 

Force 226 

Furfuroids 9 

of  crude  fiber 9 

nitrogen-free  extract 9 

Gain  of  fat,  determination  of 69,  77 

influence  of  glycogen  on  computation  of 66,  78 

potential  energy  of 244 

nitrogen  by  body 66,  67 

protein  by  body 66 

during  work 204 

potential  energy  of 244 

tissue 59 

determination  of  60 

potential  energy  of 244 

Gait,  influence  of,  on  expenditure  of  energy  in  locomotion 513 

Gases,  combustible,  composition  of 243 

potential  energy  of 243 

Gelatinoids 7 

composition  of 62 

Globulin 7 

Glutaminic  acid  formed  from  proteids 39 

an  intermediate  product  of  proteid  metabolism 44 

Glycocol,  formation  of  hippuric  acid  from 44 

oxidized  in  body 52,  53 

Glycogen,  amount  of,  in  body 66,  78 

disappearance  of,  from  liver  in  fasting 21 

formation  of,  from  artificial  hexoses 20 

carbohydrates,  hexose 20,  21 

pentose 25,  26 

dextrose 20,  21,  22,  23 

hexoses 20,  21 

pentoses 25,  26 

proteids 21, 98 

in  liver.      20, 21,  22 

muscles 23 

identical,  from  different  hexoses 20 


INDEX.  593 

PAGE 

Glycogen,  identical,  from  hexoses  and  pentoses 26 

influence  of,  on  computation  of  gain  or  loss  of  fat 66,  78 

in  muscular  tissue 64 

muscular,  disappearance  of,  in  work 23 

functions  of 222,  223 

reappearance  of,  in  rest 23 

reconversion  of,  into  dextrose 20,  22,  37 

Grade,  influence  of,  on  efficiency  of  animal 512 

utilization  of  energy  in  work  of  ascent 512 

work  of  descent 509 

Grain,  relative  effects  of  hay  and,  on  metabolism 388 

value  of  coarse  fodder  and,  for  maintenance 433,  533,  537 

work  production 533 

Gram-meter 231 

Gravity,  force  of 231 

Hair,  composition  of 63 

Hay,  relative  effects  of  grain  and,  on  metabolism 388 

Heart,  work  of,  influence  of  muscular  exertion  on 192 

Heat,  animal  source  of 261 

determination  of 245 

emission  and  heat  production 256 

influence  of  insolation  on 357 

relative  humidity  on 358 

wind  on 357 

method  of,  above  critical  temperature 355 

rate  of,  influence  of  temperature  on 350 

regulation  of 349 

from  digestive  work,  indirect  utilization  of 406 

production 338 

and  heat  emission 256 

■computation  of 249 

from  carbon  and  nitrogen  balance ....  255 

gaseous  exchange 249 

total  excreta 252 

influence  of  cleavages  on 253 

hydrations  on 253 

determination  of 245 

in  fasting 344 

constancy  of 345 

influence  of  size  of  animal  on 359 

thermal  environment  on 347 

is  a  measure  of  internal  work 344 

minimum 347,  356 

influence  of  consumption  of  food  on 338, 372,  387 

water  on 438 


594  INDEX. 

PAGE 

Heat  production,  influence  of  muscular  tonus  on 191 

species  on 369 

temperature  on 351 

thermal  environment  on 358 

time  element  on 439 

in  intermediary  metabolism 405 

on  maintenance  ration 436,  437 

relation  of,  to  mass  of  tissue 370 

surface 359 

variations  in 351 

causes  of 363 

mechanism  of 352 

regulation  of  rate  of  emission  of 349 

Heats  of  combustion 229 

computation  of 239 

of  foods 236,  237 

food  ingredients 236,  237 

organic  substances 237 

Heat,  units  of 232 

Hexosans 8 

Hexoses 8 

artificial,  formation  of  glycogen  from 20 

formation  of  glycogen  from 20,  21 

Herbivora,  determination  of  net  availability  of  energy  in 418,  427,  428 

hippuric  acid  in  urine  of 44 

metabolizable  energy  of  food  of 281 

minimum  of  proteids  for 140 

Hippuric  acid 44 

composition  of 44 

formation  of,  from  benzoic  acid 44 

glycocol 44 

in  urine 44 

loss  of  energy  in 313,  322 

non-nitrogenous  nutrients  as  source  of 45 

origin  of 44 

pentose  carbohydrates  as  source  of 46 

source  of  benzoyl  radicle  of 44,  45 

Hoof,  composition  of 63 

Horn,  composition  of 63 

Horse,  consumption  of  oxygen  in  locomotion  by 504,  506,  507 

maintenance  requirement  of 531,  537 

utilization  of  energy  by,  in  work 502 

at  a  trot 509 

walk 504 

of  ascent - 506 


INDEX.  595 

PAGE 

Horse,  utilization  of  energy  by,  in  work,  of  draft 507 

work  of  digestion  and  assimilation  in 385 

locomotion  in 504,  506,  508,  509,  510,  514,  535,  539 

Human  food,  metabolizable  energy  of 277,  280,  282 

foods,  protein  factors  for 6 

Humidity,  relative,  influence  of,  on  heat  emission 358 

Hydrations,  influence  of,  on  computation  of  heat  production 253 

Hydrocarbons  of  excreta,  determination  of 69,  72 

Hydrogen  balance 78 

in  excreta 72 

Income  and  expenditure  of  energy 3}  226 

matter 3;  5 

Individuality,  influence  of,  on  expenditure  of  energy  in  locomotion 517 

sustaining  load. .  .   518 

utilization  of  energy  in  work 517 

Indol  in  urine 46 

Ingesta,  warming,  loss  of  energy  in 374 

Insolation,  influence  of,  on  heat  emission 357 

Investigation,  methods  of 59,  234 

Katabolism 16, 17 

of  fat 35 

proteids 41 

excretory  nitrogen,  measure  of 42 

final  products  of 41 

Keratin,  composition  of 62 

Kilogram-meter 231 

Kilojoule 231,  232 

Lactic  acid,  effect  of,  on  proteid  metabolism 123 

total  metabolism 158 

production  of,  in  metabolism  of  carbohydrates 23 

replacement  value  of 158 

Lsevulose  in  blood 17 

Leucin  formed  from  proteids 39 

oxidized  in  body 52 

Ligneous  material  of  crude  fiber 9 

Liver  as  reservoir  of  carbohydrates 20 

source  of  dextrose 18, 19,  21, 49 

on  carboydrate  diet 19 

proteid  diet 19,  49 

disappearance  of  glycogen  from,  in  fasting 21 

formation  of  dextrose  in,  from  carbohydrates 19,  20,  21 

fat 21,36,37 

proteids 19,  21, 45,  50 

method  of 20 

glycogen  in 20,  21,  22 


596  INDEX. 

PAGE 

Liver,  formation  of  glycogen  in,  from  dextrose 20,  21,  22 

proteids 21 

sugar  in 18, 19,  21, 49,  50 

functions  of 18 

in  work  production 220 

glycogenic  function  of 21 

reconversion  of  glycogen  to  dextrose  in 20,  22,  37 

Live  weight,  influence  of,  on  maintenance  ration 458 

utilization  of  energy 457 

Load,  effect  of,  on  expenditure  of  energy  in  locomotion 509,  510,  515 

total  metabolism 509,  515 

utilization  of  energy 508 

in  work  of  ascent 509,  510,  515 

expenditure  of  energy  in  sustaining 508,  515 

influence  of  individuality  on .. .   518 

Locomotion,  consumption  of  oxygen  in,  by  dog 500 

horse 504,  506,  507 

expenditure  of  energy  in 499,  510 

by  dog 502 

horse  at  a  trot 509,  510,  514 

walk.. 504,  506,  508,  510, 
533,  539 

man 503 

influence  of  gait  on 513 

individuality  on 517 

load  on 59,   510,   515 

size  of  animal  on 516 

species  on 516 

speed  on  . . .   507,  508,  513 
work  of.     See  Work  of  Locomotion. 

Loss  of  fat,  determination  of 69.  77 

influence  of  glycogen  on  computation  of 66.  78 

nitrogen  by  body 66,  67 

protein  by  body 66 

tissue 59 

constant,  in  fasting 83 

determination  of 60 

Maintenance 394 

availability  of  energy  for 396,  406,  410,  413,  427,  497 

isodynamic  values  for 397 

isoglycosic  values  for 400 

ration 432 

heat  production  on 436,  437 

of  horse 531 ,  537 

influence  of  consumption  of  water  on 438 


INDEX.  597 

PAGE 

Maintenance  ration,  influence  of  fattening  on 441,  458 

live  weight  on 458 

shearing  on 436 

size  of  animal  on 440 

thermal  environment  on 435 

time  element  on 439 

relation  of,  to  net  availability  of  energy 432 

relative  value  of  grain  and  coarse  fodder  for 433,  533,  537 

value  of  carbohydrates  for 400-402 

crude  fiber 435 

fat  for 400,  402 

Man,  expenditure  of  energy  by,  in  locomotion 503 

hippuric  acid  in  urine  of 44 

metabolizable  energy  of  food  of 277,  280,  282 

utilization  of  energy  by,  in  muscular  work 503 

tissue  building 451 

work  of  ascent 503 

work  of  digestion  and  assimilation  in 382 

Mastication,  work  of 391 

Matter,  income  and  expenditure  of 3,5 

Meadow  hay,  metabolizable  energy  of 286,  290,  297,  300,  301 

utilization  of  energy  of 484,  490,  491 

Metabolic  products,  nitrogenous,  in  feces 42 

Metabolism 14 

a  gradual  process 16 

an  analytic  process 15 

a  process  of  oxidation 15 

carbon  dioxide  produced  in 14,  15 

carbon,  effects  of  muscular  exertion  on 209 

consumption  of  oxygen  in 14,  15,  16 

effects  of  muscular  exertion  on 185,  193 

non-nitrogenous  nutrients  on 114,  125 

proteid  supply  on 94,  104 

excretory  products  of 14 

fasting 80,  90,  340 

effect  of  body  fat  on 88,  90 

loss  of  protein  on 90 

ratio  of  proteid  to  total 81,  88,  89,  90,  93 

total 83,  90 

proportional  to  active  tissue 86,  93 

fat,  in  fasting 85,  88,  90 

food  increases 372 

glandular,  similar  to  muscular 344 

influence  of  food  consumption  on 387 

muscular  exertion  upon 185.  193 


598  INDEX. 

PAGE 

Metabolism  in  muscular  tonus 190 

intermediary 91 

during  muscular  exertion 219 

heat  production  in 405 

of  fat 91 

protein 91 

respiratory  exchange  in 405 

intermediate  products  in 16,  44 

muscular,  nature  of 495 

of  amides 52 

carbohydrates 15,  17 

hexose 17 

pentose 24 

production  of  carbon  dioxide  in 23,  27 

lactic  acid  in 23 

water  in 23,  27 

fat 15,29 

intermediary 91 

production  of  carbon  dioxide  in 36 

water  in 36 

non-proteids 52 

organic  acids 26 

proteids 15,  38 

products  of,  in  feces 10,  47 

determination  of 10 

influence  of,  on  determination  of  diges- 
tibility       10 

nature  of 47 

proteid 15,  38 

and  nitrogen  excretion 97 

determined  by  supply 128 

effects  of  acetic  acid  on 123 

carbohydrates  on 115 

compared  with  fat 127 

cellulose  on 117 

excess  of  proteids  on 96 

fat  on 114 

compared  with  carbohydrates 127 

lactic  acid  on 123 

muscular  exertion  on 194,  206 

influence  of  amount 

of  food  on 197 

non-nitrogenous  nutrients  on 114, 125 

duration  of .  .    128 
magnitude  of  128 


INDEX.  599 

PAGE 

Metabolism,  proteid,  effects  of  organic  acids  on ' .  .   123 

pentose  carbohydrates  on 124 

proteid  supply  on 94 

starch  on 116 

sugars  on 116 

expressed  in  terms  of  flesh 68 

glycocol  intermediate  product  of 44 

identity  of,  in  different  animals 317,  335 

on  different  feeds 322 

in  fasting 81,  90 

minimum  of  proteids  less  than 136 

tends  to  become  constant 81,  90 

two  factors  of 81,  90 

intermediate  products  in 44 

intermediary 91 

production  of  carbon  dioxide  in 42 

phosphoric  acid  in 42 

sulphuric  acid  in 42 

urea  in 42 

water  in 42 

ratio  of,  to  total,  in  fasting 86,  88,  89,  90,  93 

urea  as  measure  of 68 

relations  of,  to  food  supply 93 

relative  effects  of  hay  and  grain  on 388 

total,  computation  of 78 

effect  of  acetic  acid  on 160 

butyric  acid  on 158 

cellulose  on 162 

crude  fiber  on 161 

lactic  acid  on 1 58 

load  on 509,  515 

non-nitrogenous  ingredients  of  feeding-stuffs 

on 154 

nutrients  on 144,  154 

organic  acids  on 157 

pentose  carbohydrates  on 156 

proteid  supply  on 104 

rhamnose  on 15  " 

in  fasting 83,  90 

proportional  to  active  tissue 86,  9. 1 

nitrogen  cleavage  of  proteids  independent  of 99 

ratio  of,  to  "proteid,  in  fasting 86,  88,  89,  90 

urea  produced  in 14, 15 

water  produced  in 14,  15 

Methane,  excretion  of,  by  cattle 243 


6oo  INDEX. 

PAGE 

Methane  in  excreta,  determination  of 69,  72 

losses  of  energy  in 310,  325,  328,  330,  335 

produced  by  fermentation  of  carbohydrates 13 

Mit  hods  of  investigation 234 

Milk  fat,  formation  of,  from  carbohydrates 174 

Minimum  demands  of  vital  functions 80 

of  proteids 133 

amount  of  non-nitrogenous  nutrients  required  to 

reach 139 

effect  of  carboydrates  on 136 

fat  on ^ 135 

non-nitrogenous  nutrients  on 134 

on  health 143 

for  herbivora 140 

in  fasting 82,  83,  90,  94 

less  than  fasting  metabolism 136 

Mixed  diet,  work  of  digestion  and  assimilation  of 382,  384 

grains,  utilization  of  energy  of 483,  491 

Miickcrn  experiments 281,  455 

Motor,  efficiency  of  animal  as 498 

Mucins,  composition  of 62 

Muscle,  consumption  of  dextrose  in 22,  221 

contractile  substance  of ' 17 

efficiency  of  single 495 

extractives  of 8 

formation  of  glycogen  in 23 

respiratory  quotient  of 187 

resting,  storage  of  dextrose  in 222 

oxygen  in 222 

voluntary,  work  of 337 

Nails,  composition  of 63 

Nitrogen-free  extract,  apparent  digestibility  of 12 

influence  of  amides  on 57 

ammonium 

acetate  on.  57 
asparagin  on.  57 
non-proteids 

on 57 

carbohydrates  of 9 

digestible,  gross  energy  of 305,  306 

furfuroids  of 9 

pentose  carbohydrates  of 9 

Nitrogen  balance,  computation  of  heat  production  from  carbon  and 255 

cleavage  of  proteids 98 

cause  of 100, 101,  103 


INDEX.  60 1 

PAGE 

Nitrogen  cleavage  of  proteids,  effects  of  non-nitrogenous  nutrients  on 131 

independent  of  total  metabolism 99 

content  of  proteids 39 

equilibrium,  amount  of  proteids  required  to  reach 94 

estimation  of  protein  from 5,  6 

excretion,  effect  of  proteids  on 94,  96 

of  free 42 

proteid  metabolism  and 97 

rate  of 98 

effects  of  non-nitrogenous  nutrients  on 130 

excretory,  measure  of  proteid  katabolism 42 

factor  for  computation  of  non-proteids  from 8 

protein  from 67,  68,  77 

gain  or  loss  of,  by  body 66,  67 

income  and  outgo  of 66 

in  perspiration 48 

metabolic,  in  feces.  .'. 47 

percentage  of,  in  body  protein 62,  65 

proteids 6,  7 

protein .' 6,  62,  65 

Non-proteids 7 

asparagin  typical  of 8 

determination  of 8 

factor  for  computation  of,  from  nitrogen 8 

in  feeding-stuffs 6 

influence  of,  on  digestion 54 

apparent  digestibility  of  carbohydrates ...     57 
crude  fiber  ...   57,  58 
nitrogen-free  ex- 
tract       57 

fermentation  of  carbohydrates 55 

fermentations  in  digestive  tract 54 

of  animal  body 8 

oxidized  in  body 52 

metabolism  of 52 

nature  of 7 

not  synthesized  to  proteids 53 

produced  by  cleavage  of  proteids 7,8 

replacement  of  proteids  by 53 

resorption  of 12 

Nutrients,  available 10 

digestible,  energy  of 302,  306 

gross 302 

metabolizable 310,  332,  333 

factors  for 302,  332,  333 


602  INDEX. 

PAGE 

Nutrients,  fiber-free,  utilization  of,  in  work  production 541,  543 

computed 547 

isodynamic  replacement  of 152 

isoglycosic  replacement  of 153 

metabolizable  energy  of,  utilization  of,  in  work  production ....   545 

modified  in  digestive  tract 12 

mutual  replacement  of 148 

non-nitrogenous,  amount  of,  required  to  reach  minimum  of 

proteids 139 

as  source  of  hippuric  acid 45 

effects  of,  on  metabolism 114, 125 

minimum  of  proteids 134 

nitrogen  cleavage  of  proteids.  .  .    131 

proteid  metabolism 114, 125 

magnitude 

of 128 

duration  of.   1 28 

rate  of  nitrogen  excretion 130 

total  metabolism 144,  154 

formation  of  fat  from 162 

of  feeding-stuffs,  formation  of  fat  from 180 

mutual  replacement  of 154 

substitution  of,  for  body  fat 144 

utilization  of  excess  of 162 

percentage  of  oxygen  in 5 

relative  values  of 152 

in  work  production 522 

replacement  values  of 152,  396 

Nutrition,  function  of 2 

statistics  of 3 

Oat  straw,  metabolizable  energy  of 290,  297,  300  301 

utilization  of  energy  of 485,  490,  491 

Oil,  metabolizable  energy  of 296,  323,  332 

utilization  of  energy  of 478,  490,  491 

Organic  acids,  absence  of,  from  excreta 27 

net  availability  of  energy  of 423 

oxidized  in  body 27 

matter,  digestible,  gross  energy  of 309 

metabolizable  energy  of 297,  307 

utilization  of  energy  of 490 

total,  metabolizable  energy  of 284,  285 

utilization  of  energy  of 455,461,  490 

substances,  heats  of  combustion  of 237 

Oxidations  incomplete  in  muscular  contraction 186 

Oxygen  balance 79 


INDEX.  603 

PAGE 

Oxygen,  consumption  of,  determination  of 70,  71,  73,  79 

in  fasting 84 

locomotion,  by  dog, 500 

horse 504,  506,  507 

metabolism 14,  15,  16 

work  of  ascent  by  dog 500 

horse 506 

draft  by  dog 501 

horse 507 

not  essential  to  muscular  contraction 188 

percentage  of,  in  excreta 15 

nutrients 15 

storage  of,  in  resting  muscle 222 

Parotid  gland,  consumption  of  dextrose  of  blood  in 22 

Peanut' oil,  metabolizable  energy  of 296,  323,  332 

Pentosans 8 

oxidized  in  body 26 

Pentose  carbohydrates.     See  Carbohydrates. 

Pentoses 8 

formation  of  glycogen  from 25,  26 

in  urine 25,  26 

metabolism  of 24 

net  availability  of  energy  of 420,  428 

oxidized  in  body 25 

Peptones  absent  from  blood 40 

formed  from  proteids 12,  38,  39 

produced  during  digestion 12 

synthesis  of,  to  proteids  by  an  enzym 40 

Perspiration,  ammonium  salts  in 48 

creatinin  in 48 

nitrogen  in 48 

nitrogenous  matter  in 42 

potential  energy  of 242 

proteids  in 48 

urea  in 48 

uric  acid  in 48 

Phenols  in  urine 27,  46 

Phosphoric  acid,  production  of,  in  metabolism  of  proteids 42 

Phosphorus  balance 79 

Plastein 40 

Proteids 6 

albumoses  formed  from 38,  39 

amides  formed  from 7,  39,  52 

not  synthesized  to 53 


604  INDEX. 

PAGE 

Proteids,  amount  of,  required  to  produce  carbon  equilibrium 105 

nitrogen  equilibrium 94 

anabolism  of 38,  41 

aspartic  acid  formed  from 39 

as  source  of  muscular  energy 201,  207 

body,  and  food  proteids 40 

carbohydrate  radicle  in 50 

changes  in,  during  digestion 12 

classification  of 7 

cleavage  of,  in  digestion 12,  38 

purpose  of 38 

non-proteids  produced  by 7,  8 

differences  in 39 

effect  of  excess  of,  on  proteid  metabolism 96 

on  formation  of  hippuric  acid 463 

metabolism 94,  104 

nitrogen  excretion 94,  96 

proteid  metabolism 94 

total  metabolism 104 

food,  and  body  proteids 40 

formation  of  dextrose  from,  in  liver 19,  21,  49,  50 

fat  from 30,  50, 98, 101 

difficulty  of  proof  of 113 

equations  for 51 

later  experiments Ill 

Pettenkofer  and  Voit's  experiments.  . .  .    108 

Pfliiger's  recalculations 109 

glycogen  from 21,  98 

sugar  from 19,  21, 49,  50 

functions  of,  in  muscular  exertion 207 

gain  of,  during  work 204 

glutaminic  acid  formed  from 39 

in  perspiration 48 

intermediary  metabolism  of 91 

katabolism  of 41 

excretory  nitrogen  measure  of *. 42 

final  products  of 41 

leucin  formed  from 39 

metabolizable  energy  of 272,  276,  277 

metabolism  of.     See  Metabolism. 

minimum  of 133 

amount  of  non-nitrogenous  nutrients  required  to 

reach 139 

effect  of  non-nitrogenous  nutrients  on 134 

effects  of,  on  health 143 


INDEX.  605 


PAGE 


Proteids,  minimum  of,  for  herbivora 140 

in  fasting 82,  83,  90, 94 

influence  of  carbohydrates  on 136 

fat  on 135 

less  than  proteid  metabolism  in  fasting 136 

molecular  weight  of 15 

nature  of 39 

net  availability  of  energy  of 414,  427,  428 

nitrogen  cleavage  of 98 

cause  of 100,  101, 103 

effects  of  non-nitrogenous  nutrients  on. ...    131 

independent  of  total  metabolism 97 

content  of 6,  7,  39 

non-nitrogenous  residue  of 48,  98 

fate  of 49,  98 

formation  of  sugar  from 49,  50,  98 

non-proteids  not  synthesized  to 53 

peptones  formed  from 12,  38,  39 

percentage  of  nitrogen  in 6,  7 

proteoses  formed  from 12,  39 

putrefaction  of,  in  intestines 44, 46 

products  of '.   44,  46 

rebuilding  of,  from  cleavage  products 40 

replacement  of,  by  amides 53 

asparagin 54 

body  fat 149 

fats  and  carbohydrates  of  food 149 

non-proteids 53 

resorption  of 12 

respiratory  quotient  of 74,  75 

synthesis  of  peptones  to 40 

substituted  for  body  fat 104 

Proteid  supply,  effects  of,  on  metabolism 94,  104 

proteid  metabolism 94 

total  metabolism 104 

Proteids,  terminology  of 5,  7 

transitory  storage  of 96 

tyrosin  formed  from 39 

utilization  of  energy  of 482,  491 

excess  of 107 

work  of  digestion  and  assimilation  of. 381,  382,  384 

Protein,  circulatory 82 

composition  of 62 

digestibility  of,  real 10 

digestible,  gross  energy  of 309 


606  INDEX 


Protein,  digestible,  met abolizable  energy  of . .   310,315,317,318,320,327,332 

utilization  of  energy  of 481,  491 

estimation  of,  errors  in 6 

from  nitrogen 5,  6 

tact  or  for  computation  of,  from  nitrogen 6,  67,  68,  77 

in  human  foods ,6 

gain  or  loss  of,  by  body 66 

potential  energy  of 244 

in  feeding-stuffs 5 

loss  of,  in  fasting,  effect  of,  on  metabolism 90 

energy  of,  in  methane 310 

urine 312 

nature  of 5 

of  body,  composition  of 62,  65,  66 

percentage  of  nitrogen  in 62,  65 

organized 82 

percentage  of  nitrogen  in 6,  62,  65 

ratio  of  fat  to,  in  body  in  fasting 88,  89,  90 

real  digestibility  of 10 

storage,  cause  of 102 

extent  of 132 

terminology  of 6,  7 

Proteoses,  formed  from  proteids 8,  12,  39 

produced  during  digestion 12 

Putrefaction  of  proteids  in  intestines 44,  46 

products  of 44,  46 

Quotient,  respiratory 74 

change  in,  caused  by  work 212 

computation  from,  of  carbohydrates  oxidized 76 

fat  oxidized 76 

deductions  from 75 

during  work,  conclusions  from 75 

effects  of  muscular  exertion  on 211 

in  fat-formation  from  carbohydrates 179 

of  carbohydrates 74 

fat 74 

muscle 187 

influence  of  contraction  on 187 

proteids 74,  75 

variations  of 211 

during  work 216 

Range,  thermic 348 

Rate  of  nitrogen  excretion 98 

Ration,  maintenance.     See  Maintenance. 

Regulation  of  body  temperature 347,  348 


INDEX.  607 

VaGE 

Regulation  of  body  temperature,  chemical 352 

means  of 348 

physical 351 

emission  of  heat 349 

Rennet  ferment,  functions  of 40,  41 

Replacement,  isodynamic,  law  of 152,  399 

isoglycosic,  law  of 153,  399 

mutual,  of  fat  and  carbohydrates 151 

non-nitrogenous  ingredients  of  feeding-stuffs.  .  .    154 

nutrients 148 

of  proteids  by  amides 53 

asparagin 54 

body  fat 149 

carbohydrates  and  fat  of  food 149 

non-proteids 53 

value  of  acetic  acid 160 

butyric  acid 158 

carbohydrates 152 

cellulose 162 

crude  fiber 161 

lactic  acid 158 

non-nitrogenous  ingredients  of  feeding-stuffs 154 

nutrients 152 

organic  acids 157 

pentose  carbohydrates 156 

rhamnose 156 

Residue,  non-nitrogenous,  of  proteids 48.  98 

fate  of 49,  98 

formation  of  sugar  from 49,  50,  98 

Resorption  of  carbohydrates 12 

hexose 12,  17 

rate  of 18 

dextrose,  rate  of 18 

fat 12,30 

non-p/oteids 12 

proteids 12 

Respiration  apparatus 69 

determination  of  water  by 79 

Pettenkofer  type  of 70 

Regnault  type  of 69 

Zuntz  type  of 72 

Respiration-calorimeter .   246,  248 

Respiration,  determination  of  products  of 69,  73 

effects  of  muscular  exertion  on 192 

work  of 193  341 


608  INDEX. 


Respirator}'  exchange,  determination  of 73 

in  intermediary  metabolism 405 

Rest,  reappearance  of  muscular  glycogen  in 23 

Rhamnose,  effect  of,  on  total  metabolism 156 

replacement  value  of 156 

Rice,  utilization  of  energy  of 483,  491 

Ruminants,  utilization  of  energy  in 455,  461,  467 

S:irk< isin  oxidized  in  body 53 

Saponification  of  fat  in  digestion 12 

Schematic  body 60,  66 

Shearing,  influence  of,  on  maintenance  ration 436 

Size  of  animal,  influence  of,  on  efficiency  of  animal 515 

expenditure  of  energy  in  locomotion 516 

heat  production 359 

in  fasting 359 

maintenance  ration 440 

relation  of,  to  physiological  activities 368 

Species,  comparison  of  heat  production  of 369 

influence  of,  on  efficiency  of  animal 515 

expenditure  of  energy  in  locomotion 511 

Speed,  correction  for,  in  work  of  locomotion 507,  508 

influence  of,  on  expenditure  of  energy  in  locomotion 507,  508,  513 

utilization  of  energy  in  work 507,  513,  514 

Standing,  expenditure  of  energy  in 343,  499 

Starch,  as  source  of  muscular  energy 199 

digestible,  gross  energy  of 306 

metabolizable  energy  of 324,  332 

utilization  of  energy  of 475,  477,  490 

effect  of,  on  proteid  metabolism 116 

metabolizable  energy  of 294,  297,  301 

utilization  of  energy  of 473,  490,  491 

States,  initial  and  final,  law  of 228 

Statistics  of  nutrition 3 

Storage  of  protein,  extent  of 132 

transitory 96 

cause  of 102 

Straw,  extracted,  gross  energy  of  carbohydrates  of 308 

metabolizable  energy  of 290,  297,  300,  301 

carbohydrates  of 327 

utilization  of  energy  of 488,  490,  491 

oat,  metabolizable  energy  of 290,  297,  300,  301 

protein  of 321 

carbohydrates  of 329 

utilization  of  energy  of 485,  490,  491 

wheat,  metabolizable  energy  of 290,  297,  300,  301 


INDEX  609 

PAGE 

Straw,  wheat,  metabolizable  energy  of  protein  of 321 

carbohydrates 329 

utilization  of  energy  of 487,  490,  491 

Sugar,  effect  of,  on  proteid  metabolism 116 

formation  of,  from  non-nitrogenous  residue  of  proteids 49,  50,  98 

proteids 19,  21,  49,  50 

in  liver 18,  19,  21,  49,  50 

Sulphur  balance 79 

Sulphuric  acid,  conjugated,  in  urine 46 

production  of,  in  metabolism  of  proteids 42 

Surface  of  animal,  computation  of 364 

relation  of  heat  production  to 359 

internal  work  to 366 

to  work  of  digestion  and  assimilation 408 

Swine,  utilization  of  energy  by 452,  466 

Temperature,  body 347 

regulation  of 347,  348 

chemical 352 

means  of 348 

physical 351 

critical 353 

method  of  heat  emission  above 355 

modification  of  conception  of 357 

influence  of,  on  heat  production 351 

rate  of  emission  of  heat 350 

Thermal  environment,  critical 358 

influence  of,  on  heat  production  in  fasting 347 

maintenance  ration 435 

utilization  of  energy 471 

Thermic  range 348 

Thermo-chemistry 228 

Time  element,  influence  of,  on  heat  production 439 

maintenance  ration 439 

Timothy  hay,  metabolizable  energy  of 287,  290,  297,  301 

net  availablity  of  energy  of 424,  428 

Tissue 59 

active,  fasting  metabolism  proportional  to 86,  93 

adipose 29 

building,  expenditure  of  energy  in  digestion,  assimilation,  and  ...   491 

loss  of  energy  in 444,  447 

utilization  of  energy  in 444, 447, 448,  461 

by  carnivora 448,  466 

man 451 

ruminants 455,  461,  467 

swine 452, 466 


610  INDEX. 

PAGE 

Tissue,  building,  utilization  of  energy  in,  earlier  experiments  on 460 

effect  of  amount  of  food  on  ...  .  466 
character  of  food  on .  .  .  472 
differences  in  live  weight 

on 457 

thermal  environment  on  471 

constant  loss  of,  in  fasting 83 

gain  of  potential  energy  in 244 

gains  and  losses  of 59 

determination  of 60 

mass  of,  relation  of  heat  production  to 370 

muscular,  composition  of 63,  64 

fat  in 63,  64 

glycogen  in 64 

heat  of  combustion  of 63,  64 

Tonus,  muscular 190 

influence  of,  on  heat  production 191 

metabolism  in 190 

work  of 341 

Training,  influence  of,  on  utilization  of  energy  in  work 519 

Transformation  of  energy  in  body 2 

muscular  contraction 495 

Trot ,  expenditure  of  energy  in  locomotion  at 509,  510,  514 

utilization  of  energy  in  work  at 509,  510 

Tvrosiii,  formed  from  proteids 39 

oxidized  in  body 52 

Units  of  heat 232 

measurement  of  energy 231,  233 

Urea 42 

antecedent  of 42 

ammonium  carbonate  as 43 

lactate  as 43 

as  measure  of  proteid  metabolism 68 

in  perspiration 48 

production  of,  from  amides 52 

in  metabolism 14, 15 

of  proteids 42 

Uric  acid 43 

origin  of 43 

in  perspiration 48 

urine 43 

Urine,  aromatic  compounds  in 46 

computation  of  potential  energy  of 241,  313 

conjugated  sulphuric  acid  in ; 46 


INDEX.  611 

PAGE 

Urine,  hippuric  acid  in 44 

indol  in 46 

losses  of  energy  of  protein  in 312 

non-nitrogenous  matter  of 27,  312,  320 

amount  of 28 

derived  from  coarse  fodders 28 

non-nitrogenous  matter. .   321 

influence  of 320 

source  of 27,  321 

pentoses  in 25,  26 

phenols  in 27,  46 

potential  energy  of 272,  275,  278,  312 

computation  of 241,  277,  312 

uric  acid  in 43 

Utilization  of  energy.     See  Energy. 

Values,  isodynamic 397,  399 

isoglycosic 399,  400 

replacement 396 

modified  conception  of 405 

of  nutrients .' 396 

Variations  in  heat  production,  causes  of 363 

Walking,  consumption  of  oxygen  in,  by  horse 505 

expenditure  of  energy  in,  by  horse 504,  506,  508,  510,  533,  539 

utilization  of  energy  in,  by  horse     513 

Water,  consumption  of,  influence  of,  on  heat  production 438 

maintenance  ration 438 

determination  of,  by  respiration  apparatus 79 

production  of,  in  metabolism 14,  15 

of  carbohydrates 23,  27 

fat 36 

proteids 42 

Wheat  gluten,  digestible  protein  of,  metabolizable  energy  of 310,  317 

gross  energy  of 309 

utilization  of  energy  of 481,  491 

metabolizable  energy  of 295,  297,  301 

digestible  matter  of 301 

utilization  of  energy  of 480,  490,  491 

straw,  digestible  carbohydrates  of,  metabolizable  energy  of 329 

crude  fiber  of,  metabolizable  energy  of 330,  332 

matter  of,  gross  energy  of 310 

metabolizable  energy  of 300,  301 

utilization  of  energy  of 487 

protein  of,  metabolizable  energy  of 321,  332 

metabolizable  energy  of 290,  297,  300,  301 


612  INDEX. 

PAGE 

Wheal  straw,  utilization  of  energy  of 461,  487,  490,  491 

Wind,  influence  of,  on  heat  emission 357 

Wool,  composition  of 63 

Work.     (See  also  Excrlum,  Tnuscular) 226 

cellular 344 

change  in  respiratory  quotient  caused  by 212 

coefficient  of  utilization  in 498 

disappearance  of  muscular  glycogen  in 23 

gain  of  proteids  during 204 

glandular 343 

internal 336,  337 

fasting  heat  production  a  measure  of 344 

muscular 341 

relation  of,  to  surface 366 

kind  of,  influence  of,  on  efficiency  of  animal 512 

mechanical,  determination  of 245 

muscular,  disappearance  of  glycogen  in 23 

incidental 342 

net  available  energy  for 497 

of  ascent,  consumption  of  oxygen  in,  by  dog 500 

corrected 508 

utilization  of  energy  in 502,  503,  510 

by  dog 502 

horse 506 

man 503 

effect  of  grade  on 512 

load  on 509,  510 

circulation 191,  341 

descent 509 

influence  of  grade  on .  . ,, 509 

digestion  and  assimilation 80,  93,  337,  372,  376,  406,  493 

above  critical  point 407 

below  critical  point 406 

indirect  utilization  of  heat  from.  . .  .  406 

in  dog 378 

horse 385 

man 382 

methods  of  determining 377 

of  bone 381 

carbohydrates 379,  382,  384 

fat 378,  382,  384,  385 

mixed  diet 382.  384 

proteids 381,  382,  384 

relation  of,  to  surface 408 


INDEX.  613 

PAGE 

Work  of  digestion,  factors  of 374 

for  crude  fiber 389 

draft,  consumption  of  oxygen  in,  by  dog 501 

utilization  of  energy  in 502,  507, 513 

by  dog 502 

horse 507,  513 

heart 192,  341 

locomotion,  computation  of 512 

consumption  of  oxygen  in,  by  dog 500 

horse 505 

correction  for  speed  in 507,  508 

expenditure  of  energy  in,  by  dog 500 

horse 504,  506,  508,  509 

510,  514,  533,  539 

utilization  of  energy  in,  computed 513 

mastication 391 

muscular  tonus 341 

respiration 192,  341 

standing 343 

voluntary  muscles 337 

physiological 336 

production,  function  of  liver  in 206 

relative  value  of  nutrients  in 522 

coarse  fodder   and  grain 

for 533 

value  of  crude  fiber  for 535,  537 

fat  for 522 

utilization  of  energy  in 444,  447,  494 

by  dog 499 

horse 502 

at  a  trot 509 

walk 504 

man 502 

influence  of  fatigue  on 519 

individuality  on 517 

kind  of  work  on 512 

load  on 508 

size  of  animal  on 515 

species  on 515 

speed  on 507,  513,  514 

training  on 519 

metabolizable  energy  in 525 

methods    of    determina- 
tion   526,  528 


614  INDEX. 

PAGE 

Work,  utilization  of  metabolizable  energy  in,  Wolff's  investigations 528 

of  feeding-stuffs  in 510 

fiber-free    nutrients    in   541, 543, 
545,  547 

net  available  energy  in 497 

variations  of  respiratory  quotient  during 216 


S'"-c> 


SHORT-TITLE     CATALOGUE 

OF  THE 

PUBLICATIONS 

OF 

JOHN   WILEY   &    SONS, 

New    York. 
London:    CHAPMAN  &  HALL,  Limited. 


ARRANGED  UNDER  SUBJECTS. 


Descriptive  circulars  sent  on  application. 

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AGBICTJLTTTRE. 

Armsby's  Manual    of   Cattle-feeding. 12mo,  $1  75 

Budd  and  Hansen's  American  Horticultural  Manual: 

Part  I. — Propagation,  Culture,  and  Improvement ....  12mo,  1  60 
Part  II. — Systematic  Pomology.     (In  preparation.) 

Downing's  Fruits  and  Fruit-trees  of  America 8vo,  5  00 

Grotenfelt's  Principles  of  Modern  Dairy  Practice.   (Woll.)  ..12mo,  2  00 

Kemp's  Landscape  Gardening 12mo,  2  50 

Maynard's  Landscape  Gardening  as  Applied  to  Home  Decoration. 

12mo,  1  50 

Sanderson's  Insects  Injurious  to  Staple  Crops 12mo,  1  50 

"  Insects  Injurious  to  Garden  Crops.     (In  preparation.) 

"  Insects  Injuring  Fruits.     (In  preparation.) 

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Woll's  Handbook  for  Farmers  and  Dairymen 16mo,  1  50 

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"          Architectural  Iron  and  Steel 8vo,  3  50 

"  Compound  Riveted  Girders  as  Applied  in  Buildings. 

8vo,  2  00 
"          Planning  and  Construction  of  High  Office  Buildings. 

8vo,  3  50 

"          Skeleton  Construction  in  Buildings 8vo,  3  00 

Briggs's   Modern   American   School   Buildings 8vo,  4  00 

Carpenter's  Heating  and  Ventilating  of  Buildings 8vo,  4  00 

Freitag's  Architectural  Engineering.  2d  Edition,  Rewritten.  8vo,  3  50 

"          Fireproofing  of  Steel  Buildings 8vo,  2  50 

Gerhard's  Guide  to  Sanitary  House- inspection 16mo,  1  00 

"          Theatre  Fires  and  Panics 12mo,  1  50 

Hatfield's  American  House  Carpenter 8vo,  5  00 

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Merrill's  Stones  for  Building  and  Decoration 8vo,  5  00 

Monckton's  Stair-building 4to,  4  00 

1 


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Siebert  and  Biggin's  Modern  Stone-cutting  and  Masonry ..  8vo, 
Snow's  Principal  Species  of  Wood:  Their  Characteristic  Proper- 
tics.     (In  preparation. ) 

Wait's  Engineering  and  Architectural  Jurisprudence 8vo, 

Sheep, 
Law  of  Operations  Preliminary  to  Construction  in  En- 
gineering and  Architecture 8vo, 

Sheep, 

"       Law  of  Contracts 8vo, 

Woodbury's  Fire  Protection  of  Mills 8vo, 

Worcester  and  Atkinson's  Small  Hospitals,  Establishment  and 
Maintenance,  and  Suggestions  for  Hospital  Architecture, 

with  Plans  for  a  Small  Hospital 12mo, 

The  World's  Columbian  Exposition  of  1893 Large  4to, 


AKMY  AND  NAVY. 

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of    the    Cellulose    Molecule 12mo, 

•  Bruff's  Text-book  Ordnance  and  Gunnery 8vo, 

Chase's  Screw  Propellers  and  Marine  Propulsion 8vo, 

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Cronkhite's  Gunnery  for  Non-commissioned  Officers,.24mo,mor., 

•  Davis's  Elements  of  Law 8vo, 

•  "      Treatise  on  the  Military  Law  of  United  States.  .8vo, 

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Young's  Simple  Elements  of  Navigation ..'.  16mo,'  morocco,' 

Second  Edition,  Enlarged  and  Revised 16mo,  mor.] 

I 


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Fletcher's  Practical  Instructions  in  Quantitative  Assaying  with 

the  Blowpipe 12mo,  morocco,  1  50 

Furman's  Manual  of  Practical  Assaying 8vo,  3  00 

Miller's  Manual  of  Assaying 12mo,  1  00 

O'Driscoll's  Notes  on  the  Treatment  of  Gold  Ores 8vo,  2  00 

Ricketts  and  Miller's  Notes  on  Assaying 8vo,  3  00 

Wilson's  Cyanide  Processes 12mo,  1  50 

"        Chlorination  Process 12mo,  1  50 

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Gore's  Elements  of  Geodesy 8vo,  2  50 

Hayford's  Text-book  of  Geodetic  Astronomy 8vo,  3  00 

Merriman's  Elements  of  Precise  Surveying  and  Geodesy.  . .  .8vo,  2  50 

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Thome  and  Bennett's  Structural  and  Physiological  Botany. 

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Austen's  Notes  for  Chemical  Student-* 12mo,  1  50 

Bernadou's  Smokeless  Powder. — Nitro-cellulose,  and  Theory  of 

the  Cellulose  Molecule 12mo,  2  50 

Bolton's  Quantitative  Analysis 8vo,  1  50 

Brush  and  Penfield's  Manual  of  Determinative  Mineralogy...8vo,  4  00 
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rick — Boltwood.)    8vo,  3  00 

Cohn's  Indicators  and  Test-papers 12mo,  2  00 

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fer.)    12mo,  2  00 

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Eissler's  Modern  High  Explosives 8vo,  4  00 

Effront's  Enzymes  and  their  Applications.     (Prescott.) 8vo,  3  00 

Erdmann's  Introduction  to  Chemical  Preparations.     (Dunlap.) 

12mo,  1  25 
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Fresenius's  Manual  of  Qualitative  Chemical  Anal  vsis.  (Wells.)  8vo,  5  00 
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Descriptive.     (Wells.) 8vo,  3  00 

"  System    of   Instruction   in    Quantitative   Chemical 

Analysis.      (Allen.) 8vo,  6  00 


Fuertes's  Water  and  PuDlic  Health 12mo,     1  50 

Furman'a  Manual  of  Practical  Assaying .8vo,     6  U0 

Gill's  Gas  and  Fuel  Analysis  for  Engineers   .......... .  .  12mo,     1  25 

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Hinds's  Inorganic  Chemistry ,V?V0'     3  29 

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Hopkina's  Oil-chemists'  Handbook 8vo,    3  00 

Keep's  Cast  Iron • -8vo,     2  50 

Ladd's  Manual  of  Quantitative  Chemical  Analysis 12mo,     1  00 

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Leach's  The  Inspection  and  Analysis  of  Food  with  Special  Refer- 
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Mandel's  Handbook  for  Bio-chemical  Laboratory 12mo,     1  50 

Mason's  Water-supply.     (Considered  Principally  from  a  Sani- 
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Miller's  Manual  of  Assaying 12mo, 

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Morgan's  Outline  of  Theory  of  Solution  and  its  Besults.  .12mo, 

"        Elements  of  Physical  Chemistry 12mo, 

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O'Driscoll's  Notes  on  the  Treatment  of  Gold  Ores 8vo, 

Ost  and  Kolbeck's  Text-book  of  Chemical  Technology.      (Lo- 
renz— Bozart.)     [In  preparation.) 

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Mineral  Tests 8vo,  paper, 

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Ricketts  and  Miller's  Notes  on  Assaying 8vo, 

Rideal's  Sewage  and  the  Bacterial  Purification  of  Sewage.  .8vo, 

Ruddiman's  Incompatibilities  in  Prescriptions 8vo, 

Schimpf's  Text-book  of  Volumetric  Analysis 12mo, 

,  Spencer's  Handbook  for  Chemists  of  Beet-sugar  Houses.  16mo, 

mor., 
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ists   16mo,  morocco, 

8tockbridge's  Rocks  and  Soils 8vo, 


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75 
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*  Tillman's  Elementary  Lessons  in  Heat 8vo,  1  50 

*  "        Descriptive  General  Chemistry 8vo,  3  00 

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Whipple's  Microscopy  of  Drinking-water 8vo,  3  50 

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CIVIL  ENGINEERING. 

BRIDGES     AND     ROOFS.       HYDRAULICS.       MATERIALS  OP 

ENGINEERING.     RAILWAY  ENGINEERING. 

Baker's  Engineers'  Surveying  Instruments 12mo,  3  00 

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Freitag's  Architectural  Engineering.    2d  Ed.,  Rewritten.  .  .8vo,  3  50 

French  and  Ives's  Stereotomy 8vo,  2  50 

Goodhue's  Municipal  Improvements 12mo,  1  75 

Goodrich's  Economic  Disposal  of  Towns'  Refuse 8vo,  3  50 

Gore's  Elements  of  Geodesy 8vo,  2  50 

Hayford's  Text-book  of  Geodetic  Astronomy 8vo,  3  00 

Howe's  Retaining- walls  for  Earth 12mo,  1  25 

Johnson's  Theory  and  Practice  of  Surveying Small  8vo,  4  00 

"           Stadia  and  Earth-work  Tables 8vo,  1  25 

Kiersted's  Sewage  Disposal 12mo,  1  25 

Laplace's  Philosophical  Essav  on  Probabilities.    (Truscott  and 

Emory.) 12mo,  2  00 

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Merriman's  Elements  of  Precise  Surveying  and  Geodesy. ..  .8vo,  2  50 

Merriman  and  Brooks's  Handbook  for  Surveyors. . . .  16mo,  mor.,  2  00 

Merriman's  Elements  of  Sanitary  Engineering 8vo,  2  00 

Nugent's  Plane  Surveying 8vo,  3  50 

Ogden's  Sewer  Design 12mo,  2  00 

Patton's  Treatise  on  Civil  Engineering 8vo,  half  leather,  7  50 

Reed's  Topographical  Drawing  and  Sketching 4to,  5  00 

Rideal's  Sewage  and  the  Bacterial  Purification  of  Sewage..  .8vo,  3  50 

Siebert  and  Biggin's  Modern  Stone-cutting  and  Masonry. . .  .8vo,  1  50 

Smith's  Manual  of  Topographical  Drawing.    (McMillan.) .  .8vo,  2  50 

*Trautwine's  Civil  Engineer's  Pocket-book.  ..  .16mo,  morocco,  5  00 

Wait's  Engineering  and  Architectural  Jurisprudence 8vo,  6  00 

Sheep,  6  50 
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Sheep,  5  50 

Wait's    Law    of    Contracts 8vo,  3  00 

Warren's  Stereotomy — Problems  in  Stone-cutting 8vo,  2  50 

Webb's  Problems  in  the  Use  and  Adjustment  of  Engineering 

Instruments   16mo,  morocco,  1  25 

5 


•  Wheeler's  Elementary  Course  of  Civil  Engineering Svo,     4  00 

Wilson's  Topographic  Surveying 8vo,     3  50 

BRIDGES  AND  ROOFS. 

Boiler's  Practical  Treatise  on  the  Construction  of  Iron  Highway 

Bridges 8vo,     2  00 

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Du  Bois's  Mechanics  of  Engineering.    Vol.  II Small  4to,  10  00 

Foster's  Treatise  on  Wooden  Trestle  Bridges 4to,  5  00 

Fowler's  Coffer-dam  Process  for  Piers 8vo,  2  50 

Greene's  Roof  Trusses 8vo,  1  25 

Bridge   Trusses 8vo,  2  50 

"          Arches  in  Wood,  Iron,  and  Stone 8vo,  2  50 

Howe's  Treatise  on  Arches 8vo,  4  00 

"       Design  of  Simple  Roof-trusses  in  Wood  and  Steel.  Svo,  2  00 
Johnson,   Bryan  and   Turneaure's  Theory  and  Practice  in   the 

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Merriman  and  Jacoby's  Text-book  on  Roofs  and  Bridges: 

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Part  II  — Graphic  Statics 8vo,  2  50 

Part  III.— Bridge  Design.     Fourth  Ed.,  Rewritten 8vo,  2  50 

Part  IV.— Higher  Structure8 8vo,  2  50 

Morison's  Memphis  Bridge 4to,  10  00 

WaddelPs  De  Pontibus,  a  Pocket  Book  for  Bridge  Engineers. 

16mo,  mor.,  3  00 

"            Specifications  for  Steel  Bridges 12mo,  1  25 

Wood's  Treatise  on  the  Theory  of  the  Construction  of  Bridges 

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8vo,  2  50 

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HYDRAULICS. 

Bazin's  Experiments  upon  the  Contraction  of  the  Liquid  Vein 

Issuing  from  an  Orifice.     (Trautwine.) 8vo, 

Bovey's  Treatise  on  Hydraulics 8vo, 

Church's  Mechanics  of  Engineering 8vo, 

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paper, 
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Fol well's  Water-supply  Engineering 8vo, 

Frizell's    Water-power 8vo, 

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Ganguillet   and   Flutter's   General    Formula    for   the   Uniform 
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Hazlehurst's  Towers  and  Tanks  for  Water-works 8vo, 

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Riveted,  Metal  Conduits 8vo,     2  09 


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Wegmann's  Design  and  Construction  of  Dams 4to,  5  00 

"  Water-supply  of  the  City  of  New  York  from  1658  to 

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Wilson's  Manual  of  Irrigation  Engineering Small  8vo,  4  00 

Wolff's  Windmill  as  a  Prime  Mover 8vo,  3  00 

Wood's  Turbines 8vo,  2  60 

"      Elements  of  Analytical  Mechanics 8vo,  3  00 


MATERIALS   OF   ENGINEERING. 

Baker's   Treatise   on   Masonry   Construction 8vo, 

Black's  United  States  Public  Works Oblong  4to, 

Bovey's  Strength  of  Materials  and  Theory  of  Structures. . .  .8vo, 
Burr's  Elasticity  and  Resistance  of  the  Materials  of  Engineer- 
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Lanza's  Applied  Mechanics 8vo, 

Martens's  Handbook  on  Testing  Materials.   (Henning.).2  v.,  8vo, 

Merrill's  Stones  for  Building  and  Decoration 8vo, 

Merriman's  Text-book  on  the  Mechanics  of  Materials 8vo, 

Merriman's  Strength  of  Materials 12mo, 

Metcalf 8  Steel.    A  Manual  for  Steel-users 12mo, 

Patton's  Practical  Treatise  on  Foundations 8vo, 

Rockwell's  Roads  and  Pavements  in  France 12mo, 

Smith's  Wire:  Its  Use  and  Manufacture Small  4to, 

"       Materials  of  Machines 12mo, 

Snow's  Principal  Species  of  Wood:  Their  Characteristic  Proper- 
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Spalding's  Hydraulic  Cement 12mo, 

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Thurston's  Materials  of  Engineering 3  Parts,  8vo, 

Part   I. — Non-metallic  Materials  of  Engineering  and   Metal- 
lurgy     8vo, 

Part  II. — Iron  and  Steel 8vo, 

Part  III. — A  Treatise  on  Brasses,  Bronzes  and  Other  Alloys 

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"  Machinery  of  Transmission  and  Governors.    (Herr- 

(man — Klein.)    8vo,  5  00 

Wood's  Elements  of  Analytical  Mechanics 8vo,  3  00 

"       Principles  of  Elementary  Mechanics 12mo,  1  25 

"       Turbines  8vo,  2  50 

The  World's  Columbian  Exposition  of  1893 4to,  1  00 

METALLURGY. 

Egleston's  Metallurgy  of  Silver,  Gold,  and  Mercury: 

Vol.  I.— Silver 8vo,  7  50 

Vol.  II.— Gold  and  Mercury 8vo,  7  50 

**  Iles's  Lead-smelting 12mo,  2  50 

Keep's  Cast  Iron 8vo,  2  50 

Kunhardt's  Practice  of  Ore  Dressing  in  Lurope 8vo,  1  50 

Le  Chatelier's  High-temperature  Measurements.     (Boudouard — 

Burgess.)  12mo,  3  00 

Met  calf's  Steel.    A  Manual  for  Steel-users 12mo,  2  00 

Smith's  Materials  of  Machines 12mo,  1  00 

Thurston's  Materials  of  Engineering.    In  Three  Parts 8vo,  8  00 

Part  II.— Iron  and  Steel 8vo,  3  50 

Part  III. — A  Treatise  on  Brasses,  Bronzes  and  Other  Alloys 

and  Their  Constituents .' 8vo,  2  80 

14 


2  60 

3  00 

2  00 

4  00 

1  00 

1  25 

3  50 

L2  60 

1  00 

4  00 

1  50 

1  00 

2  00 

2  50 

MINERALOGY. 

Barringer's    Description    of    Minerals    of    Commercial    Value. 

Oblong,  morocco, 

Boyd's  Resources   of   Southwest    Virginia 8vo, 

"        Map  of  Southwest  Virginia Pocket-book  form, 

Brush's  Manual  of  Determinative  Mineralogy.     (Penfield.)  .8vo, 

Chester's  Catalogue  of  Minerals 8vo,  paper, 

Cloth, 

"         Dictionary  of  the  Names  of  Minerals 8vo, 

Dana's  System  of  Mineralogy Large  8vo,  half  leather, 

"      First  Appendix  to  Dana's  New  "  System  of  Mineralogy." 

Large  8vo, 

"       Text-book  of  Mineralogy 8vo, 

"       Minerals  and  How  to  Study  Them 12mo, 

"       Catalogue  of  American  Localities  of  Minerals .  Large  8vo, 

"      Manual  of  Mineralogy  and  Petrography 12mo, 

Egleston's  Catalogue  of  Minerals  and  Synonyms 8vo, 

Hussak'8     The     Determination     of     Rock-forming     Minerals. 

(Smith.)    Small   8vo,    2  00 

•Penfield's  Notes  on  Determinative  Mineralogy  and  Record  of 

Mineral  Tests 8vo,  paper,        50 

Rosenbusch's  Microscopical  Physiography  of  the  Rock-making 

Minerals.      (Idding's.) 8vo,    5  00 

•Tillman's  Text- book  of  Important  Minerals  and  Rocks.. 8 vo,    2  00 
Williams's  Manual  of  Lithology 8vo,    3  00 


MINING. 

Beard's  Ventilation  of  Mines 12mo,  2  50 

Boyd's  Resources  of  Southwest  Virginia 8vo,  3  00 

"        Map  of  Southwest  Virginia Pocket-book   form,  2  00 

•Drinker's     Tunneling,     Explosive     Compounds,     and     Rock 

Drills 4to,  half  morocco,  25  00 

Eissler's  Modern  High  Explosives 8vo,  4  00 

Fowler's  Sewage  Works  Analyses 12mo,  2  00 

Goodyear's  Coal-mines  of  the  Western   Coast  of   the   United 

States    12mo,  2  50 

Dilseng's  Manual  of  Mining 8vo,  4  00 

**  Hes's  Lead-smelting 12mo,  2  50 

Kunhardt's  Practice  of  Ore  Dressing  in  Europe 8vo,  1  50 

O'Driscoll's  Notes  on  the  Treatment  of  Gold  Ores 8vo,  2  00 

Sawyer's  Accidents  in  Mines 8vo,  7  00 

Walke's  Lectures  on  Explosives 8vo,  4  00 

Wilson's  Cyanide  Processes 12mo,  1  50 

Wilson's  Chlorination  Process 12mo,  1  50 

Wilson's  Hydraulic  and  Placer  Mining 12mo,  2  00 

Wilson's  Treatise  on  Practical  and  Theoretical  Mine  Ventila- 
tion   12mo,  1  25 

SANITARY  SCIENCE. 

Folwell's  Sewerage.    (Designing,  Construction  and  Maintenance.) 

8vo,  3  00 

"         Water-supply    Engineering 8vo,  4  00 

Fuertes's  Water  and  Public  Health 12mo,  1  50 

"        Water-filtration   Works 12mo,  2  60 

15 


Gerhard's  Guide   to  Sanitary  House-inspection 16mo,  1  00 

Goodrich's  Economical  Disposal  of  Towns'  Refuse.  ..Demy  8vo,  3  60 

Hazen's  Filtration  of  Public  Water-supplies 8vo,  3  00 

Kiersted's  Sewage  Disposal 12mo,  1  25 

Leach's   The   Inspection    and   Analysis    of   Food   with    Special 

Reference  to  State  Control.     (In  preparation.) 
Mason's   Water-supply.     (Considered   Principally  from   a  San- 
itary Standpoint.     3d  Edition,  Rewritten 8vo,  4  00 

"        Examination    of    Water.      (Chemical    and    Bacterio- 
logical.)     12mo,  1  25 

Merriman's  Elements  of  Sanitary  Engineering 8vo,  2  00 

Nichols's  Water-supply.     (Considered  Mainly  from  a  Chemical 

and  Sanitary  Standpoint.)      (1883.)   8vo,  2  50 

Ogden's  Sewer  Design 12mo,  2  00 

•  Price's  Handbook  on  Sanitation 12mo,  1  50 

Richards's  Cost  of  Food.    A  Study  in  Dietaries 12mo,  1  Of 

Richards  and  Woodman's  Air,  Water,  and  Food  from  a  Sani- 
tary   Standpoint 8vo,  2  00 

Richards's  Cost  of  Living  as  Modified  by  Sanitary  Science.  12mo,  1  00 

•  Richards  and  Williams's  The  Dietary  Computer 8vo,  1  50 

Rideal'a  Sewage  and  Bacterial  Purification  of  Sewage 8vo,  3  60 

Turneaure  and  Russell's  Public  Water-supplies 8vo,  5  00 

Whipple's  Microscopy  of  Drinking-water 8vo,  3  60 

Woodhull's  Notes  on  Military  Hygiene 16mo,  1  60 


MISCELLANEOUS. 

Barker's  Deep-sea  Soundings 8vo,  2  00 

Emmous's  Geological  Guide-book  of  the  Rocky  Mountain  Ex- 
cursion   of    the    International    Congress    of    Geologists. 

Large  8vo,  1  50 

FerrePs  Popular  Treatise  on  the  Winds 8vo,  4  00 

Haines's  American  Railway  Management 12mo,  2  60 

Mott's  Composition,  Digestibility,  and  Nutritive  Value  of  Food. 

Mounted  chart,  1  26 

"      Fallacy  of  the  Present  Theory  of  Sound 16mo,  1  00 

Ricketts's  History  of  Rensselaer  Polytechnic  Institute,   1824- 

1894 Small    8vo,  3  00 

Rotherham'a  Emphasised  New  Testament Large  8vo,  2  00 

Critical  Emphasised  New  Testament 12mo,  1  50 

Steel's  Treatise  on  the  Diseases  of  the  Dog 8vo,  3  60 

Totten's  Important  Question  in  Metrology 8vo,  2  60 

The  World's  Columbian  Exposition   of  1893 4to,  1  00 

Worcester  and  Atkinson.     Small  Hospitals,  Establishment  and 
Maintenance,  and  Suggestions  for  Hospital  Architecture, 

with  Plans  for  a  Small  Hospital 12mo,  1  26 


HEBREW  AND  CHALDEE  TEXT-BOOKS. 

Green's  Grammar  of  the  Hebrew  Language 8vo,  3  00 

"       Elementary  Hebrew   Grammar 12mo,  1  25 

"       Hebrew  Chrestomathy 8vo,  2  00 

Gesenius's  Hebrew  and  Chaldee  Lexicon  to  the  Old.  Testament 

Scriptures.     (Tregelles.) Small  4to,  half  morocco,  5  00 

Letteris's  Hebrew  Bible 8vo,  2  26 

16 


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